Question

the IR Spectra of butyric acid and ethyl butyrate show sharp strong singlet absorption at 1725 cm^-1 and 1740 cm^-1 respectivity by constrast, the IR spectrum of butyric anhydride shows a broad, sharp doublet at 1750 cm^-1 and 1825 cm^-1why these so different

Answer 1

The basic is lower is the double bond character, lower will be the energy required for streching. So in case of butyric acid, the oxygen lone pair of -OH of acid reduces the double bond character of C=O bond by resonance effect. So it would require less energy and thus appears at 1725 cm-1 in the IR. In ester, the effect of electronegative O gets prominent, wherein, the oxygen pulls electron off the O-C bond and thereby pulling electrons from O of the C=O bond. this makes the C=O more stable and thus appears at higher frequency in the IR spectrum. In case of butyric anhydride we can think of it as an ester in combination with an acyl group, so the ester part C=O appears at 1750 cm-1 (higher than butyric acid since it is connected to an acyl group on one side) and acyl group C=O appears the highest. the cycl group is not connected to a electronegative oxygen with no resonance effect due to the lone pair of O. Thus it appears at the highest frequency in the IR spectrum.