HYDROLYTIC CLEAVAGE OF DIESTER
The space between two head groups allows the a-nucleophiles to
solubilize itself smoothly at the interfacial region. Because of
the enhanced interfacial region, the concentration of nucleo-
phile and substrate increased and showed higher activity. There
is little information about the effect of the variation of head
groups on the micellar structure.[50–52] Table 4 clearly indicates
that the nucleophilic activity of a-nucleophiles strongly depends
on the structure and charge of the surfactant head group. In
comparison, we found higher kobs value with the pyridinium
group (CPB), approximately 10.4-fold, than the quaternary am-
monium group (CTAB), approximately 5-fold acceleration. The
results indicate that specific interactions between the cationic
surfactant and the nucleophile should be effective in determin-
ing the a-nucleophiles as the hydrophobicity together with the
size of the head groups is increased. The variation of kobs values
of the reactions depends on the micellar structure, that is, head
group, hydrophobic tail length, and counter ion. The rate
constants in a variety of association colloids are slightly higher
than in water and increase modestly with increasing micellar
media involving variations in the surfactant tail groups and
changes in the interfacial regions.[53] Analysis of kinetic data indi-
cates that CDEEAB shows higher reactivity than CTAB. The increase
of kobs values with increasing alkyl chain lengths (R = 12, 14, 16) of
the surfactants, that is, with increasing aggregation number of mi-
celle, is mainly due to the increase in the electrical surface potential
of the micelle and partially due to an increase in hydrophobicity of
the palisade layer of the micelle. In all cases, the [CDEEAB] rate
profile was maximum as compared with [TDEEAB] and [DDEEAB]
surfactants because [CDEEAB] corresponds to complete solubiliza-
tion of the substrate in the micellar pseudophase and because
alkanol groups can exert a protective effect on the positive
charge, thus lowering the attractive electrostatic interactions be-
tween the quaternary ammonium center of the micelle and the
corresponding negative charge of the dissociated acid group in
Table 6. The reactivity order follows the trend CDEEAB> TDEEAB>
DDEEAB (Fig. 6 and S3).
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Conclusion
In this study, we found that the nucleophilic activities of
a-nucleophiles are higher with the cationic micellar media toward
the BNPP cleavage. Experimental data show that the oximate ion
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hydroxamate ions (AHAꢁ, BHAꢁ, and SHAꢁ) in cationic micellar
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Acknowledgements
The authors are thankful to the University Grants Commission,
New Delhi, India, for providing financial support through Rajiv
Gandhi National Fellowship [F. 14–2 (SC)/2009 (SA–III)] to one
of the authors (B. K.) and to the Head of the School of Studies
in Chemistry, Pt. Ravishankar Shukla University, Raipur, India,
for providing laboratory facilities. This work was supported by
project grant OVUOFVZ200803 (K. K.).
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J. Phys. Org. Chem. 2012, 9999
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