CHLORINATION .I. PHYSICAL EVIDENCE FOR POLAR EFFECTS IN PRODUCTS OF CHLORINATION OF CYCLOHEXANECARBOXYLIC ACID

As a prelude to the detailed study of the directive influences of the carboxyl group upon both radical and enolic chlorination reactions, the seven monochlorocyclohexanecarboxylic acids (including the previously unreported cis-3 and cis-4 isomers) and their methyl esters have been synthesized and characterized. Nmr studies of the chloro esters at low temperatures yielded correlations between the chemical shifts of individual protons in cyclohexanecarboxylic acid and in chlorocyclohexane vs. predicted reactivity toward attack by an electrophilic radical such as Cl. Comparison of the >CHCl, >CHCO2-, and -CO2CH3 chemical shifts at low temperature showed that the resonances were dependent (a) upon the conformation (axial or equatorial) of the protons, (b) the distance to the remote polar substituent (Cl or CO2CH3), and (c) the conformation of the polar substituent. A measuee of the additivity of conformational energies (ΔG°) of two polar substituents on a cyclohexane ring was obtained by comparing the observed conformational distribution with values calculated from observed ratios in methyl cyclohexanecarboxylate(ΔG°-87° = 1.30 ± 0.14 kcal/mole) and chlorocyclohexane(ΔG°-87° = –0.67 ± 0.12kcal/mole) in perdeuterioacetone. In general, fair agreement was found for the 1,4- and 1,3-disubstituted systems, but not with the 1,2- or 1,1-systems. Methyl cis-2-chlorocyclohexanecarboxylate showed an nmr spectrum which was essentially invariant (equatorial carbomethoxy, axial chloro) over the entire temperature range studied (–87 to +26°). © 1969, American Chemical Society. All rights reserved.