Steric effects on reactivity in some naphthalene derivatives

• Main Author: Wong, E.
• Language: en
• Published: University of Canterbury. Chemistry 2013
• Online Access: http://hdl.handle.net/10092/8396

In order that a better understanding of the relative reactivity of α- and β-naphthalene derivatives be attained, two approaches have been made to evaluate the magnitude of steric effects associated with some α-naphthalene derivatives. The rates of reaction in nitrobenzene of methyl iodide with pyridine, quinoline, isoquinoline, 2-methylquinoline, and 8-methylquinoline, have been studied over the temperature range 10 - 50°. In following the kinetics of these slow reactions, a convenient and precise conductometric method has been used. Relative heats of activation were taken as quantitative measures of steric strains present in the activated complexes in these reactions. Using evidence advanced by H.C. Brown and his co-workers in their work on strained homomorphs, kinetics results obtained from the quinoline series have been used to evaluate steric strains present in the structures: 1-methylnaphthalene, 1 , 2-dimetlwlnaphthalene, and 1,8-dimethylnaphthalene.. Steric strains associated with these structures were found to be larger than those of comparable alkyl-benzene structures. This has been attributed to the rigidity of the fused ring system as compared with an alkyl group. The rates of acid-catalysed esterification of benzoic, α-naphthoic, and β-naphthoic acids in absolute methanol, and the rates of alkaline hydrolysis in 56% (w) acetone-water mixture of the methyl and ethyl esters of these acids have also been studied over the temperature range 15 - 60°. Rates were followed by acid-base titrations. For the α-derivatives the methods of R.W. Taft have been adopted to separate quantitatively the effect of the fused ring into polar and nonpolar factors. A σ*value for the fused ring in the a-naphthyl system was calculated. The apparent absence of a steric strain effect in these reactions of the α-naphthyl derivatives has been explained through considerations of steric inhibition of resonance. .An estimate of the magnitude of the steric strain in the transition states of these reactions has been made. This value is of the same order as the steric strains found for the quinoline bases in their reaction with methyl iodide. Results for the (3-derivatives were analysed by means of the Hammett equation and α σ value for the fused ring in the β-naphthyl system has been calculated. From a comparison of the σ and σ * values the conclusion was reached that the polar effects of the fused ring system on the reactivity of α- and β-naphthalene derivatives are very similar.