The Hammett equation applied to the nucleophilic displacement of ions and ion pairs on substituted benzenesulphonates

Article abstract of DOI:10.1002/poc.364

Nucleophilic substitution on meta- and para-substituted methyl benzenesulphonates was studied with two chloride salts with different structures: NBu₄Cl or KCl-Kryptofix 2,2,2. Treating the results with the Acree equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that ρ is positive and nearly the same for the two reaction paths (ρ ≈ +2). The reactivity of methyl p-nitrobenzenesulphonate was compared with that of the corresponding ethyl derivative, and it is shown that the methyl derivative reacts faster than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a negative charge is developed on the leaving group in the transition state, and that the activated complex is linear. Copyright

Full text of DOI:10.1002/poc.364

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 265–270
DOI: 10.1002/poc.364
The Hammett equation applied to the nucleophilic dis-
placement of ions and ion pairs on substituted benzene-
sulphonates
Sergio Alunni,* Monica Pica and Gustavo Reichenbach*
Dipartimento di Chimica, UniversitaÁ di Perugia, 06100 Perugia, Italy
Received 30 October 2000; revised 21 December 2000; accepted 28 December 2000
ABSTRACT: Nucleophilic substitution on meta- and para-substituted methyl benzenesulphonates was studied with
epoc
two chloride salts with different structures: NBu4Cl or KCl-Kryptofix 2,2,2. Treating the results with the Acree
equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower
one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that r is
positive and nearly the same for the two reaction paths (r ꢀ ‡2). The reactivity of methyl p-nitrobenzenesulphonate
was compared with that of the corresponding ethyl derivative, and it is shown that the methyl derivative reacts faster
than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a
negative charge is developed on the leaving group in the transition state, and that the activated complex is linear.
Copyright  2001 John Wiley and Sons, Ltd.
Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc
KEYWORDS: Hammett equation; ions; ion pairs; nucleophilic displacement
INTRODUCTION
reactivity is influenced when an ethyl group is substituted
for the methyl group of the substrate.
The importance of ion pairs in chemistry is well known.1
Several properties of a process, such as reactivity and
stereochemistry, can depend on the reacting species, ion
or ion pair.1 Studies to estimate the relative reactivities of
free ions and ion pairs have been reported in the literature
and the data were treated with the classical Acree
equation.2 Recently, we studied the reactivity of ions and
ion pairs in CH2Cl2 on a classical organic substrate
(methyl p-nitrobenzenesulphonate) by applying the
Acree equation. We used a series of chloride salts with
different requirements, and found that the ion reactivity
was always the same, and that the reactivity of the ion
pairs was lower than that of the ion and variable as a
function of the dissociation constants of the salts.3 We
also studied the reactivity of ions and ion pairs on an
organometallic substrate,4 obtaining similar results. The
aim of this research was to study how the reactivity of
ions and ion pairs varies upon changing the substituents
in the aromatic ring, determine if there is a Hammett
correlation and if the r value is different or similar for
ions and ion pairs. Moreover, we checked how the
RESULTS AND DISCUSSION
Pseudo-first-order rate costants (kobs) were measured for
the nucleophilic substitution reactions on methyl para- or
meta-substituted benzenesulfonates induced by tetrabu-
tylammonium chloride or KCl-Kryptofix 2,2,2 (KCl-
Krypto) salts at 22°C in CH2Cl2; the nucleophile can be
both the free ion (ClÀ) and the ion pair (A ClÀ). We used
‡
the solvent dichloromethane, as in previous studies,3,4
owing to its very low permittivity (e = 9.08),5 low donor
number (DN = 2) and good acceptor number
(AN = 20.04).6 We chose Bu4NCl because it has a very
low interionic distance7 with partial interpenetration
between the ions8 and KCl-Krypto because the distance
‡
between K and ClÀ is fairly high. Their dissociation
constants are 1.6 Â 10À5 and 26.1 Â 10À5 mol dmÀ3
,
respectively, at 22°C.9 In one case bis(triphenylpho-
sphoranylidene)ammonium chloride (PPNCl) was used
(KDISS = 34.1 Â 10À5 mol dmÀ3); it has a large interionic
˚
distance (11.3 A) because some solvent molecules are
interposed between the ions.7 The substituents used were
p-CH3 (1), m-NO2 (2), p-H (3), p-Cl (4) and p-Br (5), and
the data for p-NO2 were available from previous work.3
The processes involving competition between nucleo-
philes ions and ion pairs are shown in Scheme 1.
*Correspondence to: S. Alunni or G. Reichenbach, Dipartimento di
Chimica, Universita` di Perugia, 06100 Perugia, Italy.
E-mail: ormetal@unipg.it
Contract/grant sponsor: CNR.
Contract/grant sponsor: MURST.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 265–270

266
S. ALUNNI, M. PICA AND G, REICHENBACH
Table 1. Observed ®rst-order rate constants of methyl m-
nitrobenzenesulphonate with NBu4Cl in CH2Cl2 at 22°C
[NBu4Cl]
(10À3 mol dmÀ3
)
a
f
kobs(10À3 sÀ1
)
Æ
12.7
10.4
8.45
7.18
6.34
4.78
3.59
1.69
1.06
0.761
0.078
0.082
0.086
0.089
0.092
0.100
0.109
0.139
0.163
0.183
0.431
0.456
0.481
0.500
0.514
0.545
0.575
0.646
0.685
0.710
2.15
1.74
1.46
1.21
1.10
0.829
0.647
0.324
0.213
0.167
We observed a decrease in the kobs/[salt] versus [salt] plots
(see Fig. 1); this is an indication that the reaction proceeds
through two parallel paths involving both the free ions and
the ion pairs, as shown in Scheme 1. As an example, in
Fig. 1 is reported this plot for the reaction of methyl
m-nitrobenzenesulphonate with NBu4Cl.
Scheme 1. Nucleophilic substitution reaction pathsÀfor alkyl
benzenesulphonates induced by the free chloride "Cl ) or the
‡
ion pair "A ClÀ)
The data were then treated with the Acree equation,2
Kinetics with the m-NO2 derivative were followed by
monitoring the increase in absorbance at ꢀ = 280 nm,
while the decrease of absorbance was followed in all the
other substrates at ꢀ = 266 nm. In every case, good
pseudo-first-order plots were obtained. It should be
considered that the dissociation constants (KDISS) are
measured at low concentration (10À5–10À4 mol dmÀ3),
while the kinetic studies are carried out at higher
concentrations (10À3–10À2 mol dmÀ3). In order to evalu-
ate the ion and ion pair activities, the degree of
dissociation of the salt (a) and the activity coefficients
kobs ˆ ki‰ionŠ ‡ kip‰ion pairŠ
ꢁ1†
ꢁ2†
k
obs=cꢁf Æ ˆ ki ‡ kipꢁ1 À ꢁ†=ꢁf Æ
where a is the degree of dissociation, f is the activity
Æ
coefficient, evaluated as reported previously,3,4 and c is
the total concentration of the salt. A plot of kobs/caf
Æ
versus (1Àa)/af is expected to give a straight line, with
Æ
intercept ki and slope kip. The contribution of the free ion
to the total reactivity is small; nevertheless, the
consistency of the data obtained for three salts both in
this paper and in the preceding one3 confirms the
significance of the data. The kobs values for the various
substrates are given as supplementary material at the
epoc website at http://www.wiley.com/epoc. Tables 1
and 2 report the data with X = m-NO2 (2), as an example,
and the related Acree plots are shown in Figs 2 and 3.
(f ) of the free ion were calculated by the method of
Æ
successive approximations using the Debye-Hu¨ckel equa-
tion.10 In this treatment the activity coefficient of the ion
pairs is taken equal to one, owing to their neutral nature.
Table 2. Observed ®rst-order rate constants of methyl m-
nitrobenzenesulphonate with KCl-Krypto in CH2Cl2 at 22°C
[KCl]
(10À3 mol dmÀ3
)
a
f
kobs(10À3 sÀ1
)
Æ
17.8
15.1
12.0
8.88
6.22
3.56
2.67
1.78
0.889
0.524
0.518
0.513
0.509
0.511
0.527
0.541
0.567
0.623
0.160
0.176
0.201
0.236
0.280
0.353
0.391
0.445
0.534
6.78
5.87
4.87
3.64
2.60
1.57
1.07
0.815
0.414
Figure 1. Plot of kobs/[NBu4Cl] versus [NBu4Cl] for the
reaction with methyl m-nitrobenzenesulphonate
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 265–270

HAMMETT EQUATION IN NUCLEOPHILIC DISPLACEMENTS WITH IONS AND ION PAIRS
267
Figure 3. Using the Acree equation [Eqn. "2)] to determine ki
and kip for the substrate "2) with the salt KCl-Krypto
Figure 2. Using the Acree equation [Eqn. "2)] to determine ki
and kip for the substrate "2) with the salt NBu4Cl
Table 3 and Table 4 report the rate constants obtained
from the Acree equation for all the substrates with
NBu4Cl and KCl-Krypto, respectively.
From the data of Tables 3 and 4, it can be seen that the
rate constants for the ion, ki, obtained with the two salts
are in good agreement, taking into account that such data
are not very precise since they are obtained from the
small value of the intercept of the straight line. We also
applied the Acree equation to the m-NO2 derivative (2)
with bis(triphenylphosphoranylidene)ammonium chlor-
ide (PPNCl)7 and obtained consistent ki = 0.52 (0.06)
dm3 molÀ1 sÀ1 and kip = 0.63 (0.02) dm3 molÀ1 sÀ1
values. The data in Table 3 and Table 4 show that with
NBu4Cl the reactivity of the free ClÀ, ki, is always larger
than that of the ion pair, kip; this can be explained by the
lower nucleophilicity of the ion pair with respect to the
Table 3. Calculated values of ki and kip for the reaction of methyl-substituted benzenesulphonates with NBu4Cl in CH2Cl2 at
22°C "the dissociation constant of NBu4Cl in CH2Cl2 is 1.58 Â 10À5 mol dmÀ3 at 22°C)
X
kia(dm3 molÀ1 sÀ1
)
kipa(dm3molÀ1 sÀ1
)
sb
rc
d
p-NO2
1.07 (0.07)
0.12 (0.003)
‡0.78
‡0.74
‡0.22
‡0.22
0
0.998
0.999
0.978
0.965
0.994
0.936
m-NO2
p-Br
0.62 (0.04)
0.159 (0.002)
0.072 (0.033)
0.083 (0.034)
0.018 (0.006)
0.011 (0.008)
0.0167 (0.0011)
0.0137 (0.0012)
0.0053 (0.0002)
0.0027 (0.0003)
p-Cl
p-H
p-CH3
À0.16
a
Standard deviations in parentheses.
b Substituent constant from Ref. 11.
c
Correlation coefficient for the Acree plot.
d From Ref. 3.
Table 4. Calculated values of ki and kip for the reaction of methyl-substituted benzenesulphonates with KCl-Krypto in CH2Cl2 at
22°C. "the dissociation constant of KCl-Krypto in CH2Cl2 is 26.1 Â 10À5 mol dmÀ3 at 22°C.)
X
kia(dm3 molÀ1 sÀ1
)
kipa(dm3 molÀ1 sÀ1
)
sb
rc
d
p-NO2
0.98 (0.12)
0.52 (0.03)
‡0.78
‡0.74
‡0.22
‡0.22
0
0.982
0.998
0.970
0.934
0.990
0.983
m-NO2
p-Br
0.54 (0.06)
0.70 (0.02)
0.065 (0.020)
0.071 (0.025)
0.016 (0.004)
0.010 (0.002)
0.059 (0.005)
0.049 (0.006)
0.0179 (0.0008)
0.0068 (0.0004)
p-Cl
p-H
p-CH3
À0.16
a
Standard deviations in parentheses.
b Substituent constant from Ref. 11.
c
Correlation coefficient for the Acree plot.
d From Ref. 3.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 265–270

268
S. ALUNNI, M. PICA AND G, REICHENBACH
Figure 4. Hammett equation [Eqn. "3)] for the ion rate
constants, ki, with NBu4Cl
Figure 5. Hammett equation [Eqn. "3)] for the ion pair rate
constants, kip, with NBu4Cl
‡
free ion, due to the bond strength between the ion and the
counterion, which are very close to each other, as
previously discussed.3
EtOÀNa in EtOH and applied the Hammett equation to
it. They only examined four substrates and one (the
unsubstituted one) deviated from linearity. They found
two different mechanistic pathways with different r
values: ‡2.2 for the free ion and À0.6 for the ion pair, but
they did not explain this fact. However, it has to be
considered that their system is different; in fact, the effect
of the substituents is on the benzylic carbon atom,
whereas in our case the effect of the substituents is on the
leaving group.
With KCl-Krypto, ki is similar to that found with
NBu4Cl and the reactivities of free ClÀ and the ion pair
are similar to each other: this is due to the weakness of the
bond of the ion pair in this system. In fact, the distance
between the ion and the counterion is larger than in
NBu4Cl. Comparison between the kip values of the two
salts shows that the rate constant for the KCl-Krypto ion
pair is always larger than that of the NBu4Cl ion pair; this
result can also be explained by the different bond
strengths between the ion and the counterion.
Another relevant paper is that of Lee et al.,12 who
studied the reaction of substituted benzenesulphonates
with anilines, and they also found a positive r value of
about ‡1 in CH3OH and CH3CN. This result compares
well with our results. Moreover, they found that ‘the SN2
reactions of ethyl derivatives occur 10–30 times less
readily than those of methyl compounds’, and attributed
this fact to the bulkiness of the reaction centre, in
agreement with our results (see below).
Application of the Hammett equation11 [Eqn. (3)] to
the data in Table 3 and Table 4, i.e.
log ꢁkX=kH† ˆ ꢂꢃ
ꢁ3†
gives information about the structure of the transition
state for the process involving either the ion or the ion
pair. The Hammett plots for the free ion and the ion pair
with NBu4Cl are shown in Figs 4 and 5, respectively.
Good correlations were also obtained with the KCl-
Krypto system. The Hammett constants are reported in
Table 5, which shows that the r values are positive (ca
‡2). This is in agreement with the development of a
negative charge on the leaving group in the transition
state of the nucleophilic substitution reaction. Very
similar r values were obtained for the free ion and ion
pair in both salts. This is evidence that the two processes
are very similar and, even though the cation in the ion
pair influences the rate, it does not influence the
distribution of the charge developed in in the leaving
group in the transition state.
Reaction of ethyl p-nitrobenzenesulphonate 06)
with NBu4Cl or KCl-Krypto salts
In order to obtain some information about the steric
effects of the ion or ion pair nucleophile, the ki and kip
values for ethyl p-nitrobenzenesulphonate (6) with
NBu4Cl or KCl-Krypto were determined (Table 6).
Table 5. Hammett r values for ions and ion pairs
Salt
r (ion)a
r(ion pair)a
rb
rc
NBu4Cl
‡2.05(0.11) ‡1.85(0.09) 0.993
0.995
0.994
KCl-Krypto ‡2.06(0.11) ‡2.06(0.11) 0.994
To our knowledge, only one group has studied the
influence of some para and meta substituents on the rate
constants ki and kip.2b Cayzergues et al.2b studied the
nucleophilic substitution of some benzylic chlorides with
a
Standard deviations in parentheses.
Correlation coefficient of the Hammett plot for the ion.
Correlation coefficient of the Hammett plot for the ion pair.
b
c
Copyright  2001 John Wiley & Sons, Ltd.
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HAMMETT EQUATION IN NUCLEOPHILIC DISPLACEMENTS WITH IONS AND ION PAIRS
269
Table 6. Rate constants, ki and kip, for methyl and ethyl p-
nitrobenzenesulphonates
Methyl m-nitrobenzenesulphonate "2). m-Nitroben-
zenesulphonyl chloride, 2.4 g (11.5 mmol), was added
in small aliquots to a stirred solution of 1.6 ml
(11.5 mmol) of triethylammine in 100 ml of CH3OH at
0°C. The solution was left with stirring for 2 h at room
temperature. The solvent was then removed by rotary
evaporation and H2O and Et2O were added to the solid
residue. The ether phase was washed with water up to
neutrality. Evaporation of the Et2O gave a yellow solid.
Recrystallizing twice with Et2O–n-hexane at 0°C yielded
the sulphonate (1.06 g, 42%), purity by vapour-phase
chromatography (VPC) always >99%, m.p. 87–89°C.
Found: C, 38.87; H, 3.26; N, 6.94. C7H7NO5S requires C,
38.71; H, 3.25; N, 6.45%. NMR: ꢄH (200 MHz, CDCl3)
3.8 (3 H, s, Me), 7.8 (1 H, t), 8.23–8.27 (1 H, m) 8.51–
a
kia
kip
Salt
(dm3 molÀ1 sÀ1
)
(dm3 molÀ1 sÀ1
)
R
NBu4Clb
1.07 (0.07)
0.98 (0.07)
0.12 (0.003)
0.52 (0.03)
CH3
CH3
KCl-Kryptob
NBu4Cl
0.021 (0.002) 0.00624 (0.00012) CH2CH3
0.023 (0.008) 0.027 (0.002) CH2CH3
KCl-Krypto
a
Standard deviations in parentheses.
b Data from Ref. 3.
It can be seen that there is good agreement between the
reactivity of the ion, ki, determined with the two salt
solutions (0.021 vs 0.023 dm3 molÀ1 sÀ1). The reactivity
of the ion pair, kip, with KCl-Krypto is higher than that
obtained with NBu4Cl, in agreement with the data for the
methyl derivatives. The average ki(CH3)/ki(CH2CH3)
ratio is 48. The kip(CH3)/kip(CH2CH3) ratio is 19.3 for
both salts. Steric effects in the SN2 process are relevant
for both ions and ion pairs . However, the steric hindrance
of the ethyl group as compared with the methyl group is
higher for the ion than for the ion pair. An explanation of
this fact can be that the ion approaches the centre of the
nucleophilic attack more than the ion pair, and so it is
more influenced by the bulkiness of the alkyl group.
Moreover, the fact that the kip ratio is the same for both
salts can be attributed to the fact that the activated
complex is linear and that the influence of the cation on it
is very low.
If we compare our results with the theoretical results
obtained by Streitwieser13 for the reaction of alkyl
chlorides with chlorides, it can be seen that in his case the
ions react faster than the ion pairs and that in the ion pair
path the bulkier alkyl groups react faster than the smaller
groups. He interpreted these results by saying that the
activated complex for the ion pair is cyclic and influenced
by the bulkiness of the reacting substrates. The
differences between our experimental results and the
theoretical studies by Streitwieser can be explained
considering that our processes occur in solution and
solvation energies play a role. Moreover, the large
difference in the structure and size of our ion pairs can
favour the conditions for a linear transition state.
‡
8.52 (1 H, m), 8.76–8.78 (1 H, m). MS: m/z 217 (M ,
100%), 186 (34), 171 (68), 123 (88), 75 (84), 50 (83).
Methyl benzenesulphonate "3). VPC always showed
purity >99%. Found: C, 48.82; H, 4.66. C7H8O3S
requires C, 48.83; H 4.68%. NMR: ꢄH(200 MHz; CDCl3)
3.8 (3 H, s, Me), 7.5–7.95 (5 H, m, Ph).
Methyl p-chlorobenzenesulphonate "4). VPC always
showed purity >99%; m.p. 51–52°C. Found: C, 40.73;
H, 3.5. C7H7ClO3S requires C, 40.69; H, 3.41%.
NMR:ꢄH(200 MHz; CDCl3) 3.8 (3 H, s, Me), 7.51–7.89
(4 H, AaBb system, m, Ar).
Methyl p-bromobenzenesulphonate "5). VPC always
showed purity >99%; m.p. 59– 60°C. Found: C, 33.51;
H, 2.73. C7H7BrO3S requires C, 33.48; H, 2.81%.
NMR:ꢄH(200 MHz; CDCl3) 3.8 (3 H, s, Me), 7.68–7.80
(4 H, AaBb system, m, Ar).
Ethyl p-nitrobenzenesulphonate "6). M.p. 89–91°C.
Found: C, 41.35; H, 4.06; N, 6.01. C8H9NO5S requires
C, 41.56; H, 3.92; N, 6.06%. NMR: ꢄH (200 MHz;
CDCl3) 1.4 (3 H, t, Me), 4.25 (2 H, q, CH2), 8.1–8.5 (4 H,
AaBb system, m, Ar).
Kinetic measurements. All kinetic measurements were
carried out on a Kontron Uvikon 923 double-beam
spectrophotometer. The path length of the quartz cell was
1 cm. The reaction was started by adding with a
microsyringe the appropriate amount of substrate solu-
tion to the thermostatted salt solution in the cuvette. The
rate constants (kobs/sÀ1) were determined for Bu4NCl or
KCl-Krypto solutions by following the decrease in
absorbance at 266 nm due to the disappearance of the
reagent with p-CH3 (1), p-H (3), p-Cl (4) and p-Br
benzenesulphonate (5); with methyl m-nitrobenzenesul-
phonate (2) and ethyl p-nitrobenzenesulphonate (6) the
increase in absorbance due to the formation of the m- or
p-nitrobenzenesulphonate anion product was monitored
at 280 nm. For the reaction of methyl m-nitrobenzene-
EXPERIMENTAL
Materials. The solvent and salts were purified as
described previously.3 Methyl p-methylbenzene-
sulphonate (1) was a commercial product (Aldrich). All
the other substituted benzenesulphonate derivatives were
synthesized from the corresponding benzenesulphonyl
chloride (commercial) by a common procedure which
is described here for methyl m-nitrobenzenesulphonate
(2).
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 265–270

270
S. ALUNNI, M. PICA AND G, REICHENBACH
AJ, Shah SC. J. Am. Chem. Soc. 1979, 101: 5764–5769; (g)
Coddington J, Wherland S. Inorg. Chem. 1996, 35: 4023–4028; (h)
Romeo R, Arena G, Scolaro LM, Plutino MR. Inorg. Chim. Acta.
1995, 240: 81–92; (i) Harder S, Streitwieser A, Petty JT, von R.
Schleyer P. J. Am. Chem. Soc. 1995, 117: 3253–3259; (j)
Macchioni A, Bellachioma G, Cardaci G, Travaglia M, Zuccaccia
C, Milani B, Corso G, Zangrando E, Mestroni G, Carfagna C,
Formica M. Organometallics, 1999, 18: 3061–3069; (k) Jia L,
Yang X, Stern CL, Marks TJ. Organometallics, 1997, 16: 842–
857; (l) Masnovi JM, Kochi JK. J. Am. Chem. Soc. 1985, 107:
7880–7893.
2. (a) Acree SF. Am. Chem. J. 1912, 48: 352–380; (b) Cayzergues P,
Geourgoulis C, Mathieu G. J. Chim. Phys. Phys.-Chim. Biol. 1987,
84: 63–70; (c) Mathieu G. Ph.D. Thesis, University Pierre et Marie
Curie, Paris, 1985; (d) Pregel MG, Dunn EJ, Buncel E. Can. J.
Chem. 1990, 68: 1846–1858; (e) Msayb KJ, Watt CIF. Chem. Soc.
Rev. 1992, 21, 237–243; (f) Poli R, Owens BE, Link RG. Inorg.
Chem. 1992, 31: 662–667; (g) Beronius P, Pataki L. J. Am Chem.
Soc. 1970, 92: 4518–4521.
sulphonate (2) with PPNCl the increase in absorbance
was followed at 290 nm.
The classical equation ln [(A?ÀA0)/(A?ÀAt)] = kobs
t
or ln [(A0ÀA?)/(AtÀA?)] = kobst was used. Linearity was
observed up to 80% of the reaction. Duplicate runs
showed that the reproducibility was within 6%. To obtain
a pseudo-first-order condition, there was always an
excess of salt (at least 10-fold). Most of the reactions
were followed to completion. The slower reactions were
followed up to ꢀ70% and the A? value was calculated
from the data for the faster reactions. The same final
absorbance value was obtained even with different
excesses of nucleophile, which shows that the reactions
are not an equilibrium but go to completion. Blank
experiments showed that the substrates and salts were
stable under the kinetic experiment conditions.
3. Alunni S, Pero A, Reichenbach G. J. Chem. Soc., Perkin Trans. 2
1998: 1747–1750.
4. Reichenbach G, Bellachioma G, Cambi R, Cardaci G, Lanfaloni C,
Macchioni A, Ricci R, Vetralla G, Inorg. Chim. Acta, 1999, 288:
200–205.
5. Weast RC (ed). CRC Handbook of Chemistry and Physics, (67th
edn). CRC Press Inc.: West Palm Beach, FL, 1986.
6. (a) Gutman V, Mayer U. Adv. Inorg. Chem. Radiochem. 1975, 17:
189ff; (b) Gutman V. The Donor–Acceptor Approach to Molecular
Interaction. Plenum Press: New York, 1978.
7. Svorstol I, Songstad J. Acta Chem. Scand. Ser. B 1985, 39: 639–
655.
8. (a) Hirsch E, Fuoss RM. J. Am. Chem. Soc. 1960, 82: 1018–1022;
(b) Boche G. Angew. Chem Int., Ed. Engl. 1992, 31: 731–732; (c)
Abbot AP, Schriffin DJ. J. Chem. Soc., Faraday Trans. 1990, 86:
1453–1459.
Acknowledgements
Thanks are due to CNR (Rome) and the Ministero della
Universita` e della Ricerca Scientifica e Tecnologica
(MURST) for financial support. We also thank Professor
Andrew Streitwieser for suggestions and encouragement
and Dr Raimondo Germani for kindly providing some
compounds.
9. Moras D, Metz B, Weiss K. Acta Crystallogr. Sect. B 1973, 29:
383–388; Boileau S, Hemery P, Justice JC. J. Solution Chem. 1975,
4: 873–891.
REFERENCES
10. Glasstone S. An Introduction to Electrochemistry. Van Nostrand
D: New York, 1959.
11. Exner O. In Correlation Analysis in Chemistry, Chapman NB,
Shorter J (eds). Plenum Press: New York, 1978; 469–476.
12. Lee I, Choi YH, Rhyu KW, Shim CS. J. Chem. Soc., Perkin Trans.
2 1989: 1881–1889.
1. See for example (a) Szwarc M. Ions and Ion Pairs in Organic
Reactions. Wiley-Interscience: New York, 1974; (b) Reichardt C.
Solvents and Solvent Effects in Organic Chemistry. VCH:
Weinheim: 1990; (c) Lai ZG, Westaway KC. Can. J. Chem.
1989, 67: 21–26; (d) Baciocchi E, Ruzziconi R, Sebastiani GV. J.
Org. Chem. 1979, 44: 3718–3720; (e) Svoboda M, Hapala J,
Zavada J. Tetrahedron Lett. 1972, 4: 265–268; (f) Smid J, Varma
13. Streitwieser A, Choy GS, Abu-Hasanayn F. J. Am. Chem. Soc.
1997, 119: 5013–5019.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 265–270