The carbonyl group frequency. Part II. Aliphatic carboxylic esters

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Abstract
The carbonyl group frequencies of the series of methyl-substituted methyl acetates have been measured. As methyl substitution into the acyl or alkoxy-group increases, the frequencies, for vapours and solutions in non-polar solvents, decrease smoothly. These frequencies are well correlated by the sum of the Taft σ*-constants of the two alkyl groups, and this correlation can be extended to the frequencies of carboxylic acids. The less regular behaviour observed in polar solvents is ascribed to the importance of steric effects for t-butyl esters. The low solvent sensitivity of the carbonyl frequencies of t-butyl pivalate and isobutyrate is attributed to steric hindrance of specific solvation; conversely, the greater sensitivity of t-butyl acetate to polar solvents can arise from preferential solvation of the ester in the s-trans-conformation. The ability of esters to form hydrogen bonds with pyrrole, as measured by the bonded N–H stretching frequencies, is proportional to their carbonyl frequencies and is largely governed by the polar effects of the alkyl groups; for hydrogen bonding with phenol, steric effects become important for esters other than acetates.

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