The Hammett equation and micellar effects on SN2 reactions of methyl benzenesulfonates - The role of micellar polarity

Article abstract of DOI:10.1002/1099-0690(200012)2000:23<3849::AID-EJOC3849>3.0.CO;2-P

Substituent effects on the reaction of H₂O, OH^-, and Br^- with p-substituted methyl benzenesulfonates in cationic micelles of cetyl trialkylammonium ion surfactants (n-C₁₆H₃₃NR₃X, X = OH, Br, R = Me, Et, nPr, nBu) and in water were analyzed by using the Hammett equation. Values of p in the various media confirm that micellar interfacial regions are less polar than water and polarities decrease with increasing bulk of the surfactant head-group. Wiley-VCH Verlag GmbH, 2000.

Full text of DOI:10.1002/1099-0690(200012)2000:23<3849::AID-EJOC3849>3.0.CO;2-P

FULL PAPER
The Hammett Equation and Micellar Effects on S 2 Reactions of Methyl
N
Benzenesulfonates ؊ The Role of Micellar Polarity
[
a]
[a]
[a]
[a]
Lucia Brinchi, Pietro Di Profio, Raimondo Germani, Gianfranco Savelli,*
[b]
[c]
Nicoletta Spreti, and Clifford A. Bunton
Keywords: Micelles / Hammett equation / Nucleophilic substitution / Kinetics
−
−
Substituent effects on the reaction of H
2
O, OH , and Br with
by using the Hammett equation. Values of ρ in the various
p-substituted methyl benzenesulfonates in cationic micelles media confirm that micellar interfacial regions are less polar
of cetyl trialkylammonium ion surfactants (n-C16
X = OH, Br, R = Me, Et, nPr, nBu) and in water were analyzed
H
33NR
3
X,
than water and polarities decrease with increasing bulk of
the surfactant head-group.
Introduction
In these equations, subscripts W and M denote aqueous
Aqueous micelles and other association colloids can in- and micellar pseudo-phases, respectively, and the quantities
corporate reactive solutes and behave as microreactors. in squared brackets are the molar concentrations in the to-
Rate effects on spontaneous reactions depend on the distri- tal solution volume. However, Nuc
without brackets,
M
bution of substrates between water and micelles and first- Equation 1, is the local molar concentration in the interfa-
m
order rate constants in the aqueous and micellar pseudo- cial region. The second-order rate constants k
and k
,
W
2
Ϫ1 Ϫ1
phases. The latter depend on the properties of the interfa- Equation 1, have dimensions 
s , but in Equation 2, k
M
Ϫ1
cial region, whose polarity and water-content appear to be has a unit of s and is written with the concentration of
slightly lower than those of water, and this region, in some the nucleophile as a mole ratio, [Nuc
]/[Dn]. [Dn] is the
M
respects, behaves like a mixed aqueous-organic solvent.[
1]
concentration of surfactant (detergent) less the critical mi-
For bimolecular reactions, the distributions of both the re- celle concentration, cmc, which is taken as the concentra-
actants between water and micelles have to be consid- tion of monomeric surfactant.[ The association constant,
4]
[
1,2]
ered.
The concentration of reactive counterions at the
K , with respect to S, is written in terms of the concentra-
S
[
5]
surfaces of ionic micelles increases the reaction rates and tion of micellized surfactant.
, in sϪ1, and k
m
in
has to be included in any quantitative treatment. In some
cases, ionic-transfer equilibria can be monitored experi-
Second-order rate constants, k
M
2
Ϫ1 Ϫ1

s , are related by
[
3]
Ϫ
mentally, but for OH , for example, we have to use theor-
etical treatments to estimate local interfacial concentrations.
For the reactions of a substrate S with an added nucle-
ophile, Nuc, the first-order rate constant of the overall reac-
tion, kobs, is given by Equation 1 or Equation 2:[1]
m
k
2
ϭ k
M
V
M
(3)
where V is the molar volume of the interfacial reaction
M
[1]
region which is assumed to be in the range 0.14Ϫ0.37
Ϫ1
m

. As a result, comparisons of values of kw and k2 de-
pend on uncertainties regarding local concentrations. Local
ion concentrations can be estimated by solving the Poisson-
Boltzmann equation in the appropriate symmetry,[
3c,6]
but
[
a]
this method also involves assumptions. Dediazonization-
trapping provides estimates of local ion concentrations, but
cannot be used in basic solutions.
Dipartimento di Chimica, Universit a` di Perugia,
06123 Perugia, Italy
Fax: (internat.) ϩ 39-075/5855538
E-mail: savelli@unipg.it
[3a,3b]
[
[
b]
c]
Reactions are often followed in solutions of a surfactant
with both inert and reactive counterions and their competi-
Dipartimento di Chimica, Ingegneria Chimica e Materiali,
Universit a` di L’Aquila,
67100 L’Aquila, Italy
[1]
tion has to be considered. In this work we avoided the
Department of Chemistry and Biochemistry, University of
California,
complications of inter-ionic competition by using reactive-
ion micelles and defining the transfer of the nucleophile be-
tween water and the micelles in terms of Equation 4, which
has the form of a Langmuir isotherm:[
Santa Barbara, CA 93106, USA
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/eurjoc or from
the author.
7,8]
Eur. J. Org. Chem. 2000, 3849Ϫ3854

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/1223Ϫ3849 $ 17.50ϩ.50/0
3849

L. Brinchi, P. Di Profio, R. Germani, G. Savelli, N. Spreti, C. A. Bunton
FULL PAPER
water and micelles, which allowed us to obtain values of ρ
over a range of conditions and nucleophiles, as shown in
Ϫ
Ϫ
Scheme 1 for the reactions with OH and Br :
This treatment has been applied to a number of reactions
Ϫ
Ϫ
of nucleophilic anions, e.g., OH and Br , and the values
of KЈOH and KЈBr are those used earlier.[
9]
We were interested in bimolecular anionic reactions me-
diated by cationic micelles of quaternary ammonium ion Scheme 1
surfactants and, in particular, how changes in the structure
of the head-group affected the reactivities in the interfacial
ϩ
Ϫ
region. For example, if we replace the N Me head-group
Reactions of OH with 1c and 1f had been examined in
3
of
a
cetyl trimethylammonium ion surfactant (n- cationic micelles and the electronic effects on the reactivity
ϩ
[13]
C H N Me ) by bulkier alkyl groups, we change both the appeared to be higher than those in water, in agreement
1
6
33
3
affinity for anions and the properties of the interface as a with other evidence which shows that values of ρ are more
reaction region,[ e.g., by making it less polar. Therefore, positive for reactions in micelles than in water. Substitu-
effects on the observed rate constants can be due to changes ent effects on the solvolysis of substituted alkyl benzenesul-
in the partitioning of reactants between water and micelles, fonates fit the Hammett equation and ρ becomes more pos-
as well as to changes in the rate constants in the micellar itive as the solvent polarity is decreased.[ The bromide
10]
[12]
14]
pseudo-phase. As a result, a comparison of the values of k
ion is an effective nucleophile in S 2 reactions at alkyl cen-
or k , for a variety of cationic micelles with different head- ters, and although it is less reactive than OH in water, its
N
M
m
Ϫ
2
groups involves approximations and assumptions whose va- higher affinity for cationic micelles means that its micellar
lidities are uncertain.
reactions can easily be followed over a range of conditions.
It is difficult to decide whether changes in the calculated With both nucleophiles, we avoid complications of inter-
values of k 2m or kM (Equations 1 and 2), with changes in ionic competition and micellar effects on buffer equilibria.
reaction mechanism or surfactant structure, for example, In addition, spontaneous hydrolyses of sulfonic esters can
are significant, or if they are due to limitations in our kin- be followed in both micelles and homogeneous solvents
etic treatment. One way to improve the situation is to without complications by buffer effects which have to be
choose a parameter whose values are relatively independent considered in many deacylations. We can therefore use
of the imperfections of the pseudo-phase treatments, and methyl benzenesulfonates to evaluate substituent effects on
Ϫ
Ϫ
we use the Hammett equation for this purpose. Substituent the reactions of H O, OH , and Br in various media.
2
Ϫ
effects on the reaction rates and equilibria are typically ana-
Reactions of OH and thiolate ions with aryl benzoates,
lyzed by using linear free-energy relationships, e.g., the and S 2 reactions of methyl benzenesulfonates are known
Hammett equation (Equation 5) which applies to substitu- to be ‘‘well-behaved’’ in that electronic effects on these reac-
N
tions on a phenyl group:[
11]
tions in both water and micelles fit the Hammett equation
reasonably well.[
12,14]
However, in some spontaneous reac-
tions, the electronic effects of the substituents on the reac-
tions in both homogeneous solvents and micelles do not
(5) fit linear free-energy relationships. For example, substituent
0
logk/k ϭ σρ
effects on the spontaneous solvolyses of benzoyl and ben-
zenesulfonyl chlorides[
15,16]
are complex in that plots of log-
where k is the rate constant for the phenyl derivative, σ is kobs against σ are curved and may go through minima, in-
0
a substituent parameter and ρ is characteristic of the reac- dicating that electronic redistributions in the transition state
tion. Values of ρ are positive for S 2 reactions at alkyl cen- vary with changes in substrate structure and reaction me-
N
ters and increase as the polarity and water-content of solv- dium. Changes in mechanism can also generate extremes in
ents are decreased. Values of ρ for the reactions of aryl these linear free-energy plots.[
17]
Ϫ [12a,12c]
[12b]
benzoates with OH
or thiolate ion
are higher
The surfactants are: n-C H NMe X, XϭOH, Br,
16
33
3
in cationic micelles of CTABr than in water, and, for the CTAOH, and CTABr; n-C H NEt Br, CTEABr; n-
1
6
33
3
Ϫ
reaction of OH , increase with increasing bulk of the cat- C H N(nPr) OH, CTPAOH; and n-C H N(nBu) Br
showing that the micellar interfacial CTBABr. We examined spontaneous hydrolyses in (n-
region is less polar than water and that polarity decreases C H N Me )MeSO , cetyl trimethylammonium mesyl-
1
6
33
3
16 33
3
[
12c]
ionic head-group,
ϩ
Ϫ
3
16
33
3
with increasing bulk of the head-group. However, these ex- ate, CTAOMs. In these reactions, where we compared the
periments involved mixtures of inert and reactive anions, values of ρ under different conditions, errors introduced in
and in some cases, buffers,[
12a,12b]
which complicated the es- the fitting procedures, e.g., in values of KЈOH or KЈ , can-
Br
timation of rate constants in the micellar pseudo-phase be- celled out, to a first approximation, in the estimation of
cause of inter-ionic competition.[ As a simpler system, we Hammett parameters. Selection of the values of the molar
1]
examined the reactions of para-substituted methyl benzene- volume, V , of the interfacial reaction region is a vexing
M
Ϫ
Ϫ
sulfonates, p-Z-C H SO Me, with OH , Br , and H O in problem, in part because they may depend on the reaction
6
4
3
2
3850
Eur. J. Org. Chem. 2000, 3849Ϫ3854

N
The Role of Micellar Polarity Micellar Effects on S 2 Reactions
FULL PAPER
Ϫ
under consideration, but this problem should be less serious which are 55 and 25 for OH in CTAOH and CTPAOH,
Ϫ
when mechanisms and substrate structures are similar.
respectively, and 2000, 1500, 750 for Br in CTABr, CTE-
ABr, and CTBABr respectively.[ Values of kobs are cor-
9]
Results and Discussion
rected for the minor contribution of the reaction with H O
2
measured in CTAOMs (Table S9 and ref.[ ). Values of K
19]
Reactions in the Absence of Surfactant
S
are typical of compounds containing one phenyl group and
are increased by relatively hydrophobic substituents.
Values of k for spontaneous hydrolyses in water and in
obs
18]
[5]
[
mixed solvents
are listed in Table 1, and second-order
Some values of K had been estimated earlier and are sim-
Ϫ
Ϫ
S
rate constants for reactions with OH and Br are listed
in Table 2.
ilar to those used here.[ Values of kM and K are given in
13]
S
Table 2. Values of KЈOH and KЈBr decrease with increasing
Table 1. Solvolyses in the presence and absence of CTAOMs
a]
ρ[b]
Medium
Substituent[
p-NO
2
p-Br
p-Cl
H
p-Me
p-OMe
[
c]
[d]
[g]
H
2
O
7.20
14.0
65.3
61.7
32.3
1.93
3.45
15.5
1.84
1.10
1.63
7.04
6.60
3.62
0.73
0.823
1.06
4.51
4.41
2.29
0.43
0.607
3.16
1.13
1.36
1.43
1.36
1.44
1.41
(0.991)
(0.999)
(0.999)
(0.996)
(0.999)
(0.996)
[
e]
MeOH
[
f]
EtOH
[
e]
EtOH:H
2
O (1:1, v:v)
2
O (1:1, v:v)[
e]
10.5
1.70
dioxane/H
CTAOMs
5.30[
g]
1.63
0.27
[
a]
5
Ϫ1
5
Ϫ1
[b]
m
Values of 10 kobs, s , and 10 kЈ , s , in CTAOMs. Ϫ Values of correlation coefficients in parentheses. σ values used are Ϫ0.268,
c] [d]
3 3 2
Ϫ0.160, 0.227, 0.232 for OCH , CH , Cl, and Br, respecively; for NO
see text. Ϫ [ ρϭ1.10 at 25°C and 1.05 at 50°C, ref. [18b]. Ϫ
[f] [g]
[
e]
ref. [19]. Ϫ at 50°C, ref. [18a]. Ϫ at 70°C, ref. [18a]. Ϫ ref. [13].
Table 2. Nucleophilic reactions in the presence and absence of surfactants
Substituent[
p-NO
a]
ρ
r
Medium
2
p-Br
p-Cl
H
p-Me
p-OMe
Ϫ
[b]
[b]
OH , H
2
O
39.6
15.8
14.8
10.3
6.58
5.42
0.94
0.998
[
[
b]
b]
[b]
[b]
CTAOH
140 (140)
165 (165)
3.80
50.0 (55.0)
48.0 (53.0)
1.37
47.0 (51.0)
44.0 (47.0)
1.15
18.0 (18.0)
14.0 (14.0)
0.600
12.0 (14.0)
8.20 (8.80)
0.575
7.10 (7.30)
4.75 (4.75)
0.469
1.41 (1.40)
1.72 (1.72)
1.00
0.993 (0.984)
0.992 (0.988)
0.982
CTPAOH
Ϫ
2
Br , H O
CTABr
70.0
23.0
34.5 (40.0)
60.0 (87.0)
21.0
21.0 (32.0)
54.0 (78.0)
6.0
5.10
7.60 (8.60)
10.0 (13.5)
2.50
3.90 (4.25)
4.80 (6.20)
1.58
1.52 (1.58)
1.79 (1.56)
0.984
0.984 (0.986)
0.986 (0.985)
CTEABr
CTBABr
95.0 (120)
200 (300)
9.80 (11.0)
14.0 (19.0)
[
a]
4
Ϫ1 Ϫ1
4
Ϫ1
Values of second-order rate constants 10 k
w
, 
2
s
, in water and 10 k
m
, s , in surfactant; values of k
m
in parentheses are in 0.1 
Ϫ1
NaOH or NaBr. Values of K
s
,  , are: p-NO
, 58 (50 in CTABr); p-Br, 130; p-Cl, 120; H, 65; p-Me, 120; p-OMe, 90 (100 in CTAOH,
and 120 in CTPAOH). Ϫ [ ref. [13].
b]
bulk of the head-group, consistent with physical evidence
(e.g., NMR spectroscopy, conductivity)[
1c][1d]
of affinities of
Micellar Reactions
anions for micelles. Examples of the fits are shown in Fig-
ure 1 and Figure 2 for OH and Br respectively, and all
the kinetic data are available as supplementary material.
The spontaneous hydrolyses are modestly inhibited by
CTAOMs micelles (Table 1), and kobs ϭ kЈM (Equation 2),
Ϫ
Ϫ
when the substrates are fully micellar-bound. These obser-
vations are consistent with earlier results,[
13,19]
and evidence
Ϫ
Values of kM for reactions of OH with a given substrate
that micellar interfacial regions are less polar than
[
1,20]
are not very sensitive to the addition of 0.1  NaOH, but
water.
Ϫ
Ϫ
0.1  NaBr accelerates the reactions with Br (Table 2).
Cationic micelles accelerate the reactions of OH and
Ϫ
Structures of bromide ion micelles are sensitive to the addi-
Br , and values of k increase monotonically without re-
obs
Ϫ
tion of Br , which increases aggregation numbers and
aching limiting values, especially for systems with hydroxide
as counterion, as already observed in other cases.[
1b,9]
therefore may change the properties of the interfacial re-
gions, e.g., by inducing a sphere-to-rod transition;[ added
bromide may also produce an increase in interfacial brom-
ide concentration, as observed by Soldi et al.[
When
21]
NaX is added (X ϭ OH or Br), the values of kobs tend
towards limiting values, and kobs increases relative to values
without any added salt. These effects can be related to a
change in local anion concentration, when the surfactant
concentration is increased, or when salt is added, as ob-
served in cetyl trialkylammonium bromide surfactants by
21b]
Fits to the Hammett Equation
The original σm and σ values were calculated from the
p
[
21b]
Soldi et al.
The rateϪsurfactant profiles are fitted to dissociation constants of substituted benzoic acids in water
Equation 2 with experimental values of kW (Table 2) and and involved an interplay between inductive (field) effects
Ϫ1
with literature values of KЈOH and KЈBr (in units of 
)
and mesomeric (resonance) effects. There is extensive work
Eur. J. Org. Chem. 2000, 3849Ϫ3854
3851

L. Brinchi, P. Di Profio, R. Germani, G. Savelli, N. Spreti, C. A. Bunton
FULL PAPER
Ϫ
Ϫ
Values of ρ for reactions of H O, OH , and Br in mi-
2
celles were found to be more positive than in water. For the
spontaneous hydrolysis in CTAOMs, the value of ρ was
very similar to those for solvolyses in alcohols or aqueous-
organic media, which are included for comparison
Ϫ
Ϫ
(
Table 1). For reactions of OH and Br , ρ became more
positive with an increase in the bulk of the head-group, as
found earlier for ester saponification,[
12c]
corresponding to
a decrease in polarity of the interfacial region. Details of
the rate constants and values of ρ calculated by using σp
for NO are available as Supplementary material.
2
Values of ρ (Table 1 and 2) show that S 2 reactions of
N
the methyl benzenesulfonates are less sensitive to electronic
effects than the saponification of phenyl benzoates, for
Figure 1. Reaction of methyl p-methoxybenzenesulfonate in which ρ ϭ 1.86 or 1.76 in water, and 2.6Ϫ3.1 in cationic
micelles.[
12a][12c]
These differences are understandable in
CTAOH (᭹, ᭺), CTPAOH (᭿, ᮀ). Solid and open symbols refer
to reaction with and without 0.1  NaOH, respectively
view of the differences in the relative locations of the reac-
tion centers and substituents in the two sets of reactions,
which facilitate the transmission of electronic effects in the
Ϫ
reactions of the phenyl benzoates. Attack of OH on
phenyl benzoates involves a rate-limiting addition to the
acyl group, which is accelerated by electron-withdrawing
substituents in the benzoate ring, but in S 2 reactions we
N
have to consider both nucleophilic attack and loss of the
leaving sulfonate ion. Comparisons of second-order rate
constants in water and micelles depend on parameters
whose values are uncertain, e.g., those which describe inter-
ionic competition and the volume of the interfacial reaction
region.[ We therefore have to be cautious in using kinetic
data for the overall reaction to compare medium properties
of water and micellar interfacial regions, because under
some conditions, kinetic fits are indeterminate for reactions
1]
Ϫ
of weakly interacting ions (e.g., OH ) in the presence of
strongly interacting ions.[ However, as discussed above, a
comparison of the values of ρ eliminates some of these un-
symbols refer to reaction with and without 0.1  NaBr, respectively certainties.
3c]
Figure 2. Reaction of methyl p-methoxybenzenesulfonate in
CTABr (᭺), CTEABr (᭢, ᭞), CTBABr (᭜, ᭛). Solid and open
The Interfacial Reaction Region
on the separation of these effects, including estimations of
Micellar acceleration of bimolecular reactions is due larg-
σ values for special situations,[11] and Yukawa and Tsuno ely to the concentration of the reactants in the interfacial
have introduced an additional parameter to allow for the region where local second-order rate constants are often
relative importance of inductive and mesomeric effects.[
22]
similar, but not identical, to those in water. If they were
[1]
There were negative deviations with σ ϭ 0.78 for the p- identical, substituent effects would be the same in water and
p
NO group in all our micellar reactions. There are devi- micelles, which is not the case.
2
ations from the simple form of the Hammett equation for
Comparison of second-order rate constants in water and
the p-NO2 group in a number of reactions,[11a,11b] de- micelles requires concentrations to be expressed in the same
pending to some extent on the reaction medium, and sim- units, and conventionally molar concentrations are used in
Ϫ1
ilar deviations were found for other groups, e.g., OR, where solution reactions. Elsewhere we have taken VM ϭ 0.14 
resonance interactions were possible. This question is con- (Equation 3), although it may depend on the micellar head-
m
sidered later. In our work, fits were improved when we used group and on the nature of the reactants. Values of k /k
2
W
Ϫ1
the σ value for p-NO , which considers only inductive ef- calculated with VM ϭ 0.14  for all the surfactants range
I
2
Ϫ
fects, (based on the correlation coefficient, r), and this ap- from approximately 0.1Ϫ0.5 for reactions of OH , and
proach has been applied to electronic effects of p-OMe in 0.4Ϫ1 for reactions of Br . They are not very sensitive to
which are accelerated by electron dona- the head-group bulk for reactions of OH , but an increase
tion. Most of the data in the literature are for p-substitu- in bulk increases k /k for the reaction of Br (assuming
Ϫ
[
23]
Ϫ
some reactions
m
Ϫ
2
W
Ϫ1
ents, except for reactions in organic or mixed solvents where VM ϭ 0.14  ). These differences are consistent with vari-
the m-NO2 derivative was also used.[ unless specified, ations in ρ (Table 2). Reactions of OH and Br probably
18]
Ϫ
Ϫ
data were collected at 25.0 °C.
differ in the extents of charge dispersion in transition-state
3852
Eur. J. Org. Chem. 2000, 3849Ϫ3854

N
The Role of Micellar Polarity Micellar Effects on S 2 Reactions
FULL PAPER
formation, but differences in average locations of the very more informative than simple rate constants because they
Ϫ
Ϫ
hydrophilic OH and the less hydrophilic Br in the inter- were less sensitive to assumptions regarding interfacial vol-
facial region may also affect relative rate constants. The as- umes and transfer equilibria of ions and nonionic sub-
sumption that the micellar interface is a uniform reaction strates. Values obtained in this way were more positive than
region is applicable only as a first approximation, but it those in water, indicating that the polarities of the interfa-
rationalizes extensive rate data.[ In addition, the increasing cial regions are lower than that of water and decreased with
1]
(
positive) values of ρ for the S 2 reactions in micelles, or increasing bulk of the head group.
N
in solvents of lower polarity than water, can be rationalized
by considering the reaction region as uniform, and they are
understandable in terms of JencksϪMore O’Ferrall[ free- Experimental Section
24]
energy diagrams and qualitative descriptions of medium ef-
fects on organic reactions which give evidence on the ex- Materials: Surfactants were samples used earlier,[9b,13] as was 1c.
Compounds 1b and 1f were obtained from Aldrich. The other sul-
fonates were prepared from the sulfonyl chloride in MeOH with
tents of bond-making and -breaking in transition
[
11,12]
states.
On the basis of data from dediazonization,
3
Et N at 0° C, except for 1a, where we used NaOMe in MeOH. Ϫ
Soldi et al. concluded that values of VM are larger than
1
For the substrates 1a, 1d, and 1e, H NMR spectra were recorded
those used here and increase significantly with increasing
on a 80 MHz Bruker instrument, in order to confirm the structures.
bulk of the head-group.[
21b]
In that event, values of k2m
1
H NMR (CDCl
3
), δ: 1a: 3.7 (s, 3 H, CH
3
), 3.9 (s, 3 H, CH
), 7.5Ϫ8.0
), 7.4Ϫ7.8 (m, 2 H, 2 CH),
3
), 7.1
would be higher than those quoted here, especially with
CTPAOH and CTBABr, but the Hammett parameters
(
m, 2 H, 2 CH), 7.9 (m, 2 H, 2 CH); 1d: 3.7 (s, 3 H, CH
m, 4 H, 4 CH); 1e: 3.7 (s, 3 H, CH
3
(
3
should not be affected, provided that VM is independent of 7.8Ϫ7.9 (m, 2 H, 2 CH). Melting points: 1d, m.p. 51Ϫ52 °C; 1e,
m.p. 59Ϫ60 °C (ref. 60.5 °C)[
18a]
.
the substrate.
Kinetics: Reactions were followed spectrophotometrically at 25.0°
Electronic Effects of the Nitro Groups
C, as described[
13,16]
on HP8452 or Shimadzu 160 A instruments
We saw negative deviations from Equation 5 when we at the following wavelengths: 1a, 246; 1b, 236; 1c, 266; 1d,e, 242,
used the σ value of 0.78 for the p-nitro group. For example, 1f, 280 nm. The substrate was added in MeOH, and the reaction
p
Ϫ4
Robertson’s very precise data for solvolyses of substituted mixture contained substrate (10 ) and MeOH (1 vol%). Good
methyl benzenesulfonates in water and mixed solvents[
18]
first-order kinetic plots were obtained with all substrates. Reactions
were followed in redistilled, CO -free water. The least reliable data
2
were for the reaction with Br in water, which was not much faster
than the spontaneous hydrolysis. There may be a salt effect of NaBr
3
on this reaction, because 0.1  MeSO Na decreases kobs for the
hydrolysis of 1a from 6.07·10 to 5.75·10
crease the second-order rate constants for all the substrates to sim-
ilar extents. We did not have this problem with the reactions with
generally gave slightly better fits to the Hammett equation
Ϫ
when the σ value of 0.65 for p-NO was used, and ρ values
I
2
then became more positive, by approximately 0.1 units, than
for the reactions with OH and Br (Tables 1, 2, and S10).
Some of these values are from rate data at 50° or 70° C,
but the quality of the fits for reaction in H O at 25° or 50°
Ϫ
Ϫ
Ϫ6
Ϫ6 Ϫ1
s , but it should de-
2
Ϫ
Ϫ
C with either the σ or σ parameters for the p-NO group OH or with Br in micelles which were much faster than those
I
p
2
were essentially identical, although ρ became slightly less with water.
positive with an increase in temperature. Our value of ρ for
Data Fitting: The first-order rate constants were corrected for the
minor contribution of reaction with H O, based on reactions in
CTAOMs (Table S9) and quoted values of kobs refer to reactions
the hydrolysis in water was similar to that from the earlier
data (Table 1). There are examples of reactions in solution
for which deviations from linearity of Hammett plots are
evident for strongly electron-donating or -withdrawing
groups with resonance interactions, when the original σp
2
Ϫ
Ϫ
with OH or Br . In fitting the kinetic data to Equation 1, we used
[9]
literature values of KЈOH and KЈBr

.
Values of K
S
were 50Ϫ130
Ϫ1
, as is typical of compounds with this general structure, and we
values are used. It is possible that electronic effects of the assumed that hydrophobic substituents would modestly increase
K
S
. Estimated values of k
M
were insensitive to changes in K
S
of ca.
p-NO group are sensitive to the reaction medium, e.g., the
asymmetry of the micellar, interfacial, region may orient
2
4
1
0%. Values of 10 cmc, were taken as: 7, 5, 8, 6, 2.8  for CTAOH,
CTPAOH, CTABr, CTEABr, and CTBAbr, respectively, and as
zero in 0.1  NaOH or NaBr with very little effect on fits, especially
in 0.1  NaOH or NaBr. There were deviations between the ob-
served and predicted rate constants in dilute surfactant, especially
the p-NO group so as to disfavor electron withdrawal by
2
resonance, and a decrease in solvent polarity should have a
similar effect. We noted that good fits to the Hammett
equation have been obtained for reactions of other p-nitro-
[9]
with the hydroxide ion surfactants, as in other systems. They are
phenyl derivatives in micelles,[
12a][12b]
although here the pos-
due to several factors, including assumptions regarding concentra-
tions of monomeric surfactant, as given by the cmc, perturbation
of micellar structure by the substrate and possible intervention by
premicelles. Addition of OH or Br improves the fits, probably
by promoting micellization.
itive ρ values were higher than in our work and deviations
from linearity would be less evident.
Ϫ
Ϫ
Conclusions
Acknowledgments
Application of the Hammett equation to S 2 reactions
N
Ϫ
Ϫ
of H O, OH , and Br with p-substituted methyl benzene- Support of this work by Consiglio Nazionale delle Ricerche, Rome,
2
sulfonates in cationic micelles gave values of ρ which were and by the Ministero dell’Universita’ e della Ricerca Scientifica,
Eur. J. Org. Chem. 2000, 3849Ϫ3854
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