Thursday, 13 December 2012

Amide infrared spectra

Amides show a very strong C=O peak at 1680-1630.

Primary amines give two N-H stretch peaks, one near 3350 and one near 3180, from asymmetric and symmetric vibrations respectively.

Secondary amines give one N-H stretch peak at 3300

N-H bending at 1640-1550 for both secondary and primary amides.

N-methyl acetamide

Note how the C=O partially overlaps the N-H bend.

For dilute solutions the C=O is found at 1690. This is for the same reason as carboxylic acids - hydrogen bonding normally lowers the C=O frequency.

Like other carbonyl compounds, the C=O stretch frequency increases with decreasing ring size, by about 40 per carbon removed.

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