Abstract
The 13C chemical shifts of carbonyl carbons are greatly influenced by neighbouring functional groups and the resonance signals appear at the low-field range in all sorts of organic compounds. Previously, the authors reported the graphic representation of the chemical shifts of carbonyl compounds in the 2nd paper in this series. More detailed charts of carbonyl carbons in esters, carboxylic acids and amides are presented here.
The chemical shifts (from TMS) for carbonyl carbons in esters, carboxylic acids and amides lie in the ranges, 163 to 179 ppm, 161 to 186 ppm and 151 to 180 ppm respectively, but the values for some triazol derivatives are found at higher field, 140 ppm region (Figs. 3 to 5).
In methyl esters of benzoic acids, the chemical shifts of carbonyl carbons for compounds with the electron-withdrawing substituents resonate at the higher field with respect to the shift of benzoic acid, while the electron-donating substituents result in the lower field shift (Fig. 3). The same tendency in chemical shift is observed for the α-substituted compounds in uracils (Fig. 5).
In α, β-unsaturated carboxylic acids, in which halogen atoms are bonded to the α-carbon, the resonances for the carbonyl carbons are gradually shifted to higher field in the order of I, Br and Cl (Fig. 4), while the order is reversed for acid halides in which the halogen atoms are bonded directly to the carbonyl carbon.
The carbonyl carbons of cis-isomers in esters and carboxylic acids resonate at higher field than that of trans-isomer by (1.5, 5) ppm, and it is considered to be due to the effect of steric compression between the oxygen atom of the carbonyl group and the substituents.
