MICELLAR CATALYSIS OF ORGANIC-REACTIONS .34. NUCLEOPHILIC AROMATIC-SUBSTITUTION REACTIONS IN MICELLES WITH BULKY HEAD GROUPS

Observed second-order mte constants for the hydroxydechlorination of 1 -chloro-2,4-dinitrobenzene (2), 2-chloro-3,5-dinitrobenzoate ions (3) and 4-chloro-3,5-dinitrobenzoate ions (4) in micelles of cetyltrialkyl ammonium bromide (C16H33NR3Br, where R = Me, Et, n-Pr and n-Bu) are re ported. For substrate 2, the observed catalysis increased as the size of the micellar head group was increased. This was shown to be primari ly due to an increase in the rate of reaction in the micellar pseudo-p hase (k2M). For substrates 3 and 4, the observed catalysis decreased a s the size of the micellar head group was increased. Much smaller chan ges in k2M were observed in these reactions, which lead to a dianionic intermediate which is more sensitive to polarity effects at the micel le surface than is the monoanionic intermediate formed from 2. These r esults support the contention that as the size of the micellar head gr oup is increased, water is squeezed away from the micelle surface, res ulting in a less polar reaction environment.