Micellar inhibition of SN1 reactions of sterically hindered compounds

Abstract

Aqueous cationic micelles of cetyltrimethylammonium surfactants (CTAX; X = Br, Cl, or 0.5SO₄) and dodecyltrimethylammonium bromide, and anionic micelles of sodium lauryl sulphate (NaLS) inhibit S_N1 hydrolyses of sterically hindered arenesulphonates and chlorides. The substrates were 2-adamantylp-bromobenzenesulphonate (brosylate) and p-nitrobenzenesulphonate,1,2,2-trimethylpropyl (pinacolyl) tosylate, brosylate, benzenesulphonate, and p-methoxy- and p-nitrobenzenesulphonate, and 2,2-dimethyl-1-phenylpropyl chloride and tosylate. Micellar inhibition increases with increasing substrate hydrophobicity but is always larger with cationic than with anionic micelles by factors of between 2 and 5 for the arenesulphonate, and 10 for 2,2-dimethyl-1-phenylpropyl chloride. The difference is much larger for hydrolysis of diphenylmethyl chloride and bromide. For hydrolysis of 1-benzylbutyl tosylate, with nucleophilic participation by water, micellar inhibition is smaller than with the hindered substrates but greater with cationic than with anionic micelles. These micellar inhibitions are compared with those on deacylations which are dominated by bond-making and where reaction is faster in cationic than in anionic micelles. Micellar medium effects are related to the mechanisms of these spontaneous hydrolyses and to substrate structure. Substituent effects were examined for S_N reactions in water; ρ≈ 1.5 for hydrolyses of pinacolyl arenesulphonates, and is similar for hydrolyses in micelles.