Carboxylic acid derivatives carbonyl stretching frequencies

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm-1 (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. The C=0 bond of the amide carbonyl is somewhat less than a full double bond. Because it is not as strong as the C=0 bond in a simple ketone or carboxylic acid, the amide C=0 has a lower stretching frequency. 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. 

Recall that a carbonyl group produces a very strong signal between 1650 and 1850 cm in an IR spectrum. The precise location of the signal depends on the nature of the carbonyl group. Table 21.3 gives carbonyl stretching frequencies for each of the carboxylic acid derivatives. 

The acid chloride carbonyl is the strongest of the acid derivatives, with the most double-bond character, so its C O stretch appears at the highest frequency. Anhydrides, esters, and carboxylic acids are next. Amides, in which the contribution from the dipolar resonance form is strongest, and the sing/e-bond character of the carbonyl group is greatest, have the lowest carbonyl stretching frequencies (Table 18.2). 

Carbonyl Stretching Frequencies of Carboxylic Acid Derivatives RC—L... 

Infrared spectroscopy can also be used to probe resonance in carboxylic acid derivatives. The dipolar resonance structure weakens the C=0 bond and causes a corresponding decrease in the carbonyl stretching frequency (Table 20-2). The IR data for carboxylic acids reported in Section 19-3 refer to the common dimeric form, in which hydrogen bonding reduces the stretching frequencies of both the 0-H and C=0 bonds to about 3000 and 1700 cm respectively. A special technique—vapor deposition at very low temperature—allows the IR spectra of carboxylic acid monomers to be measured, for direct comparison with the spectra of carboxylic acid derivatives. Monomeric acetic acid displays vc=o at 1780 cm similar to the value for carboxylic anhydrides, higher than that for esters, and lower than that of halides, consistent with the degree of resonance delocalization in carboxylic acids. 

Carbonyl stretching frequencies in the IR spectra are diagnostic of the carboxylic acid derivatives Acyl chlorides absorb at 1790-1815 cm , anhydrides at 1740-1790 and 1800-1850 cm , esters at 1735-1750 cm and amides at 1650-1690 cm . ... 

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