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

Abstract

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.