Collect. Czech. Chem. Commun. 1979, 44, 1273-1295

The syn-anti dichotomy in bimolecular elimination of alkyltrimethylammonium salts: Steric and polar effects on rates and orientation in the competing pathways

Jiří Závada and Magdalena Pánková

Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6


The title problem has been investigated in two homologous series of positionally isomeric alkyltrimethylammonium salts, RCH2CHXC5H11 and RCHXCH2C5H11 (R = H, CH3, C2H5, n-C3H7, iso-C3H7 and tert-C4H9; X = N(+)(CH3)3Cl)(-), employing potassium tert-butoxide in tert-butanol and also potassium methoxide in methanol as the base-solvent combination. The overall rate constants of the individual olefin-isomer formation have been determined in these series and dissected, approximately, into the syn- and anti-components. The alkyl structure-reactivity relationship is thus obtained separately for the two complementary mechanisms of bimolecular elimination. The long-standing question whether steric or polar influence of alkyl group dominates in the complex reaction is re-examined on these grounds. Contrary to earlier extreme views it is shown that both the steric as well as the polar effects are very important, the former being operative mainly in the stereochemical (syn-anti) whereas the latter in the orientational (Hofmann-Saytzeff) control of the dichotomous reaction.