Studies in nuclear magnetic resonance spectroscopy. Part VII. 13C and 1H spectra of aliphatic ketones and secondary alcohols. The negative inductive effect of methyl groups

Abstract

¹³ C and ¹H N.m.r. parameters have been obtained for the series of aliphatic ketones, acetone to 2,2,4,4-tetramethylpentan-3-one, and secondary alcohols, propan-2-ol to 2,2,4,4-tetramethylpentan-3-ol. The downfield shift of the carbinol carbon atoms with increasing substitution is explained in terms of a negative inductive effect (–l) of a methyl group. These shifts can be predicted by use of a modification of a known empirical relationship. The resonances of the carbonyl carbons of the ketones also move downfield with increasing substitution as shown previously. Values of J(CH) for the α-positions of the ketones were found to be insensitive to methyl substitution. However, the geminal HH coupling constants for the α-positions were found to be greater in ethyl ketones (J_av–17·6 Hz) than in methyl ketones (J_av–14·5 Hz), indicating that ethyl ketones exist predominantly in a conformation in which the methyl group eclipses the plane of the carbonyl double bond. Populations of conformations of the secondary alcohols have been determined from the values of the vicinal coupling constants. From these populations, values of the carbinol methine proton chemical shift have been estimated from additivity relations and shown to agree closely with the observed values.