SN2 displacement by bromide ions in dichloromethane. The role of reverse micelles

Reverse micellar systems are of interest as reaction media because they are powerful models for biological compartmentalization, enzymatic catalysis and separation of biomolecules. Solutions of ionic surfactants in apolar solvents may contain reverse micelles, but they may also contain ion pairs, or small clusters, with waters of hydration. We studied the bimolecular reaction in CH2Cl2 solutions of cationic tetraalkylammonium bromide salts (onium salts), such as cetyltrimethylammonium bromide (CTABr), cetyltripropylammonium bromide (CTPABr) and tetra-n-butylammonium bromide (TBABr). Methylnaphthalene-2-sulfonate (β-MeONs), its 6-sulfonate derivative (β-MeONsS–) as the 2,6-lutidinium salt and methyl-5-N,N,N,trimethylammonium naphthalene-1-sulfonate (α-MeONsNT+) as the trifluoromethanesulfonate salt react with Br– in CH2Cl2. First-order rate constants, kobs, increase linearly and similarly for the three substrates with increasing concentrations of the onium salts. Reactions are faster with TBABr than they are with CTPABr and CTABr, and the reactivity of the three substrates is in the order: α- MeONsNT+ >> β-MeONsS– > β-MeONs. The reactions are inhibited by the addition of H2O, but CTABr tolerates H2O in large excess. At [H2O]/[CTABr] = w0 = 6, “water-pool” reverse micelles form, and kobs for all three substrates is then independent of w0.