Salt effects on the rates and mechanisms of solvolysis reaction of organic halides and water structure distortion in N,N-dimethylformamide-and N,N-dimethylacetamide-water mixed solvents

Article abstract of DOI:10.1246/bcsj.20090329

In 50% (v/v) DMFand DMAH₂O mixed solvents, salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX) have been examined for further exploring. Compared with previous results obtained in other organic solvent systems, such as 50% (v/v) acetone, 1,4-dioxane, or sulfolaneH₂O, no exceptional behavior was observed in the "pseudo" first-order rate constants (k/s^-1) of all the typical SN¹ and SN² substrates by the addition of alkali metal and alkaline earth metal perchlorates or many kinds of tetraalkylammonium salts in 50% (v/v) DMFand DMA-H₂O mixed solvents. The detailed examination of Δlog(k/s^-1)/Δ[LiClO₄] for 1-adamantyl bromide vs. the contents of organic solvents (CH₃CN, DMA, and DMSO) suggested that the observed different salt effects were caused by the different solvation abilities of the solvents toward the leaving-group anion as well as the metal cation. As a new highlight in the present paper, we were able to demonstrate a proportionality or correlation between the LiClO₄ effects in the solvolysis rates and the carbocation stabilities expressed by the Gibbs free energy values (ΔG^o) of RX in the gas phase. Based on the Raman spectra of DMA-H₂O and DMF-D₂O as well as DMF-D₂O mixed solvents, we discuss the distortion of the bulk water structure.

Full text of DOI:10.1246/bcsj.20090329

©
2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 4, 401–414 (2010)
401
Salt Effects on the Rates and Mechanisms of Solvolysis
Reaction of Organic Halides and Water Structure
Distortion in N,N-Dimethylformamide­ and
N,N-Dimethylacetamide­Water Mixed Solvents
Masashi Hojo,* Tadaharu Ueda, Eriko Ueno, Tensei Hamasaki, and Toshiyuki Nakano
Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520
Received December 11, 2009; E-mail: mhojo@cc.kochi-u.ac.jp
In 50% (v/v) DMF­ and DMA­H2O mixed solvents, salt effects on the solvolysis reaction rates of aliphatic halides
and related compounds (RX) have been examined for further exploring. Compared with previous results obtained in other
organic solvent systems, such as 50% (v/v) acetone­, 1,4-dioxane­, or sulfolane­H2O, no exceptional behavior was
¹1
observed in the “pseudo” first-order rate constants (k/s ) of all the typical SN1 and SN2 substrates by the addition of
alkali metal and alkaline earth metal perchlorates or many kinds of tetraalkylammonium salts in 50% (v/v) DMF­ and
¹1
DMA­H2O mixed solvents. The detailed examination of ¦log(k/s )/¦[LiClO4] for 1-adamantyl bromide vs. the
contents of organic solvents (CH3CN, DMA, and DMSO) suggested that the observed different salt effects were caused by
the different solvation abilities of the solvents toward the leaving-group anion as well as the metal cation. As a new
highlight in the present paper, we were able to demonstrate a proportionality or correlation between the LiClO4 effects in
o
the solvolysis rates and the carbocation stabilities expressed by the Gibbs free energy values (¦G ) of RX in the gas
phase. Based on the Raman spectra of DMA­H2O and DMA­D2O as well as DMF­D2O mixed solvents, we discuss the
distortion of the bulk water structure.
The ionic strength influences the kinetics of reactions
pairs as shown in eq 1,
between ions and neutral molecules. It is believed that at
higher ionic concentration, the rate constant may change
because of changes in the activity coefficients of the ions (not
given by the Debye­Hückel theory) and because the activity
coefficients of neutral molecules are affected by higher ionic
R kX þ M kClO4 ꢀ R kClO4ꢀ þ M kX
þ
ꢀ
þ
ꢀ
þ
þ
ꢀ
ð1Þ
and his interpretation has been applied to so many salt
effects on solvolysis reactions. Nevertheless, a quantitative and
comprehensive treatment has never succeeded in accounting
for the concentrated salt effects on solvolysis reactions. The
1
strengths.
9
In organic chemistry, nucleophilic substitution reactions,
solvent and salt effects in organic reactions have been
1
0,11
especially solvolysis reactions have been vastly studied and
widely discussed.
ysis supplied the necessary tool to study chemical systems on
the nanosecond and picosecond time scales relevant to ion-pair
The development of laser-flash photol-
2
discussed. It is known that the rates of S 1 solvolysis of alkyl
N
halides and related compounds are subject to positive salt
3
4
12
effects. Bunton et al. reported exponential increases in the
dynamics. Bockman and Kochi mentioned, however, the
methanolysis rates of S 1 substrates with increasing concen-
original goal of finding a spectroscopic distinction between
contact ion pairs (CIPs) and solvent-separated ion pairs (SSIPs)
remains unfulfilled.
N
+
tration of salts and they insisted small cation effects (for Li ,
+
+
5
Na , and Et N ). However, Allen et al. have reported a great
4
1
3­16
difference in the salt effects of LiClO and n-Bu NClO on the
In previous studies,
we have observed the solvolysis rate
4
4
4
solvolysis of 4-methoxybenzyl chloride in 85% (v/v) aqueous
dioxane. Many negative salt effects for unimolecular heteroly-
sis have been reviewed.6 Ingold and Hughes7 carried out
detailed kinetic and stereochemical investigations on nucleo-
philic substitution at saturated carbon and polar elimination
reaction.
constant of typical SN1 substrate, such as 1-adamantyl halides
or tert-butyl chloride, increases with increasing concentration
of alkali metal (M ) and alkaline earth metal (M ) perchlo-
+
2+
+
+
2+
2+
rates: cation effects increase as Na < Li < Mg , Ba in
1
3
80% (v/v) MeOH­H O (¾ = 49), 50% (v/v) acetone­H O
2
r
2
1
4
15
(¾ = 55), 50% (v/v) 1,4-dioxane­H O (¾ = 44), and
50% (v/v) sulfolane­H O solvent systems. The main points
2
proposed in our previous solvolytic studies
r
2
r
8
16
Winstein and co-workers proposed an ion-pair scheme for
1
3­16
solvolyses in an ionizing but only slightly dissociating solvent,
especially, acetic acid (¾r = 6.2). According to Winstein’s
interpretation, the “normal” salt effect on the acetolysis rate is
due to acceleration of the substrate ionization step leading to
are summariz-
ed as: (1) The addition of highly concentrated salts as well as
organic solvents would reduce the properties of bulk water
(H­O­H) into those of a non-aqueous solvent, such as an
alcohol (R­O­H) or even an ether (R­O­R). (2) In water-mixed
organic solvents containing concentrated metal perchlorates
+
¹
the contact ion pair (RX ꢀ R , X ) while the “special” salt
effect is the result of an exchange reaction between two ion
Published on the web March 26, 2010; doi:10.1246/bcsj.20090329

4
02
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
H O
Salt Effects on the Rates and Mechanisms
2
covalent bonding or coordinating) as well as Coulombic
interactions with many simple anions, not only in low
C
C
C
24
permittivity media (¾ < 10) but also in higher permittivity
r
+
+
HO
22,23
media (20 < ¾ < 65) with poor solvating ability;
where
r
-
X
the simple anions include halides, carboxylates, phosphates,
sulfonates, etc. Coordination chemistry of the lithium ion with
3
1
simple anions as well as cyclic ligands is reviewed.
3
2
+
2+
Olah reviewed the significance of carbocations in chem-
istry; in general, stable carbocations are prepared in superacidic
M , M
3
3
media. Schaller and Mayr made it possible to monitor
photometrically the formation of the carbocation of 4,4¤-
bis(morpholino)benzhydryl carboxylates in aqueous acetone
and acetonitrile. However, we have discovered that stable
carbocations can be produced from trityl halides by the
Scheme 1. Proposed mechanism for the metal cation effect
on a typical SN1 solvolysis.
(
0­3.0 mol dm¹3), the exponential increases in solvolysis rates
of S 1 substrates are caused by the favorable formation of
carbocations through direct “chemical” interaction between
metal cations and the leaving group of a substrate in the
modified solvent. (3) The solvolysis rates of S 2 substrates
N
+
+
2+
2+
addition of the perchlorates salts of Li , Na , Mg , (Ca ,
2
+
2+
25­27a
Sr ,) and Ba in acetonitrile;
which have been detected
by UV­visible, H and 13C NMR spectroscopies.
2
5 1
26,27a
For a
N
are decelerated by concentrated salts because of the decrease
in water activity. (4) Positive or negative salt effects by
fluoran-based black color former in acetonitrile, the color
has been developed by the addition of the alkali metal or
2
7a
LiClO can be correlated to the m-values (by Grunwald and
alkaline earth metal perchlorates as well as phenol which
is used as a normal color developer. In addition, reversible
4
1
7
Winstein) of the substrates. The mechanism of the metal
cation effects can be illustrated by Scheme 1.
2
+
decolorization through the interaction between Mg
and
At a first glance, however, the proposed scheme might be
judged to be impossible because no one can deny that a diluted
alkali metal halide (e.g., NaCl) in aqueous solution would
dissociate completely. Difficulties involved in the mechanism
arise from the following two points: (a) the coordination ability
of alkali metal or alkaline earth metal ions must be quite
polyamines (acyclic and macrocyclic) made it possible to
reconfirm that the colored zwitterion is produced by the direct
chemical interaction with Mg(ClO ) in the fluoran dye­
4
2
2
7b
acetonitrile solution.
In order to support our proposal of previous solvolytic
1
3­16
studies
and to explore further, we examined the effects of
1
8
small because of no d-electrons. Cotton and Wilkinson
various types of salts, MClO , M(ClO ) , and R NX (R = Et,
4
4 2
4
describe that in main elements, s- and p-orbitals, especially
p-orbitals, are the important key and d-orbitals play a secondary
role, sometimes just a negligible role. (b) In aqueous solution,
strong solvation (hydration) may shade completely the chemi-
cal interaction sites on alkali metal or alkaline earth metal
cations as well as simple anions even though they have
some potential for such chemical interaction in addition to
Coulombic interaction.
Pr, n-Bu, X = ClO , Cl, Br, OTs) on the solvolysis reactions of
typical SN1 and SN2 substrates in 50% (v/v) DMF­ and DMA­
4
H O solvents. The basicities of DMF and DMA are rather
2
3
4
large, i.e., Gutmann’s donor numbers (DN) = 26.6 and 27.8,
respectively, while the acidities of the solvents are small, i.e.,
the acceptor numbers (AN) of DMF and DMA are reported to
3
4
be 16.0 and 13.6, respectively. Acetonitrile and DMSO were
also used as solvents, and the role of the organic solvent in
water mixed solvents was discussed. The substitution (or
“anion exchange”) of leaving groups with added salt anions for
Nevertheless, we have postulated that a very small but direct
+
2+
“
chemical” interaction can operate between M or M and
simple anions, such as halide, even in aqueous or organic­
aqueous solutions if they are under the “dihydrogen ether”
typical S 2 substrates were examined thoroughly in the 50%
N
(v/v) DMF­ as well as the DMA­H O solvent system. The
2
1
3­16
(
[R](H)­O­(H)[R])
conditions. As for “dihydrogen ether,”
solvolyses accompanied by the anion exchanges for benzyl
halides (S 1­S 2 borderline substrates) were examined also in
1
9
13,15
Reichardt et al. described citing our papers
that, at high
N
N
¹3
salt concentrations (c > 5 mol dm ), region C of ion solva-
tions, according to Frank and Wen,20 can be abolished and only
regions A and B survive, resulting an aqueous solvent called
the same solvent. A linear relationship was observed between
the stabilities of carbocations and the LiClO effects on the
4
solvolysis rate constants of the various substrates in 50% (v/v)
DMA­H2O. Raman spectra of DMA­H2O and DMA­D2O in
the presence of various salts were measured to evaluate the
degree of distortion of bulk water properties. Raman spectra of
“
dihydrogen ether.”
In addition, Reichardt et al.19 have described that the adverse
influence of tetramethylammonium chloride is more difficult to
explain and that tetraalkylammonium salts are usually regarded
as possessing structure-making properties. However, we have
DMF­D O were also measured to obtain further confident
2
evidence.
to stress that tetraalkylammonium halides at higher concen-
trations do distort the bulk water structure1
5,21
Results and Discussion
even though the
salt at low concentrations is regarded as possessing structure-
Exponential Increases in the Solvolysis Rate of 1-
Adamantyl Chloride by the Addition of Metal Perchlorates.
Figure 1 shows the changes in the solvolysis rate of 1-
making properties.
By means of voltammetry,22 conductometry,
23,24
and UV­
visible and NMR spectroscopies,2
5­30
we have certainly
adamantyl (=1-tricyclo[3.3.1.1 ]decyl) chloride with the
3,7
demonstrated that alkali metal and alkaline earth metal cations
possess “weak” but unexpectedly distinct chemical (i.e.,
addition of alkali metal or alkaline earth metal perchlorates
in a 50% (v/v) N,N-dimethylacetamide (DMA)­water mixed

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
403
Figure 1. Exponential increases in the solvolysis rate
constant of 1-adamantyl chloride with the addition of
various metal perchlorates in 50% (v/v) DMA­H2O at
Figure 2. Differences of slopes in the solvolysis rate
constant of 1-adamantyl chloride with LiClO in various
4
contents of DMA in DMA­H O mixed solvents at 65 °C:
2
65 °C: ( ) NaClO4, ( ) LiClO4, ( ) Mg(ClO4)2, and ( )
( ) 50, ( ) 60, and ( ) 75% (v/v) DMA.
Ba(ClO4)2.
+
¹
solvent at 65 °C. The “pseudo” first-order reaction rate constant
between Li and Cl may be encouraged to produce greater
¹1
(
k/s ) increased exponentially with increasing concentration
amount of carbocations at the higher LiClO concentrations.
4
of LiClO , NaClO , Mg(ClO ) , or Ba(ClO ) , i.e., linearity
Similar phenomena were observed for 1-adamantyl bromide
in 60, 70, and 90% (v/v) DMA­water mixed solvents. More
and less distinct slope changes for 1-adamantyl bromide were
4
4
4 2
¹
4 2
1
was observed between log(k/s ) and the concentration of
MClO or M(ClO ) . The effects of metallic ions increased as
4
4 2
2+
+
+
2+
Na < Li < Ba < Mg . The difference in the salt effects
between Mg and Ba is small and some contradictory
results have been observed. In 50% (v/v) DMF­water or 50%
v/v) DMSO­water, obvious superiority of Ba over Mg
was observed, while the superiority between Mg and Ba
obtained in CH CN­ and DMSO­water systems, respectively.
3
2+
2+
The solvation ability toward the metal cations of CH3CN
3
4
34
(DN = 14.1) is much smaller than DMSO (DN = 29.8),
2
+
2+
(
although the solvation abilities of both solvents toward anions
must be similar, judging from the same acceptor numbers
(AN = 19.3). The experimental results will be discussed in the
next section.
2
+
2+
¹
3
(
5
of up to 1.5 mol dm salt concentration) was not observed in
0% (v/v) sulfolane­water solvent.16
The Arrhenius plots of the solvolysis reaction (55­80 °C) of
Contents of Organic Solvents and the Role of Their
1
8
-adamantyl chloride in 50% (v/v) DMA­H O solvent gave
Solvation Abilities. The effects of LiClO on the solvolysis
rate constant of 1-adamantyl bromide were examined in 60,
2
4
¹
1
9.0, 94.3, and 109.1 kJ mol activation energies (Ea) for no
¹
3
salt, 1.0 and 2.0 mol dm LiClO , respectively. The values
were 95.1 and 98.7 kJ mol for 1.0 and 2.0 mol dm NaClO4,
respectively.
75, and 90% (v/v) DMA­H O mixed solvents: the linear
4
2
¹
1
¹3
¹1
increases in log(k/s ) with the increasing concentration of
LiClO indicated the positive salt effect. However, the value
4
¹
1
Figure 2 shows exponential increases in the solvolysis rate
of ¦log(k/s )/¦[LiClO ] varied with increasing DMA con-
4
¹1
constant (k/s ) of 1-adamantyl chloride with LiClO in
various contents of DMA (50, 60, and 75% (v/v)) in DMA­
H O mixtures at 65 °C. In the absence of LiClO , the solvolysis
tents: 0.30, 0.46, and 0.51 in 60, 75, and 90% (v/v) DMA,
respectively. As described above, for the chloride substrate,
4
¹1
¦log(k/s )/¦[LiClO ] = 0.32, 0.38, and 0.51 in 50, 60, and
2
4
4
reaction constants decreased with increasing content of DMA:
75% (v/v) DMA­H2O mixed solvents (Figure 2). Obviously,
the slopes for the bromide substrate are smaller than those
¹
1
log(k/s ) = ¹4.22, ¹5.03, and ¹6.39 in 50, 60, and 75%
v/v) DMA­H O, respectively. The deceleration in the
+
¹
(
for the chlorides. The interaction of Li with Br should
2
¹
solvolysis should be caused by the decreases in the water
activity with increasing contents of DMA. However, the slope
be smaller than that with Cl in those solvents. In 75 and
¹1
90% (v/v) DMSO­water mixed solvents, the ¦log(k/s )/
¹
1
of log(k/s ) vs. the LiClO concentration increased with
¦[LiClO ] values were 0.32 and 0.39 for 1-adamantyl bromide.
4
4
¹
1
increasing content of DMA: ¦log(k/s )/¦[LiClO ] = 0.32,
In the 50% (v/v) CH CN­water mixed solvent, a linear
4
3
0
.38, and 0.51 for 50, 60, and 75% (v/v) DMA­H O, respec-
increase in the solvolysis constant was observed for 1-
adamantyl bromide with increasing LiClO4 concentration up
2
tively. Increasing contents of the organic solvent cause the
decrease of the activity of water, by which the solvolyses are
decelerated. However, both Li and Cl (of the leaving group)
are so much less solvated in water mixed solvents of lesser
water contents (e.g., 25%) that the chemical interaction
¹3
to 2.0 mol dm . In 60, 75, and 90% (v/v) CH CN­water
3
+
¹
¹1
mixed solvents, however, the relation of log(k/s ) vs. LiClO
4
concentration gave not linear but convex curves over lower
LiClO concentrations (Figure 3). For lower LiClO concen-
4
4

4
04
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
Salt Effects on the Rates and Mechanisms
Scheme 2. Involvement of acceptor (AN) and donor (DN)
numbers of an organic solvent toward the leaving-group
anion as well as the metal cation.
It is true that in every solvent system the salt effect becomes
more significant with increasing content of organic solvent.
¹
1
However, the fact that the slope of ¦log(k/s )/¦[LiClO ] (·)
4
vs. the content of an organic solvent differs one another
suggests that the different salt effects are caused by the different
solvation abilities of the solvents. Scheme 2 shows the
involvement (or the solvation) of an organic solvent toward
the leaving-group anion from the substrate as well as toward
the cation of the salt. Both acceptor (AN) and donor (DN)
numbers of an organic solvent should be involved for the
metal salt effect, where the perchlorate ion is regarded to
have no specific effect other than Coulombic interaction. An
inverse proportional relationship between ¦·/¦ (contents of
organic solvent) and the square roots of donor and acceptor
Figure 3. Changes in solvolysis rate constant of 1-ada-
mantyl bromide with the addition of LiClO4 in CH3CN­
H2O mixed solvents: ( ) 50% (30 °C), ( ) 60% (30 °C),
(
) 75% (45 °C), and ( ) 90% (60 °C) CH3CN.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
numbers ( DN AN) for CH CN (16.5), DMA (19.4), and
3
ꢁ
DMSO (24.0) supported our proposed mechanism illustrated
in Scheme 2. Figure 4 provides us with the ¦log(k/s )/
¹1
¦
[LiClO4] (·) values at 50% (v/v) organic solvents: 0.44, ca.
0
.25, and ca. 0.2 for CH CN, DMA, and DMSO, respectively.
3
Now, we can rather directly compare these values to previous
1
4
15
data: log(k /k ) = 0.39 (at 50 °C) and 0.48 (at 50 °C) for
1
0
5
0% (v/v) acetone­ and 1,4-dioxane­water mixed solvents,
where the k and k values represent the solvolysis rate con-
1
0
¹
3
stants of 1-adamantyl bromide at 0 and 1.0 mol dm LiClO4,
respectively. We may note that the DN
and 1,4-dioxane are 14.6 and 12.6, respectively.
Deceleration in the Solvolysis Rate of a Typical S 1
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢁ
AN values of acetone
N
Figure 4. The changes in salt effects, ¦log(k/s¹1)/¦[Li-
ClO4] (·), with increasing contents of organic solvents.
The temperatures at which the rate constants were observed
are 45, 60, and 80 °C for 60, 75, and 90% DMA­H2O, 60
and 80 °C for 75 and 90% DMSO­H2O, respectively; cf.
Figure 3 for CH3CN­H2O.
Substrate by the Addition of Non-Metallic Salts.
The
effects of non-metallic salts on the S 1 solvolysis rate were
N
examined in the 50% (v/v) DMA­H O solvent. Figure 5 shows
2
the change in the solvolysis rate of 1-adamantyl chloride with
+
¹
¹
¹
¹
¹
increasing concentration of Et N X (X = ClO , Cl , Br ,
4
4
¹
and TsO ) at 65 °C. All the tetraethylammonium salts caused
the deceleration in the solvolysis rate; the tosylate with the
greatest bulk among the four anions caused the most remarkable
decrease in the ¦log(k/s ) value of ¹0.59, i.e., the solvolysis
rate decreased to one-fourth at 1.0 mol dm of the salt, com-
¹
3
¹1
tration ranges (0­0.5 mol dm ), the ¦log(k/s )/¦[LiClO ]
4
¹
1
values were evaluated to be 0.44, 0.54, 0.66, and 0.85 in 50, 60,
¹
3
7
5 and 90% (v/v) CH CN­water solvents, respectively.
Figure 4 shows the relationship between the ¦log(k/s )/
3
¹
1
¹1
pared with that in the absence of the salt [log(k/s ) = ¹4.22] at
¦
[LiClO ] (·) values and the contents of the organic solvents
65 °C. The tetraethylammonium salts caused the deceleration
effect in the order of ClO4 µ Br < Cl < TsO for the
4
¹
¹
¹
¹
(0.5, 0.6, 0.75, and 0.9) in the water mixed solvents. It is
¹
¹
¹
obvious that at 75% (v/v) organic solvent contents, for
instance, the ¦log(k/s )/¦[LiClO4] (·) value increases
as DMSO < DMA < CH CN and also that the slope of
¦
anions. A similar and distinct order (ClO < Br < Cl <
4
¹
1
¹
+
TsO ) of Et4N salts has been observed in 50% (v/v) sulfolane­
water. In 50% (v/v) DMF­H O, the order of ClO
Br < Cl was observed for 1-adamantyl chloride at 70 °C and
also for 1-adamantyl bromide at 45 °C: the deceleration effect
by Et NCl was larger than that by Et NBr (Table S1).
1
6
¹
<
4
3
2
¹
1
¹
¹
log(k/s )/¦[LiClO ] (·) vs. contents of the organic solvents
4
become more steep as DMSO (0.46) < DMA (0.73) < CH CN
3
(1.0).
4
4

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
405
The deceleration effect must be brought by the decrease in
the activity of water mixed with DMF or DMA: the solvolyses
Similar but more remarkable deceleration in the solvolysis
rate was observed with other tetraalkylammonium bromide
of typical S 1 substrates should be initiated by the attack of
(R NBr: R = Pr and n-Bu). The longer the chain in tetraalkyl-
4
N
water molecules toward the substrates in order to produce the
carbocations. The solvation abilities of the anions may result in
the difference of “effective” water activities. The distortion of
water structure with the presence of tetraalkylammonium salts
will be discussed based on our Raman spectral data in the final
section.
ammonium bromide, the more remarkable deceleration was
+
+
+
caused of the solvolysis rate as Et N < Pr N < n-Bu N :
4
4
4
¹
1
the changes in the logarithm factors [¦log(k/s )] were
¹0.2, ¹0.63, and ¹1.02 with the 1.0 mol dm salts at 65 °C,
¹
3
respectively. We noticed that the effect of Pr NBr was close to
4
¹3
that of Et NOTs for the whole range of 0­1.5 mol dm (e.g.,
0.63 and ¹0.59 at 1.0 mol dm for Pr NBr and Et NOTs,
4 4
4
¹
3
¹
respectively, cf. Table 1).
On the Solvolysis Rate of 2-Adamantyl Bromide.
Another typical S 1 substrate, 2-adamantyl bromide, in the
N
same solvent system was subject to similar acceleration and
deceleration effects by added salts as shown in Figure 6.
Without a salt, the solvolysis rate constant of the substrate was
¹
1
¹1
observed to be log(k/s ) = ¹5.50 at 80 °C. The log(k/s )
value increased linearly with increasing concentration of
LiClO . The lithium ion should assist the formation of a stable
4
carbocation from 2-adamantyl bromide in a manner similar to
1
-adamantyl chloride. However, the carbocation of a secondary
carbon may be less stable than of a tertiary carbon. Therefore,
the degree in acceleration caused by the metal salts in the
solvolysis rate for 2-adamantyl bromide should be less
than that for 1-adamantyl chloride (apart from the difference
in the leaving group). For instance, the solvolysis rates
of the 1- and 2-derivatives increased with the logarithm
¹
1
¹3
factors [¦log(k/s )] of +0.97 and +0.56 at 3.0 mol dm
LiClO , respectively. The deceleration effects on the solvolysis
4
of 2-adamantyl bromide in the presence of non-metallic
salts, Et4NX (X = Cl and Br) and n-Bu4NBr, were observed
Figure 5. Decreases in the solvolysis rate constant of 1-
adamantyl chloride with the addition of various salts in
¹1
50% (v/v) DMA­H2O at 65 °C: ( ) Et4NClO4, ( )
as Et NBr µ Et NCl < n-Bu NBr: the ¦log(k/s ) values of
4 4 4
¹
3
Et4NBr, ( ) Et4NCl, ( ) Et4NOTs, and ( ) n-Bu4NBr.
¹0.16, ¹0.16, and ¹0.79, respectively, for the 1.0 mol dm
Table 1. “Pseudo” First-Order Reaction Rate Constants of the Solvolyses for Typical SN1 Substrates in the
Presence of Various Salts in the 50% (v/v) DMA­H2O Solvent
¹1 b)
Substrate
1-Adamantyl chloride
Temp/°C
Salta)
k/s¹1
log(k/s¹1
)
¦log(k/s )
¹
¹
¹
¹
¹
¹
¹
¹
5
4
5
4
4
5
5
5
65
No salt
LiClO4
6.07 © 10
1.41 © 10
9.62 © 10
2.15 © 10
1.68 © 10
3.31 © 10
2.58 © 10
3.79 © 10
1.55 © 10
1.40 © 10
5.77 © 10
¹4.22
¹3.85
¹4.02
¹3.67
¹3.77
¹4.48
¹4.46
¹4.42
¹4.81
¹4.85
¹5.24
®
65
65
65
65
65
65
65
65
65
65
+0.37
+0.20
+0.55
+0.45
¹0.26
¹0.24
¹0.20
¹0.59
¹0.63
¹1.02
NaClO4
Mg(ClO4)2
Ba(ClO4)2
Et4NClO4
Et4NCl
Et4NBr
Et4NOTs
Pr4NBr
n-Bu4NBr
c)
¹5
¹
5
6
¹
¹6
¹6
¹6
¹6
¹7
2-Adamantyl bromide
80
No salt
LiClO4
Et4NCl
Et4NBr
n-Bu4NBr
3.13 © 10
4.56 © 10
2.20 © 10
2.18 © 10
5.07 © 10
¹5.50
¹5.34
¹5.66
¹5.66
¹6.29
®
80
80
80
80
+0.16
¹0.16
¹0.16
¹0.79
¹
4
4
tert-Butyl chloride
20
No salt
LiClO4
1.41 © 10
1.87 © 10
¹3.85
¹3.73
®
+0.13
¹
2
0
a) The salt concentration of 1.0 mol dm 3. b) ¦log(k/s¹1) = log(k1/s¹1)[with salt] ¹ log(k0/s¹1)[no salt] for
¹
each substrate. c) p-Toluenesulfonate (tosylate) salt.

4
06
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
Salt Effects on the Rates and Mechanisms
[MX]/[M ][X ]) of transition-metal halides. We have report-
+
¹
salts. The solvolysis rates of typical S 1 substrates, 1-
adamantyl chloride and 2-adamantyl bromide, were decelerated
by the non-metallic salts. The difference in deceleration effects
of Cl and Br was not so clear for 2-adamantyl bromide while
the deceleration effect of Cl on 1-adamantyl chloride was
obviously larger than that of Br .
Now we may talk about the interaction between halide
anions from substrates and other species in solution. Rudakov
and Kozhevnikov have reported exponential increases in
solvolysis rates of tert-butyl halides in EtOH­H O mixed
N
1
6
ed an extremely large positive “salt” effect of Ni(ClO ) on
4 2
the solvolysis rate constant of 1-adamantyl chloride in 50%
¹
¹
36
(v/v) sulfolane­H O. Cox and Maskill have discovered
2
¹
solvolyses of tert-butyl iodide catalyzed by I2 in aqueous
ethanol; the same effect, but to a smaller extent, is also found
for tert-butyl bromide in the solvent. We would like to note that
¹
¹
¹
the polyhalogen ion formation, such as I and I Br (e.g.,
3
2
3
5
¹
¹
I + Br ꢀ I Br ), in the solution can account for the catalytic
2
2
effects by I . The stability of the polyhalogen ions has been
2
2
¹
¹
¹ 37
solvents with the complex formation constants (e.g., K =
reported to decrease as I3 > I Br > Br .
2 3
Effects of LiClO on the Solvolyses of S 2 Substrates,
4
N
Hexyl Chloride, Bromide, and Tosylate. The changes in the
¹
1
solvolysis rates, log(k/s ) were examined for hexyl chloride,
¹3
bromide, and tosylate with 0­2.0 mol dm LiClO4 in 50%
v/v) DMA­H O at 80 °C. As contrasted with the cases of the
(
2
typical S 1 substrates, the solvolysis rates of the S 2 substrates
N
N
were not accelerated but decelerated by the addition of
¹1
the metal perchlorate. The log(k/s ) value (¹3.68) of hexyl
tosylate decreased linearly with increasing LiClO concentra-
4
¹
1
¹3
tion: ¦log(k/s ) = ¹0.17 and ¹0.33 at 1.0 and 2.0 mol dm
LiClO . The decrease for the bromide substrate was remark-
able: ¦log(k/s ) = ¹0.23 and ¹0.50 at 1.0 and 2.0 mol dm
4
¹
1
¹3
LiClO , respectively (Table 2 and Table S3). While the rate
4
constant of the chloride substrate increased slightly up to 0.5
¹
3
mol dm LiClO , and then it began to decrease with increasing
4
concentration of the salt. A similar minor increase was observed
for hexyl chloride in 50% (v/v) DMF­H O at 80 °C; the rate
2
¹
3
constant of the substrate increased slightly up to 1.0 mol dm
LiClO (Table S2), and then it decreased. The cause of these
4
irregular increases for the chloride substrate is not clear but
because of the long alkyl chain (causing some stability of the
Figure 6. Changes in the solvolysis rate of 2-adamantyl
bromide with the addition of various salts in 50% (v/v)
DMA­H2O at 80 °C: ( ) LiClO4, ( ) Et4NBr, ( ) Et4NCl,
and ( ) n-Bu4NBr.
carbocation), the some S 1 scheme might bring additional
N
contribution to the very slow S 2 solvolysis of the substrate.
N
Table 2. “Pseudo” First-Order Reaction Rate Constants of the Solvolyses for Typical SN2 Substrates in the Presence
of Various Salts in 50% (v/v) DMA­H2O Solvent at 80 °C
Salta)
k/s¹1
log(k/s¹1
)
¦log(k/s )
¹1 b)
Substrate
Temp/°C
¹
¹
¹
6
6
6
Hexyl chloride
80
No salt
LiClO4
Et4NCl
Et4NBr
1.01 © 10
1.16 © 10
1.29 © 10
1.56 © 10
1.29 © 10
¹5.99
¹5.94
¹5.89
¹4.81
¹5.89
®
c)
80
80
80
80
(+0.05)
+0.10
+1.18
+0.30
d)
¹5
¹6
e)
e)
e)
Et4NOTs
¹
5
5
Hexyl bromide
Hexyl tosylate
80
No salt
LiClO4
3.07 © 10
1.80 © 10
8.24 © 10
3.08 © 10
2.46 © 10
¹4.51
¹4.74
¹6.08
¹4.51
¹4.61
®
¹0.23
¹1.57
0
¹
80
80
80
80
d)
¹7
Et4NCl
¹5
Et4NBr
Et4NOTs
¹5
+0.10
¹
4
4
80
No salt
LiClO4
2.09 © 10
1.41 © 10
6.98 © 10
2.50 © 10
1.27 © 10
¹3.68
¹3.85
¹6.16
¹4.60
¹3.89
®
¹
80
80
80
80
¹0.17
¹2.48
¹0.92
¹0.21
d)
¹7
¹5
¹4
Et4NCl
d)
Et4NBr
Et4NOTs
a) The salt concentration of 1.0 mol dm 3. b) ¦log(k/s¹1) = log(k1/s¹1)[with salt] ¹ log(k0/s¹1)[no salt] for each
¹
¹1
¹3
substrate. c) The slope ¦log(k/s )/¦[LiClO4] = ¹0.14 for whole the range of 0, 0.5, 1.0, 2.0, and 3.0 mol dm
LiClO4, cf. Table 4. d) Only the linear part in the ln[S] vs. t plot was evaluated because an exchange reaction
occurred between the leaving group and the added salt anion. e) p-Toluenesulfonate (tosylate) salt.

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
407
Solvolyses of S 2 Substrates Accompanied by Anion-
N
Exchange Reactions.
Solvolyses accompanied by “anion
exchange” reactions for typical S 2 substrates, such as hexyl
N
chloride, bromide, and tosylate were examined in the DMF­
(
Table S2) and DMA­H O solvents (Table 2 and Table S3).
2
The anion-exchange reaction prior to or in parallel to the
solvolysis of hexyl chloride, bromide, and tosylate has been
suggested to cause remarkable acceleration or deceleration in
the solvolysis rate, depending on the anion of added tetra-
ethylammonium salts.16 It is well known that the order of
nucleophilicity for halide ions in aprotic solvents, such as
¹
¹
¹
¹
¹
¹
acetone, is Cl > Br > I , whereas Cl < Br < I in aque-
ous solution.
3
8
The whole arguments for the solvolysis and exchange
reactions for the SN2 substrates in the 50% (v/v) DMA­H2O
solvent system has been verified also in the 50% (v/v) DMF­
H O solvent system. In 50% (v/v) DMF­H O, we examined
2
2
n-butyl bromide and tosylate as well as hexyl-compounds.
However, n-butyl chloride was excluded because the boiling
point of the compound (78.4 °C)39 is lower than the reaction
temperature. The possible formation of n-butyl chloride
through the anion exchange of n-butyl bromide in the presence
of the chloride ion (Et4NCl) at 80 °C brought a partial escape
of the chloride compound from the reaction vessel in order
to give an imprecise value for the solvolysis rate constant:
Figure 7. Changes in the solvolysis rate constant of SN1­
SN2 borderline substrates with the addition of LiClO4 in
50% (v/v) DMA­H2O: ( ) 4-methylbenzyl bromide, ( )
4
-methylbenzyl chloride at 35 °C, ( ) benzyl bromide, ( )
benzyl chloride at 50 °C, ( ) 4-nitrobenzyl bromide, ( )
-nitrobenzyl chloride at 70 °C.
4
¹
1
¦
log(k/s ) = ¹0.99 (too small) for n-BuBr, cf. ¹1.55 for
As displayed in Figure 7, on the other hand, all the benzyl
bromides gave negative slopes: ¹0.31, ¹0.27, and ¹0.16 for 4-
nitrobenzyl (at 75 °C), benzyl (at 50 °C), and 4-methylbenzyl
bromides (at 35 °C), respectively. Compared with chlorides, the
bromides gave larger negative values. As shown above, a
similar trend was observed for the typical SN2 substrates, hexyl
halides: the bromide gave a larger negative value than the
chloride substrate.
¹3
n-HexylBr in the presence of 1.0 mol dm Et NCl (Table S2).
In the 50% (v/v) DMA­H O system, however, ¦log(k/s )
was ¹1.57 for hexyl bromide in the presence of 1.0 mol dm
4
¹1
2
¹3
Et NCl (Table 2).
4
The solvolyses in the presence of added salt anions were
examined for phenacyl chloride and bromide (2-chloro- and 2-
bromoacetophenone) in 50% (v/v) DMA­H O to demonstrate
2
1
4
that another type of S 2 substrate is subject to anion-exchange
In the previous study, we observed that the solvolysis rate
constants of N,N-dimethylsulfamoyl and p-toluenesulfonyl
N
reactions. The detailed results will be reported in a paper. Now,
we may go on to the solvolyses of S 1­S 2 borderline
chlorides in 50% (v/v) acetone­H O decreased with increasing
N
N
2
substrates.
Solvolyses of Benzyl Chlorides and Bromides, S 1­S 2
LiClO4 concentration but increased with the presence of
Et NBr. At that time, the phenomena were not fully interpreted.
N
N
4
4
0
Borderline Substrates. Benzyl halides are regarded as SN1­
SN2 borderline substrates in solvolysis reactions. Figure 7
shows the changes of solvolysis rate constants of benzyl
halides in the presence of LiClO in 50% (v/v) DMA­H O.
Recently, Kevill et al. demonstrated that the pathways for
solvolyses of N,N-dimethylsulfamoyl and p-toluenesulfonyl
chlorides are indeed S 2 and not S 1 (earlier suggestions).
N
N
Therefore, the acceleration in solvolysis rates of these
4
2
¹1
The log(k/s ) of benzyl chloride at 50 °C decreased linearly
substrates by the addition of Et NBr should be caused by the
4
¹
¹
with increasing concentration of LiClO , giving a slope,
anion exchange between Cl from the substrates and Br of the
added salt.
4
¹
1
¹3
¦
log(k/s )/¦[LiClO ] = ¹0.16, cf. ¹0.19 at 1.0 mol dm
4
LiClO in Table 3. At 70 °C, 4-nitrobenzyl chloride, having
Solvolysis reactions of the S 1­S 2 borderline substrates,
4
N
N
more SN2 tendency because of its electron-withdrawing
group, gave a steeper slope (¹0.23) than that of benzyl
chloride (¹0.16). The log(k/s ) of 4-methylbenzyl chloride
which may also be accompanied by anion exchange, were
examined in 50% (v/v) DMA­H O. Figure 8 shows the
2
¹
1
changes in solvolysis rate constants of benzyl chloride and
bromide at 50 °C. Almost no change in the rate constant was
with greater S 1 tendency, however, increased with increas-
N
ing concentration of LiClO , after showing a minimum value
observed when Et NCl was added to benzyl chloride. Sim-
4
4
¹
3
¹1
at 0.5 dm LiClO . The slope, ¦log(k/s )/¦[LiClO ], over
ilarly, no change was observed when Et NBr was added to
4
4
4
¹3
the range of 0­2.0 mol dm LiClO was small but definitely
benzyl bromide. Upon the addition of a salt having a different
type of anion, however, the solvolysis rates of the substrates
4
a positive value, +0.04. In 50% (v/v) DMF­H O, the
2
¹
1
substrate gave a similar minimum and a totally positive
slope. We have discussed, in the previous sections, that
typical S 1 and S 2 substrates give positive and negative
altered remarkably. For instance, log(k/s ) of benzyl bromide
was decreased from ¹3.72 to ¹4.42 (to a one-fifth) and ¹5.00
¹
3
(to ca. 1/20) by the addition of 0.05 and 0.2 mol dm Et NCl,
N
N
4
¹
3
slopes of rate constants, respectively, with increasing concen-
tration LiClO4.
respectively. In the presence of 1.0 mol dm Et NCl, at last,
4
the rate constant, log(k/s ) of benzyl bromide reached to that
¹
1

4
08
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
Salt Effects on the Rates and Mechanisms
Table 3. “Pseudo” First-Order Reaction Rate Constants of the Solvolyses for SN1­SN2 Borderline Substrates in the
Presence of Various Salts in 50% (v/v) DMA­H2O Solvent
Salta)
k/s¹1
log(k/s¹1
)
¦log(k/s )
¹1 b)
Substrate
Temp/°C
¹
¹
¹
6
6
6
Benzyl chloride
50
No salt
LiClO4
Et4NCl
Et4NBr
No salt
LiClO4
8.07 © 10
5.30 © 10
7.71 © 10
1.79 © 10
1.89 © 10
1.01 © 10
7.82 © 10
2.07 © 10
¹5.09
¹5.28
¹5.11
¹3.75
¹3.72
¹3.99
¹5.11
¹3.69
®
50
50
50
¹0.19
¹0.02
+1.34
®
c)
¹4
¹
4
4
Benzyl bromide
50
¹
50
50
50
¹0.27
¹1.39
+0.03
c)
¹6
Et4NCl
Et4NBr
¹4
¹
¹
¹
6
6
6
4
4
-Methylbenzyl chloride
-Methylbenzyl bromide
35
No salt
LiClO4
Et4NCl
Et4NBr
No salt
LiClO4
9.28 © 10
9.15 © 10
7.54 © 10
1.38 © 10
2.10 © 10
1.51 © 10
7.14 © 10
2.00 © 10
¹5.03
¹5.04
¹5.12
¹3.86
¹3.68
¹3.82
¹5.15
¹3.70
®
(¹0.01)
¹0.09
+1.17
®
d)
35
35
35
c)
¹4
¹
4
4
35
¹
35
35
35
¹0.14
¹1.47
¹0.02
c)
¹6
Et4NCl
Et4NBr
¹4
¹
¹
¹
5
6
5
4
-Nitrobenzyl chloride
-Nitrobenzyl bromide
75
No salt
LiClO4
Et4NCl
Et4NBr
No salt
LiClO4
1.16 © 10
7.09 © 10
1.34 © 10
2.47 © 10
2.14 © 10
1.05 © 10
1.40 © 10
2.54 © 10
¹4.94
¹5.15
¹4.87
¹3.61
¹3.67
¹3.98
¹4.85
¹3.59
®
75
75
75
¹0.21
+0.07
+1.13
®
c)
¹4
¹
4
4
4
75
¹
75
75
75
¹0.31
¹1.18
+0.08
c)
¹5
Et4NCl
Et4NBr
¹4
a) The salt concentration of 1.0 mol dm 3. b) ¦log(k/s¹1) = log(k1/s¹1)[with salt] ¹ log(k0/s¹1)[no salt] for
¹
each substrate. c) Only the linear part in the ln[S] vs. t plot was evaluated because an exchange reaction occurred
¹1
between the leaving group and the added salt anion. d) The slope ¦log(k/s )/¦[LiClO4] = +0.04 for whole
¹3
the range of 0, 0.25, 0.5, 1.0, and 2.0 mol dm LiClO4, see the text.
of benzyl chloride without a salt, i.e., ¹5.11 (to ca. 1/25). Just
oppositely, the rate constant of benzyl chloride increased as
¹3
high as ca. 25 times at 1.0 mol dm Et NBr.
4
For the 4-methyl- and 4-nitro-derivatives of benzyl halides,
solvolysis reactions accompanied by the anion exchange were
also examined. The changes of rate constants at 35 °C of 4-
methylbenzyl chloride and bromide with Et NX (X = Cl and
4
¹
1
Br) are shown in Figure 9. The rate constant, log(k/s ), of 4-
methylbenzyl bromide decreased to the value of 4-methylben-
¹
3
zyl chloride (intrinsic value) by the addition of 0.2 mol dm
¹
3
Et NCl. In the presence of 0.5 and 1.0 mol dm Et NCl, the
4
4
rate constant of 4-methylbenzyl bromide further decreased to
lower values than that of 4-methylbenzyl chloride. Conversely,
¹
1
the increases in the rate constant, log(k/s ), of 4-methylbenzyl
chloride were observed by the addition of Et NBr. How-
4
ever, the rate constant remained just below the value of 4-
¹
3
methylbenzyl bromide (¹3.68) even at 1.0 mol dm Et NBr.
4
The addition of even a small amount of Et NCl to the
4
solvolysis system of 4-nitrobenzyl bromide (at 75 °C) caused a
remarkable decrease in the rate constant (Table S4). On the
other hand, a very remarkable increase was observed for 4-
nitrobenzyl chloride upon the addition of a small amount of
Figure 8. Changes in the solvolysis rates of benzyl halides
with the addition of Et4NX in 50% (v/v) DMA­H2O at
Et NBr, for instance, more than a three-fold rate constant at
4
¹
3
50 °C: ( ) Et4NBr, ( ) Et4NCl for benzyl bromide, ( )
0.01 mol dm Et NBr. The greater S 2 tendency, based on the
4 N
Et4NBr, ( ) Et4NCl for benzyl chloride.
nitro-group, may promote more sufficient anion-exchange

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
409
Figure 9. Changes in the solvolysis rates of 4-methylbenzyl
halides with the addition of Et4NX in 50% (v/v) DMA­
H2O at 35 °C: ( ) Et4NBr, ( ) Et4NCl for 4-methylbenzyl
bromide, ( ) Et4NBr, ( ) Et4NCl for 4-methylbenzyl
chloride.
Figure 10. Relationship between the LiClO effects on the
4
solvolysis rate constants in 50% (v/v) DMA­H O and the
2
stabilities of carbocations (the standard Gibbs energy
changes for eq 2): (1) hexan-1-ylium, (2) hexan-2-ylium,
reactions. Contrastingly, the greater S 1 tendency, based on the
methyl-group on benzyl halides, caused less anion-exchange
N
(3) butan-2-ylium, (4) cyclopentylium, (5) adamantan-2-
ylium, (6) 2-methylpropan-2-ylium, (7) norbornan-2-
ylium, (8) 3-methylpentan-3-ylium, (9) adamantan-1-
ylium, (10) 4-nitrobenzylium, (11) benzylium, (12) 1-
phenylethan-1-ylium. The stability of 1-adamantyl cations
is taken as the standard value (¦G = 0). Open and solid
symbols represent chloride and bromide compounds,
respectively. cf. Table 4 for the temperatures of solvolyses
examined.
reaction. At last, the typical S 1 substrates, tert-butyl or 1-
N
adamantyl halides exhibit no apparent anion-exchange property
(vide supra).
Relationship between Effects of LiClO and Stabilities of
4
Carbocations. Bingham and Schleyer41 reported an attempt
to rationalize the solvolysis rate constants of tertiary bridge-
head halides (such as 1-adamantyl chloride) with the stabilities
of carbocations by empirical calculation. Abboud and co-
4
2
workers have developed a method to determine the stability
of carbocations in the gas phase by dissociative proton
attachment (DPA) using Fourier transform ion cyclotron
resonance (FTICR) spectroscopy. Carbocations are ranked
according to their stabilities relative to the 1-adamantyl cation,
based on eq 2,
(the difference of ca. 0.1) the line of the corresponding or
related chlorides. Just 2-adamantyl bromide was above the line
of the chlorides. Benzyl halides and a benzyl-derivative
(1-chloro-1-phenylethane) gave another correlation group
below the group of aliphatic halides. Generally speaking, the
more stable carbocation is apt to react through S 1 rather S 2
N
N
o
pathway. Therefore, it is obvious that LiClO gives positive and
4
þ
þ
1
-Ad­ClðgÞ þ R ðgÞ ! 1-Ad ðgÞ þ R­ClðgÞ; ꢀG ð2Þ
negative effects on the solvolysis rate constants of S 1 and S 2
N
N
o
The Gibbs free energy values (¦G ) of RX in the gas
phase have been found to give excellent correlations with the
log(k/s ) values of RX in solution, where RX are not only
bridgehead chlorides (such as 1-adamantyl) but also bi- and
tricyclic secondary (such as 2-norbornyl and 2-adamantyl)
chlorides.
Figure 10 shows the relationship between the standard
Gibbs energy changes for eq 2 and the LiClO effects on the
solvolysis rate constants of various chloride and bromide
substrates in 50% (v/v) DMA­H O (Tables S3 and S5).
substrates, respectively. It must be quite natural that the salt
effects on the S 1­S 2 borderline substrates are close to zero.
N
N
¹
1
The general principle may be that the faster solvolysis rate, the
larger extent of the LiClO effect.
4
¹1
Despite higher temperature (¦T = 10 °C), the log(k/s ) of
cyclohexyl chloride (¹5.65) and bromide (¹4.32) at 80 °C
were still much smaller than those of cyclopentyl chloride
(¹4.93) and bromide (¹3.52) at 70 °C. However, the LiClO4
effects on the rate constant of the cyclohexyl substrates (0.07
for the chloride) were slightly larger (+0.06) than those of the
cyclopentyl substrates (0.01 for the chloride), cf. Table 4. Now,
we have to discuss in detail the relationship between the
4
2
Aliphatic chlorides gave a good proportionality or correla-
tion between ¦Go and ¦log(k/s )/¦[LiClO ]; aliphatic
chlorides examined are 1-hexyl, 2-hexyl, cyclopentyl, tert-
butyl (2-chloro-2-methylpropane), and exo-2-norbornyl (=exo-
¹1
4
¹
1
solvolysis rates, log(k/s ), and the extent of LiClO4 effects,
¹1
¦log(k/s )/¦[LiClO ].
4
2
-bicyclo[2.2.1]heptyl) chlorides, 3-chloro-3-methylpentane,
and 1-adamantyl chloride. Aliphatic bromides, i.e., 1-hexyl,
-butyl, and cyclopentyl bromides, gave a straight line below
It is well known that the solvolysis rate of tert-butyl chloride
is much faster than that of 1-adamantyl chloride in polar
solvents, hydrogen bonding donating solvents. For instance,
2

4
10
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
Salt Effects on the Rates and Mechanisms
Table 4. The Stabilities of Carbocations in the Gas Phase and the LiClO4 Effects on the Solvolysis Rate
Constants of Various Chloride and Bromide Substrates in the 50% (v/v) DMA­H2O Solvent
a)
LiClO4 effectb)
¦log(k/s¹1)/¦[LiClO4]
Standard Gibbs energy ,
Cations
o
¹1
¦G /kcal mol
Hexan-1-ylium (1)
¹30 (an arbitral value)c)
chloride: ¹0.14 (80 °C)
bromide: ¹0.24 (80 °C)
chloride: ¹0.03 (80 °C)
bromide: ¹0.12 (80 °C)
Hexan-2-ylium (2)
Butan-2-ylium (3)
Cyclopentylium (4)
¹18.9 (a loaned value)d)
¹18.9
¹13.6
chloride:
bromide: ¹0.07 (70 °C)
chloride: 0.07 (80 °C)
bromide: ¹0.01 (80 °C)
0.01 (70 °C)
Cyclohexylium
data not found
Adamantan-2-ylium (5)
¹7.6
¹6.0
¹3.0
¹1.1
bromide:
chloride:
chloride:
chloride:
chloride:
0.19 (80 °C)
0.15 (20 °C)
0.21 (60 °C)
0.24 (20 °C)
0.32 (65 °C)
2
-Methylpropan-2-ylium (6)
Norbornan-2-ylium (7)
-Methylpentan-3-ylium (8)
Adamantan-1-ylium (9)
3
0.00 (by definition)
4
-Nitrobenzylium (10)
Benzylium (11)
-Phenylethan-1-ylium (12)
¹17.2
¹5.8
chloride: ¹0.23 (75 °C)
bromide: ¹0.31 (75 °C)
chloride: ¹0.16 (50 °C)
bromide: ¹0.27 (50 °C)
1
1.7
chloride:
0.09 (20 °C)
+
+
a) Standard Gibss energy change for a reaction, 1-AdCl(g) + R (g) ¼ 1-Ad (g) + RCl(g), cf. Ref. 42b.
¹1
b) The LiClO4 effect (¦log(k/s )/¦[LiClO4]) for each substrate was evaluated over a concentration range
¹3
¹1
¹1
of 0­2.0 mol dm LiClO4. c) Methylium: ¹77.4 kcal mol . d) Butan-2-ylium: ¹18.9 kcal mol
.
solvolysis rate of tert-butyl chloride is roughly 1000 times
to ionizing solvent power Y (m = 1 or 1.2 for 1-AdCl),46 and
may vary not only with experimental conditions but also with
the different definitions. In the present paper, however, we were
able to demonstrate quantitatively the correlation between the
4
3
faster than that of 1-adamantyl chloride in 80% ethanol­H O.
2
The solvolysis rate constant of 1-norbornyl derivative has been
found significantly below the correlation line.44 The cause
of the much faster solvolysis of tert-butyl chloride has been
explained by nucleophilic solvent participation45 or other
reasons, e.g., increased hydrogen bond donation to the leaving
group of tert-butyl chloride.43 In the present study, accordingly,
the solvolysis experiment of tert-butyl chloride (20 °C) was
performed at much lower temperature than that of 1-adamantyl
chloride (Table 4). Regardless of the solvolysis rate of tert-
butyl chloride is much faster than that of 1-adamantyl chloride,
LiClO effects and the carbocation stabilities expressed by the
4
o
Gibbs free energy values (¦G ) of RX in the gas phase, where
RX are not only typical S 1 halides but also non-, mono-, bi-,
N
and tricyclic secondary (such as 2-hexyl, cyclopentyl, 2-
norbornyl, and 2-adamantyl) and, finally, typical S 2 halides.
N
Raman Spectra for Observing Water Structure Distor-
tion. Raman spectroscopy is a technique preferably used for
studies of water structure because the proportions of intensities
¹1
the extent of LiClO effect on tert-butyl chloride was smaller
of main OH stretching modes (around 3200 and 3400 cm ) are
thought to reflect well a network of hydrogen bonds as well as
4
than that on 1-adamantyl chloride in good accordance with the
stabilities of their carbocations (Table 4).
4
7
its disturbance by the presence of some solute. Scherer et al.
Generally, the solvolysis rate of a bromide substrate is 20­30
times faster than that of the corresponding chloride (Tables 2,
have reported Raman spectra of liquid D O from ¹10 to 90 °C;
the (isotropic) spectra show the development of a band around
2
¹
1
¹1
3
, and 4). Nevertheless, the effect of LiClO on the solvolysis
2500 cm at the expense of the band around 2400 cm with
increasing temperature. These bands have been attributed to the
OD stretching vibration of the symmetrically hydrogen-bonded
4
rate constants was found to be somewhat independent from the
¹
1
solvolysis rate constants themselves: the ¦log(k/s )/¦[Li-
¹
1
ClO ] value for a bromide substrate was smaller (ca. 0.1) than
complex (tetrahedral: 2400 cm ) and of the single-handed
4
¹1
48
that of the corresponding chloride, as shown in Figure 10. The
complex (2500 cm ). Tokushima et al. estimated a 2:1 ratio
between distorted and tetrahedral species at room temperature
by means of high-resolution X-ray emission spectroscopy of
liquid water.
apparent difference in the LiClO effects may be caused by the
4
+
difference in the interaction of Li (increasing concentration)
¹
¹
with the leaving groups (Cl and Br ) in 50% (v/v) DMA­
H O.
Figure 11 shows the change in Raman spectra of H O with
2
2
In previous papers,1
3­15
we reported the linearity between the
increasing contents of DMA (25­90% (v/v)) at room temper-
LiClO4 effects and the Grunwald­Winstein m values17 for SN1
ature. The band intensity at ca. 3200 cm decreased remark-
¹1
¹1
to S 2 substrates in methanol­water, acetone­water, and 1,4-
ably compared with that at ca. 3400 cm . Both band shifted
toward higher wavenumber with increasing contents of DMA
N
dioxane­water mixtures. The salt effects were expressed by
the changes in solvolysis rates, log(k /k ), on the addition of
¹
1
and reached 3280 and 3460 cm , respectively, at 90% (v/v)
DMA. The increased distortion of water structure with the
1
o
¹
3
1
.0 mol dm LiClO . The m value is the reaction’s sensitivity
4

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
411
Figure 13. Raman spectra of 50% (v/v) DMA­D2O mixed
solutions containing concentrated Et4NCl at room temper-
ature: (1) R = ¨ (no salt); (2 - - - -) R = 20; (3 - - -)
R = 10; where R = [DMA + D2O]/[Et4NCl].
Figure 11. Raman spectra of DMA­H2O mixture at room
temperature: (1 - - - -) 0, (2 ­ ­ ­) 50, (3) 75, (4) 90% (v/v)
of DMA.
¹
3
2
.5 mol dm ) in 50% (v/v) DMA­H O. The decrease in the
2
Raman intensity ratio (I1/I2) with the metal perchlorates was
2+
+
+
2+
ordered as Mg < Li < Na < Ba . Barium perchlorate
caused the most remarkable distortion in the water structure and
+
MgClO the least, and the effect of Li was smaller than that of
4
+
Na . This order is not consistent with the order in the metal
effect on the solvolysis rate of the typical S 1 (Na < Li <
+
+
N
2+
2+
Ba < Mg , cf. Figure 1). The coordination abilities of the
metal ions in non-aqueous solvents, such as acetonitrile, have
+
+
2+
2+ 27a,30
been found to be Na < Li < Ba < Mg
.
We have
insisted that the metal effect on the S 1 solvolysis rate of RX
N
¹
must be based on the direct interaction between X of the
substrate and the added metal cation in organic solvent­H O
2
¹
+
mixtures. The degree in the chemical interaction of X ­M or
¹
2+
X ­M should depend on the intrinsic coordination ability of
a metal ion as well as the trivialized solvation ability of water
¹
+
2+
toward both anions (X ) and cations (M and M ).
Figure 12. Relationship between the ratio of intensities in
Raman OH stretching bands (I1/I2) and concentrations
In Figure 13 are shown the Raman spectra of 50% (v/v)
DMA­D O containing concentrated Et NCl. With increasing
¹1
2
4
of metal perchlorates where I1: ca. 3200 cm , I2: ca.
¹
1
¹1
concentration of Et NCl, the intensity of the band at 2500 cm
4
3
400 cm : ( ) LiClO4, ( ) NaClO4, ( ) Ma(ClO4)2, ( )
¹
1
increased at the expense of that at 2400 cm , accompanied by
almost no wavenumber shift. The intensity changes by the
Ba(ClO4)2.
addition of DMA to H O was demonstrated by the remarkable
addition of the salt to the DMA­D O mixture were similar to
2
2
¹1
decrease of the band intensity at ca. 3200 cm compared with
those of D O with temperature increase reported by Scherer
2
¹1
47
that at ca. 3400 cm . Similar Raman band changes (2400 and
et al. The change in the Raman intensity ratio (I /I ) between
1
2
¹
1
¹1
2
500 cm ) were observed also for DMF­D2O mixed solvents.
ca. 2400 and ca. 2500 cm , caused by the addition of Et4NCl,
is shown in Figure 14. Tetraethylammonium salts caused
changes in the Raman intensity ratio (I /I ) as Cl < OTs <
1 2
Br . Tetrabutylammonium bromide caused much larger effect
than Et NBr. The effect by LiClO is also shown in Figure 14
We must just take into account that the intensity ratios of
¹1
¹
¹
the main Raman components (around 3200 and 3400 cm ) of
¹
H O have been reported to alter with the different excitation
2
wavelengths in Raman spectroscopy.49 In the present work, a
single wavelength (514.5 nm) was utilized for excitation of
4
4
as a reference. By the addition of all the tetraalkylammonium
salts of higher concentrations, the water structure in the DMA­
D O mixtures was found to be distorted at a great extent.
2
the Raman spectroscopy. Based on the IR spectra of H O
2
5
0
containing various amount of DMF, Biliškov and Baranović
have evaluated the fraction of water OH oscillators that are
hydrogen bonded to other water molecules.
In principle, the decreases in the solvolysis rates of SN1
substrates with the addition of tetraalkylammonium salts
Figure 12 shows the decreases in the ratio (I /I ) of Raman
OH band intensities around 3200 and 3400 cm with increas-
ing concentrations of various metal perchlorates (up to ca.
(R NX) are based on the decrease of water activity in DMA­
1
2
4
¹
1
H O. Therefore, the order shown in Figure 5 may indicate the
2
true order of the water activity decrease in the DMA­H O
2

4
12
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
Salt Effects on the Rates and Mechanisms
8
Winstein’s interpretation of “special” salt effect but we have
attempted to provide concrete evidence for the exchange
reaction between two ion pairs without invoking the contact
and solvent-separated ion pairs (CIP and SSIP). The obvious
motive force for the ion-pair exchange should be based on the
direct chemical interaction between the metal cations and the
leaving-group anions of substrates under “dihydrogen ether”
conditions, in which, solvation ability of H O should be
2
weakened toward both anions and cations in the solution.
Experimental
Materials.
N,N-Dimethylformamide (DMF) and N,N-di-
methylacetamide (DMA) as the solvents, purchased from Wako,
were used as received. The substrates from TCI were used
without further purification: 1-adamantyl chloride (>98%) and
bromide (>97%), 2-adamantyl bromide (>97%), tert-butyl chlo-
ride (>98%), hexyl chloride (>95%), bromide (>98%), and
p-toluenesulfonate (tosylate) (>98%), 2-bromobutane (>98%), 2-
chlorohexane (>98%), cyclopentyl chloride (>98%) and bromide
(>98%), cyclohexyl chloride (>98%) and bromide (>95%),
benzyl chloride (>99%) and bromide (>98%), (1-chloroethyl)-
benzene (>97%). Aldrich chemicals were used also as received:
4-methylbenzyl chloride (99%) and bromide (97%), 3-chloro-3-
methylpentane (97%), exo-2-chloronorbornane (98%).
Figure 14. Relationship between the ratio of intensities in
Raman OD stretching bands (I1/I2) and concentrations
¹1
¹1
of R4NX where I1: ca. 2400 cm , I2: ca. 2500 cm
) LiClO4, ( ) Et4NCl, ( ) Et4NBr, ( ) Et4NOTs, ( )
n-Bu4NBr.
:
(
Salts of NaClO4, Et4NBr, Et4NOTs, and Pr4NBr from Aldrich as
well as LiClO4 and Et4NCl from Wako, and n-Bu4NCl form TCI
were used. Tetraethylammonium perchlorate (Et NClO ) was
solutions containing concentrated R NX at 65 °C (Et NClO ,
4
4
4
Et NBr < Et NCl < Et NOTs < n-Bu NBr). The water activi-
4
4
4
4
4
4
ty in concentrated salt solution should be influenced by both the
partial volume of a salt and the solvation of the anion. On the
other hand, the degree of distortion of water structure evaluated
by Raman intensity ratio (I /I ) was different (Et NCl <
prepared by adding an equivalent amount of HClO4 to Et4NBr in
water. The precipitates were filtered, washed with cold water and
were recrystallized from water several times, followed by drying at
70 °C in vacuo. Tetrabutylammonium bromide was prepared as
follows: tributylamine and butyl bromide in acetonitrile solvent
were refluxed for 36 h, followed by evaporation of the solvent to
dryness; the n-Bu4NBr precipitates were recrystallized from ethyl
acetate twice and dried at 70 °C in vacuo.
1
2
4
Et4NOTs < Et4NOBr < n-Bu4NBr) from the order in the
solvolysis deceleration. The discrepancy between the orders
¹
1
based on the solvolysis deceleration ¦log(k/s ) and on
Raman intensity ratio (I /I ) might be caused by the fact that
1
2
¹
Kinetics and Raman Spectra.
The solvolysis rates were
Cl can interact with the proton of H O to affect the OH
2
¹3
¹
51
determined by titration with 0.0025 mol dm standard aqueous
sodium hydroxide using phenolphthalein as the indicator. The
initial concentrations of the substrates were all kept at 0.010
stretching while the Br ion interacts weakly. Roscioli et al.
have reported the hydrogen-bonding interaction between
¹
anions and H O (X ­H O: X = OH, F, Cl, and Br) in the
2
2
¹3
gas phase. Takekiyo and Yoshimura52 discussed the hydration
mol dm in the reaction vessels. All the substrates were dissolved
in acetonitrile (Aldrich, anhydrous 99.8%) at first. In principle,
structures of the tetraethylammonium cation in water.
0
­90% consumption of a substrate was followed for evaluating the
One may accept that the properties of bulk water are based
on the (huge) network of hydrogen bonding. As discussed
above, the water structure (of bulk water) is distorted by the
addition of organic solvents and concentrated salts and also by
increasing temperature, which makes the bulk water turn to
rate constant, except for some cases; in SN2 substrates containing
Et4NX salts, only the linear parts in the ln[S] vs. t curve were
utilized. Details for the other reagents and the procedure for the
13­15
kinetic experiments have been described previously.
Raman
spectra of H2O and D2O (99.9% atom D from Aldrich) mixed with
DMF or DMA were recorded with a Horiba-Jobin Yvon LabRam
Spectrograph HR-800 at room temperature (ca. 23 °C). Excitation
for the Raman spectroscopy was provided by a 514.5 nm argon
laser. The Raman intensity was standardized with the CD
stretching band of 5% (v/v) CD OD­D O as an internal intensity
“
dihydrogen ether” conditions. Note that all the salts concerned
are not only alkali metal and alkaline earth metal salts but also
tetraalkylammonium salts at higher concentrations. The dis-
tortion of bulk water structure may bring us unexpected high
reactivity of solutes: we have reported a great enhancement in
3
2
the oxidation ability of dilute HNO as well as concentrated
standard; CD3OD, 99.8 atom % D from Aldrich was used. For
DMF­H2O mixed solvents, signals of DMF were utilized as the
Raman intensity standard.
3
5
3
HNO in nanoscale water-droplets of reverse micelle systems.
3
Hydrogen bond dynamics of both water in highly concentrated
NaBr solution and in reverse micelle showed a similar
5
4
behavior.
We are grateful to Prof. Herbert Mayr, Ludwig-Maximilians
University, Germany for his valuable comments to the effects
of tetraalkylammonium salts on the solvolysis rates of SN1
substrates. We are also deeply indebted to Professor Emeritus
Tadashi Okuyama of Hyogo Prefectural University, Japan for
Conclusion
We are confident that the proposed scheme for salt effects
on solvolyses in the present work is never contradictory to

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
413
his encouraging us to examine the relationship between effects
133.
of LiClO in solvolyses and stabilities of carbocations in gas
21 A. Nose, M. Hojo, T. Ueda, J. Phys. Chem. B 2004, 108,
4
7
98.
phase.
22
3
M. Hojo, Y. Imai, Bull. Chem. Soc. Jpn. 1983, 56, 1963.
M. Hojo, T. Takiguchi, M. Hagiwara, H. Nagai, Y. Imai, J.
Supporting Information
2
Phys. Chem. 1989, 93, 955; M. Hojo, H. Hasegawa, Y. Miyauchi,
H. Moriyama, H. Yoneda, S. Arisawa, Electrochim. Acta 1994, 39,
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the presence of various salts in 50% (v/v) DMF­H2O are given in
Tables S1 and S2. Tables S3­S5 are some results in 50% (v/v)
DMA­H2O. This material is available free of charge on the web at:
http://www.csj.jp/journals/bcsj/.
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