Micellar Effect upon Spontaneous Hydrolyses and Their Relation to Mechanism

Article abstract of DOI:10.1021/ja00388a030

Cationic micelles of cetyltrimethylammonium surfactants (CTAX; X = Cl, Br, OMes, 0.5SO4) and anionic micelles of sodium lauryl sulfate (NaLS) typically inhibit spontaneous hydrolyses.The substrates were (4-YC6H4CO)2O (Y = t-Bu, H, NO2, CN), (4-YC6H4O)2CO and 4-YC6H4OCOCl (Y = H, NO2), PhCOCl, PhCH2Br, PhSO3Me, and o-C6H4(CO)2O.Hydrolysis of 4-O2NC6H4OCOCl is slightly speeded by CTABr, but the other hydrolyses are inhibited by both anionic and cationic micelles.Micellar inhibition of anhydride hydrolyses with increasing hydrophobicity of the substrate, wich will cause it to reside more deeply in the micelle, away from the water-micelle interface.There is also a mechanistic effect, because hydrolyses of acyclic anhydrides, carbonates, and chloroformates are faster in cationic than in anionic micelles, which is consistent with the transition state having anionic character.The S_N2 hydrolysis of PhSO3Me is also faster in CTAOMes than in NaLS.Hydrolyses of PhCH2Br, PhCHMeCl, Ph2CHCl, Ph2CHBr, and PhCOCl are faster in NaLS than in cationic micelles, and the effect of micellar charge is largest for reactions of Ph2CHCl and Ph2CHBr, where the transition state has the greatest carbocationic character.Halide micellar counterions do not react with the acyl or alkyl halides, but they attack PhSO3Me.Carbocations generated by heterolysis of Ph2CHCl or Ph2CHBr are not trapped by halide counterions.Relative rates in cationic and anionic micelles are intensitive to substrate hydrophobicity but apper to be indicative of reaction mechanism.