Ketones other characteristic frequencies

Among the characteristics of charge on C, the most adequate one seems to be the chemical shift of C, in the NMR- C spectrum (see Sect. 1II.2). However, other characteristics should not be excluded. For example, a linear relation exists between the activation parameters of the 1,2-migrant shift (R = CH3, CgHj) in arenium ions and the C=0 vibrations frequencies in the IR spectra of ketones formally obtained by substituting the fragment >C, —R for the group >C=0 3oi.569)... 

Characteristic ketonic frequencies other than the C=0 stretching absorption... 

In the consideration of carbonyl frequencies of hydroxy-, amino-or ketonic acids, allowances must also be made for the possibility that ring closure has occurred with the formation of a lactone, lactam or lactol. The characteristic carbonyl frequency of the carboxyUc acid then disappears. Penicillic acid [28] has been shown to be such a case, and it exists as a free acid only under certain limited physical conditions. Similarly, acetophenone ort/zo-carboxylic acid absorbs at 1732 cm". This corresponds to a five-membered ring lactol frequency rather than that of an aryl acid. ort/zo-Formylbenzoic acid (1738 cm" also exists as the lactol rather than as the free acid [17], and numerous other cases are known. In some cases, such as benzil-ort/io-carboxylic acid, both forms are known. The keto-form has absorptions at 1698 cm" and 1683 cm", corresponding to the aryl ketone and to the acid, whilst the lactol form absorbs at 1692 cm" and 1745 cm", corresponding to the aryl ketone and the five-membered ring lactol carbonyl absorption. 

All esters have two strong characteristic bands, one due to the C=0 stretching vibration and the other due to the C-O stretching vibration. The frequency of the C=0 stretching vibration for esters is influenced in a very similar way to that observed for ketones, except that the decrease in wavenumber for aliphatic esters due to the presence of a, -unsaturation is less, being approximately 10-20cm . ... 

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