Infrared active bond carboxylic acids

Dissociated fatty acids (soaps) have quite different features. Delocalisation of the C=0 double bond across the CO2 group depresses the frequency of the stretching vibration. Stretching of the two C-O bonds is coupled (i.e. the vibration of one influences the vibration of the other). Both asymmetric and symmetric vibrations occur and are infrared active. Bands appear between 1600 and 1560 cm (asymmetric), and 1420 and 1340 cm (symmetric), with short-chain carboxylates absorbing at the higher end of the range. 

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