Effects of micellar head group structure on the spontaneous hydrolysis of methyl naphthalene-2-sulfonate. The role of perchlorate ion

Article abstract of DOI:10.1039/a705316i

The spontaneous (S_N2) hydrolysis of methyl naphthalene-2-sulfonate (MeONs) in water is inhibited by cationic, anionic and zwitterionic micelles of the following surfactants, CTAOMs, n-C₁₆H₃₃N⁺Me₃MeSO₃ ^-; CTPAOMs, n-C₁₆H₃₃N⁺Pr₃MeSO₃ ^-; SDS, C₁₂H₂₅OSO₃^-Na⁺; SB3-14, n-C₁₄H₂₉N⁺Me₂(CH₂) ₃SO₃^-;-SBBu3-14, n-C₁₄H₂₉N⁺Bu₂(CH₂) ₃SO₃^-; DMMAO, n-C₁₄H₂₉N⁺Me₂O^-; DPMAO, n-C₁₄H₂₉N⁺Pr₂O^-. Rate constants, k_rel, relative to those in water, are in the range 0.55-0.63 for all the cationic and zwitterionic micelles including the protonated amine oxides. The value of k_rel in anionic micelles of SDS is 0.22, but NaClO₄ sharply decreases k_rel in SB3-14 from 0.56 to 0.15. These rate effects are not related to variations in substrate binding but depend upon interactions of the head groups with the initial and transition states.

Full text of DOI:10.1039/a705316i

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Effects of micellar head group structure on the spontaneous
hydrolysis of methyl naphthalene-2-sulfonate. The role of perchlorate
ion
Lucia Brinchi,a Pietro Di Profio,a Raimondo Germani,a Gianfranco Savelli,a
Nicoletta Spretib and Clifford A. Buntonc
a Dipartimento di Chimica, Università di Perugia, 06100 Perugia, Italy
b Dipartimento di Chimica, Ingegneria Chimica e Materiali, Università di L’Aquila,
67010 L’Aquila, Italy
c Department of Chemistry, University of California, Santa Barbara, CA 93106, USA
The spontaneous (SN2) hydrolysis of methyl naphthalene-2-sulfonate (MeONs) in water is inhibited by
؊
cationic, anionic and zwitterionic micelles of the following surfactants, CTAOMs, n-C16H33N؉Me3MeSO3 ;
؊
CTPAOMs, n-C16H33N؉Pr3MeSO3 ; SDS, C12H25OSO3؊Na؉; SB3-14, n-C14H29N؉Me2(CH2)3SO3؊;-
SBBu3-14, n-C14H29N؉Bu2(CH2)3SO3؊; DMMAO, n-C14H29N؉Me2O؊; DPMAO, n-C14H29N؉Pr2O؊. Rate
constants, krel, relative to those in water, are in the range 0.55–0.63 for all the cationic and zwitterionic
micelles including the protonated amine oxides. The value of krel in anionic micelles of SDS is 0.22, but
NaClO4 sharply decreases krel in SB3-14 from 0.56 to 0.15. These rate effects are not related to variations in
substrate binding but depend upon interactions of the head groups with the initial and transition states.
Although charge affects rates of spontaneous hydrolyses at
Introduction
micellar surfaces there is limited evidence regarding effects of
head group size or structure.10 Engberts and co-workers found
that micelles and other association colloids modestly inhibit
spontaneous hydrolyses of acyltriazoles and the extent of inhib-
ition is sensitive to colloid structure.5b,c Spontaneous hydrolyses
of benzenesulfonyl chlorides are micellar inhibited and inhib-
ition is greater for anionic than for cationic micelles, but zwit-
terionic sulfobetaine and cationic micelles behave similarly.12
Similar micellar charge effects are seen in spontaneous hydrol-
yses of carboxylic anhydrides and acyl chlorides.5d,8a
Aqueous micelles affect rates of nonsolvolytic bimolecular
reactions by controlling concentrations of the two reagents at
the micelle–water interface. Therefore overall rate constants
depend upon transfer equilibria between water and micelles and
second-order rate constants in each region.1 Nucleophilic reac-
tions have been widely studied and second-order rate constants
at micelle–water interfaces are typically similar to, or slightly
lower than, those in water. For SN2 reactions of BrϪ or ClϪ
mediated by cationic micelles of quaternary ammonium ion
surfactants rate constants at the micelle–water interface
increase modestly with increasing head-group bulk of the sur-
factant,2 but the effect is inverted for reactions of OHϪ.3 Rates
of intramolecular SN2-like cyclisations at micellar surfaces also
increase modestly with increasing head group size.4
In the present work we examine micellar effects on the spon-
taneous (SN2) hydrolysis of methyl naphthalene-2-sulfonate
(MeONs), Scheme 1.
–
SO3Me
+
SO3
Spontaneous, bimolecular, hydrolyses of alkyl halides and
sulfonate esters are mechanistically simple, because the solvent,
e.g., water, participates only nucleophilically and by solvating
leaving groups, and not as a general acid–base catalyst as in
some deacylations. In the transition state positive charge
develops on the nucleophilic water molecule and is dispersed
into the solvent by hydrogen-bonding. However, despite
involvement of water molecules as nucleophiles or general
bases, aqueous micelles do not strongly affect rates of either
deacylations or SN2 hydrolyses, indicating that the micellar
interfacial region is ‘water-rich’.1,5 There could also be effects
due to the high ionic concentration in this region, as well as its
slightly lower polarity relative to water.1,6,7 This region is elec-
trically asymmetric in ionic and zwitterionic micelles, which
may affect hydrolyses in which local charges are developed in
the transition state, and inhibition of spontaneous hydrolyses is
larger in anionic than in cationic micelles.5c,7 Inhibition is larger
for SN1 than for spontaneous, bimolecular hydrolyses and then
the more effective inhibitors are cationic, rather than anionic,
micelles.5a,d,8 These different charge effects can be rationalized
in terms of interactions of the micellar head groups with local
charges in the transition states of uni- and bi-molecular
hydrolyses at acyl or alkyl centers. Similar interactions with
head groups are apparently important in anionic decarboxyl-
ations9 and dephosphorylations10 and E1cB reactions11 in ionic
and zwitterionic micelles.
H2O
+ + MeOH
H+
Scheme 1
This reaction is a mechanistically simple, concerted dis-
placement, whereas hydrolyses of acyl derivatives may be con-
certed or step-wise,13 and the hydrophobic substrate binds
strongly to micelles. As a result we obtain limiting values of the
rate constants in relatively dilute surfactant so that changes due
to micellar growth are unimportant. We used cationic, anionic
and zwitterionic surfactants and varied the head group struc-
tures. The amine oxides are useful because protonation converts
them from zwitterions, which are nucleophilic,14 into cations,
and protonation in dilute acid is well established.15 Surfactants
usedinthisworkarecetyltrimethylammoniummethanesulfonate
Ϫ
(CTAOMs) n-C16H33NϩMe3MeSO3 ; cetyltripropylammonium
methanesulfonate (CTPAOMs) n-C16H33NϩPr3MeSO3 ; sodium
Ϫ
dodecyl sulfate (SDS) C12H25OSO3Na; 3-(N,N-dimethyl-
myristylammonio)propanesulfonate (SB3-14) n-C14H29Nϩ-
Ϫ
Me2(CH2)3SO3 ;
3-(N,N-dibutylmyristylammonio)propane-
sulfonate (SBBu3-14) n-C14H29NϩBu2(CH2)3SO3 ; 1,1-dimeth-
ylmyristylamine oxide (DMMAO) n-C14H29NϩMe2OϪ; 1,1-
dipropylmyristylamine oxide (DPMAO) n-C14H29NϩPr2OϪ.†
Ϫ
† Cetyl = hexadecyl, myristyl = tetradecyl.
J. Chem. Soc., Perkin Trans. 2, 1998
361

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Table 1 Rate constants of the spontaneous hydrolysis of MeONs in
cationic surfactants
Table 3 Rate constants of the spontaneous hydrolysis of MeONs in
amine oxide surfactants
CTAOMs
CTPAOMs
DMMAO
DPMAO
[Surfactant]/10Ϫ3
[Surfactant]/10Ϫ3 mol dmϪ3
kobs/10Ϫ6 sϪ1
kobs/10Ϫ6 sϪ1
mol dmϪ3
kobs/10Ϫ6 sϪ1 a
kobs/10Ϫ6 sϪ1 b
kobs/10Ϫ6 sϪ1 a
0
3
5
10
30
50
100
12.3
9.30
8.83
7.82
7.35
7.34
7.32
12.3
8.04
7.58
7.27
0
2
6
13.4
9.74
13.3
9.03
13.4
9.08
7.42
7
7.97
7.64
7.61
7.20
10
12
18
21
31
35
7.12
7.04
7.07
6.90
6.96
6.96
7.47
6.90
6.72
7.06
Table 2 Rate constants of the spontaneous hydrolysis of MeONs in
sulfobetaine surfactants
a In 0.05 mol dmϪ3 MeSO3H. b In 0.1 mol dmϪ3 MeSO3H.
SB3-14
SBBu3-14
kobs/10Ϫ6 sϪ1
12.3
Table 4 Rate constants of the spontaneous hydrolysis of MeONs in
[Surfactant]/10Ϫ3 mol dmϪ3
kobs/10Ϫ6 sϪ1
SDS
0
1
3
5
8
10
30
50
100
12.3
9.15
8.12
[SDS]/10Ϫ3 mol dmϪ3
kobs/10Ϫ6 sϪ1
8.50
8.07
0
10
20
30
50
70
100
12.5
7.43
3.54
3.10
2.85
2.93
2.69
7.19
7.09
6.89
7.17
7.11
7.69
7.70
7.75
The counterion in the cationic surfactants was mesylate
rather than halide ion to limit nucleophilic participation,2 as in
solutions of HBr and DMMAO or DPMAO,16 and therefore
we used MeSO3H to protonate the amine oxides.
Results and discussion
Micellar kinetics
Micelles and water are treated as distinct reaction media, i.e., as
pseudophases,1 and the observed first-order rate constant, kobs
,
is given by eqn. (1):
kЈW ϩ kЈMKS[Dn]
(1)
kobs
=
1 ϩ KS[Dn]
Substrate, S, is rapidly partitioned between water and
micelles, designated by subscripts W and M respectively, with a
binding constant, KS, with respect to micellized surfactant
(detergent), Dn, whose concentration is the total less that of the
monomer, i.e., the critical micelle concentration, c.m.c., under
the kinetic conditions.17
The value of KS of MeONs in cationic micelles2,3 is ca. 103
dm3 molϪ1 and this hydrophobic substrate probably promotes
micellization, so that, except in dilute surfactant, MeONs is
fully micellar bound and kЈM ≈ kobs. We observed limiting rate
constants in the range 0.03–0.1 mol dmϪ3 surfactant, consistent
with quantitative substrate binding (Tables 1–4). The electrolyte
concentration was varied by addition of MeSO3Na or MeSO3H
and NaClO4 with SB3-14, and the hydrolysis rate in water is
almost unaffected by up to 1 mol dmϪ3 electrolyte. Rate data
with added MeSO3Na and MeSO3H are available as sup-
plementary material and in Fig. 1.‡ Added C1O4Ϫ binds very
strongly to sulfobetaine micelles as shown by 35Cl NMR spec-
troscopy and strong inhibition of the reaction of BrϪ with
Fig. 1 Rate constants of the spontaneous hydrolysis of MeONs in
0.05 mol dmϪ3 SB3-14 with addition of MeSO3Na (᭹) and NaClO4 (᭿)
MeONs.16 It therefore converts a zwitterionic into an anionic
micelle, which should affect rates of spontaneous hydrolyses.
Values of kЈM are given in Table 5 and variations of kobs with
[surfactant] are given in Tables 1–4. Values of kЈM were
obtained by fitting rate data to eqn. (1) as described.1,2
Effect of micellar charge
The spontaneous hydrolysis of methyl benzenesulfonate is
inhibited by both CTAOMs and SDS and values relative to
reaction in water, krel, are 0.70 and 0.40 respectively.5d,18 Values
of krel for reaction of MeONs in surfactants (Table 5) are simi-
lar to, but slightly lower than, those for reaction of methyl
benzenesulfonate, despite differences in hydrophobicities of
these substrates (for the benzenesulfonate18 KS = 50–70 dm3
molϪ1). Both substrates reside in a similar region of the micelle
‡ This material has been deposited in the Supplementary Publications
Scheme (SUPPL NO. 57320, 3 pp.) For details of the deposition
scheme, see ‘Instructions for Authors’, J. Chem. Soc., Perkin Trans. 2,
available via the RSC Web pages (http://www.rsc.org/authors).
362
J. Chem. Soc., Perkin Trans. 2, 1998

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polarity of the solvent.22 Extension of these rules to micellar
Table 5 Rate constants of the spontaneous hydrolyses in the micellar
pseudophasea
surfaces is satisfactory for a number of reactions1d,5 and we
therefore expected that polarity and availability of water at
micellar surfaces, and therefore rate constants, would be
affected by bulky alkyl groups.
Surfactantb
kЈM/10Ϫ6 sϪ1
CTAOMs
7.33 (0.59)
7.23 (0.58)
6.93 (0.55)
7.01 (0.56)
7.03 (0.56)
7.94 (0.64)
7.27 (0.58)
1.85 (0.15)
7.73 (0.62)
7.40 (0.59)
7.22 (0.58)
6.81 (0.54)
7.06 (0.56)
2.81 (0.22)
CTAOMs ϩ MeSO3Nab
CTPAOMs
Similarities in kinetic behaviours of quaternary ammonium
and sulfobetaine micelles are understandable. The charge in
ionic micelles is extensively neutralized by counterions1 and
if the trimethylene tether in our sulfobetaine surfactants is
extended23 the sulfonate residue should not strongly perturb the
environment of the quaternary ammonium ion. As noted
earlier protonated amine oxides are cationic surfactants, but
although they can strongly hydrogen-bond to water they appear
to have no special effect on its reactivity towards MeONs
(Table 5).
CTPAOMs ϩ MeSO3Nab
SB3-14
SB3-14 ϩ MeSO3Nab
SB3-14 ϩ MeSO3Hb
c
SB3-14 ϩ NaClO4
SBBu3-14
SBBu3-14 ϩ MeSO3Nab
DMMAOHϩ ϩ MeSO3Hd
DMMAOHϩ ϩ MeSO3Hb
DPMAOHϩ ϩ MeSO3Hd
SDS
Hϩ
RЈR2Nϩ᎐OϪ
RЈR2Nϩ᎐OH
a At 25.0 ЊC, with kЈW = 1.25 × 10Ϫ5 sϪ1. Values in parentheses are rela-
tive rate constants in micelles and water, krel.
b 0.1 mol dmϪ3 MeSO3Na
The nature of the reaction region
or MeSO3H. c 1–1.6 mol dmϪ3 NaClO4 (Fig. 1). d 0.05 mol dmϪ3
MeSO3H.
Rate enhancements of nonsolvolytic bimolecular reactions at
micellar surfaces are fitted quantitatively by models that take
into account the high concentration of reagents in the inter-
facial region, which is often identified as the Stern layer, a few
Angstroms thick, where counterions are concentrated.1 Based
on other kinetic evidence, the properties of this region as a
reaction medium should depend on the head group structure,
cf., refs. 2–5, 8–10, in contrast to the apparent insensitivity of
our spontaneous SN2 hydrolyses (Table 5). The hydrophobic
naphthalene group of MeONs will orient itself towards the
quaternary ammonium ions and the attached alkyl tails,19,24 but
the methylsulfonate residue can extend into a more aqueous
region (the question of the ‘wetness’ of the micellar surface and
the extent of water–hydrocarbon contact has been discussed in
terms of various micellar models1,5a,7,25). In the transition state
negative charge should build up on the naphthalenesulfonate
residue and interact unfavorably with anionic head groups in
SDS micelles, consistent with the difference in krel for these and
the cationic or zwitterionic micelles. (Table 1 and refs. 5d, 8, 11).
Inhibitions are similar with cationic and zwitterionic micelles,
which have similar charge asymmetry in the interfacial region.
There is no indication that changes in the average location of a
substrate in the micellar interfacial region, as related to its
hydrophobicity, have a major effect on the rate of spontaneous
hydrolysis.
which, based on NMR data with MeONs, is the micelle–water
interface adjacent to the ionic head groups.19 However,
increased penetration of the more hydrophobic substrate,
MeONs, should partially shield it from water molecules.19
Zwitterionic and cationic micelles, including protonated
amine oxides, behave differently from anionic micelles (Table 5).
Neutral amine oxides can increase kobs by nucleophilic dis-
placement on MeONs14 and we discuss these reactions else-
where. The values of kЈM in Table 5 do not seem to be related to
the hydrogen bonding of water to anionic or zwitterionic head
groups. Hydrogen bond donation to sulfate or sulfonate res-
idues might activate water as a nucleophile,20 but this effect does
not explain our results, because reactions are slower in anionic
than in cationic micelles, and water at the micellar surface is
apparently not deactivated by hydrogen-bonding to protonated
amine oxides (Table 5). However, analyses of micelle–solute
interactions in terms of linear solvation free energy relation-
ships indicate the importance of hydrogen bonding,7 and the
specificity of micelle–ion interactions1 is related to the ease of
partial ionic dehydration.21
Effect of head group structure
The insensitivity of kЈM to head group bulk and structure (Table
5) was unexpected because of the observed acceleration in
spontaneous anionic decarboxylations and dephosphorylations
in cationic and sulfobetaine micelles with increasing head group
bulk.9b,10
The role of perchlorate ion
The hydrolysis rate of MeONs in SB3-14 is almost unaffected
by the addition of up to 1.5 mol dmϪ3 MeSO3Na (with a slight
increase), but we observe a large effect upon addition of
NaClO4 (Fig. 1). As noted, perchlorate ion interacts strongly
with sulfobetaine micelles,16 and in effect, C1O4Ϫ converts a
zwitterionic sulfobetaine into an anionic micelle. This gener-
ation of head group charge inhibits the spontaneous hydrolysis
of MeONs (Table 5 and Fig. 1). However, the inhibition by
NaClO4 is higher than expected for an anionic micelle, with
krel = 0.15 as compared with 0.22 in SDS. It is difficult to
explain inhibition by C1O4Ϫ solely in terms of micellar charge
because micellized SB3-14 ϩ C1O4Ϫ should not be more
‘anionic’ than SDS. Chevalier and co-workers have suggested
that the interfacial region of betaine micelles can be very
open,23,26 which allows water molecules to penetrate these
regions. However, C1O4Ϫ interacts with the ammonium centers,
based on changes in the 14N NMR spectrum,16 and it will then
expel water from this region and thus inhibit spontaneous,
bimolecular hydrolyses (Table 5). The insolubility of cationic
surfactants ϩ C1O4Ϫ prevents our examining this system.
Cyclization of the phenolic derivatives, 1, (Scheme 2) is also
O–
O
(CH2)3
+
X–
O
O(CH2)3X
1a X = Br
b X = I
Scheme 2
accelerated by an increase in head group bulk,4 as are SN2 reac-
tions of ClϪ and BrϪ with MeONs.2 However, an increase in the
bulk of cationic head groups slightly decreases second-order
rate constants for reactions of OHϪ at micellar surfaces.3 Bulky
alkyl groups at the ammonium ion center may partially exclude
water molecules from the micellar surface and decrease its
polarity and hydrogen bonding of interfacial water to anions.
The absence of such an acceleration of reactions of OHϪ is
understandable because this ion has a high affinity for water
and its hydration should be unaffected by an increase in head
group bulk. The Hughes–Ingold rules predict that SN2 sol-
volyses should be inhibited by a decrease in water content or
Conclusions
Our evidence regarding effects of cationic, betaine sulfonate
J. Chem. Soc., Perkin Trans. 2, 1998
363

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and anionic micelles on a spontaneous SN2 hydrolysis fits the
generalization that cationic and betaine micelles behave simi-
larly as reaction media.9b,10,11 Rate enhancements of reactions
of anionic reagents are generally lower with betaine sulfonate
than with cationic micelles but these differences are due to
relatively weak binding of anions to betaine micelles, and not to
reactivities at micellar surfaces.27 We can explain rate effects on
a variety of spontaneous reactions in terms of a lower polarity
and water availability in the interfacial region, relative to bulk
water,6,7,23 together with electrical asymmetry in this region,
which generates the differences between cationic and anionic
micelles.5d
The E1cB reaction of the carbanion of a fluorenyl carboxylic
ester also illustrates the role of charge asymmetry at micellar
surfaces. In this reaction negative charge is dispersed out of the
hydrophobic fluorenyl group in the ketene-like transition state.
Rate effects are small, but reaction is inhibited by cationic and
betaine micelles and accelerated by anionic and phospholipid-
derived micelles.11
We note that micellar effects on rates of spontaneous reac-
tions may be very large, as in decarboxylations,9 or small as in
these SN2 hydrolyses or the E1cB reaction.11 They are quali-
tatively understandable in terms of a simple model of the
micelle–water interface and evidence on its hydration and polar-
ity.6,7 These considerations should also apply to reactions in
other association colloids, e.g., microemulsions and vesicles.1,5c
However, changes in the properties of the interfacial region,
induced, for example, by changes in the head group struc-
ture, can induce modest rate effects depending on reaction
mechanism.
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Experimental
Materials
Preparation and purification of MeONs and the surfactants
have been described.2,10 Critical micelle concentrations (c.m.c./
mmol dmϪ3) of the zwitterionic surfactants were: SB3-14, 0.29;
SBBu3-14, 0.11; DMMAO, 0.14; DPMAO, 0.05. They were
measured by surface tension and there were no minima in
the relevant plots.1a Reactions were carried out in redistilled,
deionized water.
Kinetics
Reactions were followed at 25.0 ЊC in a Shimadzu UV-160 A or
an HP 8452 spectrophotometer by following decreasing
absorbance at 326 nm with 10Ϫ4 mol dmϪ3 MeONs, as
described.2 The slower reactions could not be followed to 10
half-lives and kobs was then calculated by a nonlinear, least-
squares fitting of the variation of absorbance with time to a
first-order rate equation. For the faster reactions values of kobs
from this method and those based on an infinity absorbance
agreed. Values of kobs over a range of [surfactant] are in Tables
1–4 and supplementary material. Addition of MeSO3H to solu-
tions of the sulfobetaines had little effect on kobs showing that
there is no reaction with OHϪ.
22 C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd
edn., Cornell University Press, Ithaca, New York, 1969, p. 457 ff.
23 Y. Chevalier and P. Le Perchec, J. Phys. Chem., 1990, 94, 1774.
24 (a) S. J. Bachofer and U. Simonis, Langmuir, 1996, 12, 1744; (b) P. J.
Kreke, L. J. Magid and J. C. Gee, Langmuir, 1996, 12, 699.
25 (a) P. Fromherz, Ber. Bunsenges. Phys. Chem., 1981, 85, 891; (b) J. A.
Butcher and G. W. Lamb, J. Am. Chem. Soc., 1984, 106, 1217;
(c) B. Jonsson, P.-G. Nilsson, B. Lindman, L. Guldbrand and
H. Wennerstrom, in Surfactants in Solution, ed. K. L. Mittal and
B. Lindman, Plenum Press, New York, 1984, vol. 1, p. 3.
26 Y. Chevalier, N. Kamenka, M. Chorro and R. Zana, Langmuir,
1996, 12, 3225.
Reactions in solutions of amine oxides were followed in
excess MeSO3H where protonation is quantitative.15 The elec-
trolyte concentration was varied, in the absence of surfactants,
by addition of MeSO3H, MeSO3Na and NaClO4; the hydrolysis
rate in water is almost unaffected by added salt. In 0.05–0.1 mol
dmϪ3 MeSO3H kobs = 1.33 0.01 × 10Ϫ5 sϪ1; in 0.1–1 mol dmϪ3
MeSO3Na kobs = 1.25 0.07 × 10Ϫ5 sϪ1; in NaClO4 0.1–1 mol
Ϫ3
dm
k
= 1.14 0.40 × 10Ϫ5 sϪ1.
obs
27 (a) M. da Silva Baptista, I. Cuccovia, H. Chaimovich, M. J. Politi
and W. F. Reed, J. Phys. Chem., 1992, 96, 6442; (b) C. A. Bunton,
M. M. Mhala and J. R. Moffatt, J. Phys. Chem., 1989, 93, 854.
Acknowledgements
Support of this work by Consiglio Nazionale delle Ricerche,
Rome, the Ministero dell’Università e Ricerca Scientifica e
Tecnologica, Rome and the U.S. Army Office of Research is
gratefully acknowledged.
Paper 7/05316I
Received 23rd July 1997
Accepted 25th September 1997
364
J. Chem. Soc., Perkin Trans. 2, 1998