RATE STUDIES OF THE REACTION BETWEEN ALCOHOLS AND BROMINE IN THE PRESENCE OF A SILVER COMPOUND

Treatment of secondary and tertiary alcohols with bromine in the presence of silver salts results in the production of substituted tetrahydrofuran derivatives through abstraction of a (delta)-hydrogen atom, and, in the case of secondary alcohols, the corresponding ketones can also be produced by abstraction of the (alpha)-hydrogen. A study of the kinetics of the reactions that occur in the absence of light during treatment of secondary and tertiary aliphatic alcohols with bromine in the presence of silver salts was conducted. Kinetically controlled experiments were conducted in the presence of silver oxide, silver carbonate and with no silver salts, with a ten-fold excess of the alcohol over the other constituents. Spectrophotometric measurements revealed that with an excess of the alcohol in the presence of either silver salt, the bromine was rapidly consumed in a first order manner to form the hypobromite from the alcohol. Rates of hypobromite decomposition were obtained by spectrophotometric measurements and by indirect iodometry. Comparison of the relative pseudo-first order rates of hypobromite decomposition determined for each alcohol in the presence of each silver salt indicated that an increase in the degree of substitution at the carbon (delta) to the hypobromite group decreased the stability of the hypobromite. This increase in decomposition rate was less pronounced when silver oxide was used than in the presence of silver carbonate. Product analysis by gas chromatography and gas chromatography-mass spectrometry indicated that the cyclic ethers, ketones and olefins were produced, with differences in the product distributions observed with each silver salt. The activity of the silver oxide in acting as a heterogeneous reactant surface was found to be much greater when prepared from silver nitrate and base just prior to use. The highly covalent nature of the silver oxide crystal accounts for the observed differences in surface activity from the other silver salts used to promote the cyclization of alcohols. The results of this study suggest that the product distributions obtained under kinetically controlled conditions may not agree with those obtained under other conditions.