Micellar Catalysis of Organic Reactions. XXXI. A Study of the Effects of Micelles of Cetyltrimethylammonium Bromide on Some S_NAr Reactions in Aqueous Binary Mixtures

Abstract

The effect of micelles of cetyltrimethylammonium bromide ( ctab ) on the S_NAr reactions of 1-fluoro-2,4-dinitrobenzene (1), 2-fluoro-5-nitrobenzoate (2) and 4-fluoro-3-nitrobenzoate (3) with sodium hydroxide in aqueous binary mixtures with several alcohols has been studied. Two products were detected in all of these reactions: the phenol from reaction with hydroxide ions, and an aryl alkyl ether from reaction with the alkoxide ions. Micelles of ctab increased the percentage yield of the ether product at the expense of the phenol for compounds (1) and (2) in most of the binary mixtures used. For compound (3), however, micelles of ctab had little effect on the product distribution.

These differences were attributed to differences in the orientation of substrates (2) and (3) when solubilized by micelles of ctab . Very high yields of the ether were obtained for the reaction of compound (1) with hydroxide ions in trifluoroethanol /water mixtures, and this was attributed to the considerable ionization of trifluoroethanol which was the most acidic alcohol used in this work. The lowest yield of the ether product was obtained in reactions of compound (1) with hydroxide ions in propan-2-ol/water mixtures since propan-2-ol was the least acidic alcohol used.

These results are compared with those previously reported for the reaction of compound (1) in the presence of hydroxy-functionalized micelles and β- cyclodextrin.

In the presence of micelles of ctab the aryl alkyl ethers derived from compounds (1) and (2) underwent a subsequent S_NAr reaction with hydroxide ions during which the alkoxide ion was displaced. For compound (3) no subsequent reaction of the ether was detected in the presence of micelles of ctab . This was also attributed to the orientation of this product within the micelle. The reaction centre was buried in the micellar interior, and hence was shielded from a subsequent reaction.

The rates of this subsequent reaction for compound (1), and of the decomposition of micellar aryl ethers and of cyclodextrin aryl ethers derived from compound (1), are also compared.

The increase in the yield of the ether product in the presence of micelles was attributed to the increased ionization of alcohols in the presence of cationic micelles.