would be likely to be small. The apparent kinetic orders with
respect to water for the uncatalysed hydrolysis of the C12 SXS
in solutions of concentration 1–20% are much higher, typic-
ally around 30 (see Fig. 3), and bear no relation to expect-
afforded SMS as colourless crystals in 70% yield. Analysis:
found C 49.98, H 8.74%; C H SO Na requires C 49.77, H
8.77%.
12
25
4
ation on the basis of the simple S 2 mechanism proposed
N
Sodium cycloundecylmethyl sulfate (SCS). The commercial
alcohol was purified by distillation under reduced pressure and
sulfated as described for the preparation of SMS. After
recrystallisation the product SCS (51% yield) was in the form
of colourless crystals. Analysis: found C 49.70, H 7.80%;
C H SO Na requires C 50.33, H 8.10%.
earlier. A possible explanation for the steep dependence may
be that, as [SXS] increases, the reactant, whether the sulfate
head-group of SXS or the benzoyl group of BPT, occupies an
environment which becomes increasingly hydrophobic,
thereby experiencing a lower reactant water activity than
12
23
4
indicated by [H O], the macroscopic water concentration cal-
2
culated from the composition of the whole reaction mixture.
In addition, the hydrophobic microenvironment will exert a
substantial medium effect by virtue of its reduced charge
solvating capability; indeed, in the case of the putative SN2
displacement of sulfate ion by a molecule of water in the
uncatalysed SXS hydrolysis, not only will there be the devel-
opment of two additional charges in the transition state, but
there will also be an additional, developing electrostatic
repulsion between the departing sulfate ion and the negatively
charged Stern layer of the micellar aggregates.
Sodium 2-pentylheptyl sulfate (SPS). Iodopentane (2.2 molar
equivalents) was added to a cooled ethanol solution in which
diethyl malonate had been converted into its sodium salt using
a 7-fold excess of sodium ethoxide; the mixture was refluxed for
1
6 hours. Excess aqueous sodium hydroxide was then added
and refluxing continued for a further 4 hours. After cooling
to room temperature the mixture was carefully acidified with
concentrated hydrochloric acid, and most of the ethanol was
removed under reduced pressure until crude 2,2-dipentyl-
malonic acid precipitated as a yellow solid. Decarboxylation by
heating of the solid at 170 ЊC for 24 hours afforded crude
2-pentylheptanoic acid which was distilled; the fraction of the
distillate collected at 170 ЊC/7 mmHg was a colourless liquid.
Analysis: found C 71.74, H 11.97%; C H O requires C 71.94,
Conclusions
The pattern of kinetic behaviour observed in the hydrolysis of
aqueous sodium C -alkyl sulfates at concentrations up to 70%
12
24
2
12
H 12.08%. Reduction of the acid was achieved using lithium
aluminium hydride in diethyl ether first at room temperature
and then at reflux overnight. An aqueous work-up, followed by
ether extraction and distillation under reduced pressure yielded
the pure 2-pentylheptanol as a colourless liquid, bp 170 ЊC/1
mmHg. Analysis: found C 77.20, H 14.17%; C H O requires C
is broadly the same as previously described for SDS. The same
kinetic form—autocatalysis by protons generated as hydrogen
sulfate is produced—is observed, but there are substantial
variations in the reactivity as the alkyl structure changes;
β-branching reduces the reactivity, an effect that is particularly
pronounced in SMS. Notwithstanding the similarity of the
patterns of reactivity for the different SXS hydrolysing by
the uncatalysed and hydrogen-ion catalysed pathways, it is
12
26
7
7.34, H 14.07%. Sulfation of the alcohol to produce SPS was
achieved in 50% yield as described above for SMS. Analysis:
found C 49.70, H 8.70; C H SO Na requires C 49.77, H
12
25
4
argued that the results are consistent with an S 2 mechanism
N
8
.77%.
for uncatalysed hydrolysis and the concerted SO -cleavage
3
(
-transfer)/proton transfer mechanism for the catalytic route.
Kinetic measurements
Changes in the microenvironment of the sulfate group in aggre-
gates formed from the different SXS are seen as being respon-
sible for much of the rate variation. Using the formal water
concentration in SXS solutions at concentrations just above the
c.m.c. leads to values of ∆(log k)/∆(log [H O]) that are much
larger than reaction orders in H O expected on the basis of the
proposed mechanisms. Neutral BPT hydrolysis appears not to
be a useful guide to water activity in SXS solutions; again the
apparent orders of reaction are generally much larger than the
value of 2, expected on the basis of the well-established reaction
mechanism, as a result of the superimposed medium effect,
particularly of the sulfate head-groups in the Stern layer of
the surfactant aggregates, which changes substantially from
one SXS to another. The effects of surfactant aggregation on
the microenvironment in which chemical reactions take place
appear to be the dominant kinetic influence.
The procedures for following the hydrolysis of SMS, SPS and
SCS and the computer modelling method for extracting the rate
coefficients for the uncatalysed and catalysed reactions were as
1
2
previously described for SDS.
2
The hydrolysis of BPT was initiated by injection of a small
volume of an acetonitrile solution of the triazole into the SXS
solution whose pH had been adjusted to 4 by addition of a
small volume of dilute aqueous hydrogen chloride. The pro-
gress of the reaction was followed by the decreasing absorbance
(
A) of BPT at 273 nm. Plots of ln (A) vs. time were accurately
linear over five half-lives, and first order rate constants (k) were
derived by linear regression analysis. Reactions were conducted
in duplicate or triplicate and, for SDS solutions, reproducibility
of rate coefficients was good (within 2% of the mean); for the
other SXS, the reproducibility was unaccountably somewhat
poorer, but usually rate coefficients were within 5% of the mean
value.
Experimental
Materials
References
Water was repeatedly distilled in an all glass apparatus and was
free of CO contamination. BPT was a pure sample available
from earlier studies in the Groningen laboratory.
1
D. Bethell, R. E. Fessey, E. Namwindwa and D. W. Roberts,
J. Chem. Soc., Perkin Trans. 2, 2001, preceding paper (DOI:
2
1
0.1039/b102957f).
(a) W. Karzijn and J. B. F. N. Engberts, Tetrahedron Lett., 1978,
787; (b) W. Karzijn and J. B. F. N. Engberts, Recl. Trav. Chim.
2
Sodium 2-methylundecyl sulfate (SMS). SMS was prepared
by dropwise treatment of 2-methylundecanol (obtained by
reduction of commercial 2-methylundecanal with ethanolic
sodium borohydride) with a dichloromethane solution of
chlorosulfonic acid at 0 ЊC, followed by careful neutralisation
using ethanolic sodium hydroxide. Removal of the solvents
in vacuo, followed by washing of the solid salt with a little
dichloromethane and three recrystallisations from propan-2-ol
1
Pays-Bas, 1983, 102, 513; (c) J. R. Haak, J. B. F. N. Engberts and M.
J. Blandamer, J. Am. Chem. Soc., 1985, 107, 6031.
3
4
5
W. H. Noordman, W. Blokzijl, J. B. F. N. Engberts and M. J.
Blandamer, J. Org. Chem., 1993, 58, 7111.
N. J. Buurma, A. M. Herranz and J. B. F. N. Engberts, J. Chem. Soc.,
Perkin Trans. 2, 1999, 113.
D. W. Roberts, Comun. Jorn. Com. Esp. Deterg., 1992, 23, 81.
6 B. D. Flockhart, J. Colloid Sci., 1961, 16, 484.
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1501