Section 20.21 Spectroscopic Analysis of Carboxylic Acid Derivatives

Infrared Spectroscopy C=O stretching frequency depends on whether the compound is an acyl chloride, anhydride, ester, or amide. C=O stretching frequency n CH3CCl O CH3COCCH3 O CH3COCH3 O CH3CNH2 O 1822 cm-1 1748 and 1815 cm-1 1736 cm-1 1694 cm-1 6

Infrared Spectroscopy Anhydrides have two peaks due to C=O stretching. One results from symmetrical stretching of the C=O unit, the other from an antisymmetrical stretch. C=O stretching frequency n CH3COCCH3 O 1748 and 1815 cm-1 6

Infrared Spectroscopy Nitriles are readily identified by absorption due to carbon-nitrogen triple bond stretching in the 2210- 2260 cm-1 region. 6

1H NMR readily distinguishes between isomeric esters of the type: RCOR' O R'COR O and O C H O C H is less shielded than 6

1H NMR For example: CH3COCH2CH3 O O and CH3CH2COCH3 Both have a triplet-quartet pattern for an ethyl group and a methyl singlet. They can be identified, however, on the basis of chemical shifts. 6

O O Figure 20.9 CH3CH2COCH3 CH3COCH2CH3 1 Chemical shift (d, ppm) 1.0 1.0 2.0 3.0 4.0 5.0 1.0 2.0 3.0 4.0 5.0 Chemical shift (d, ppm) 1

13C NMR Carbonyl carbon is at low field (d 160-180 ppm), but not as deshielded as the carbonyl carbon of an aldehyde or ketone (d 190-215 ppm). The carbon of a CN group appears near d 120 ppm.

UV-VIS nÆp* absorption: lmax CH3CCl O CH3COCCH3 O CH3COCH3 O CH3CNH2 O 235 nm 225 nm 207 nm 214 nm 6

Mass Spectrometry Most carboxylic acid derivatives give a prominent peak for an acylium ion derived by the fragmentation shown. RCX •• O • • RCX •+ O • • RC O + • • + • • • X

Mass Spectrometry Amides, however, cleave in the direction that gives a nitrogen-stabilized cation. RCNR'2 •• O • • •+ RCNR'2 O • • •• + O C • • NR'2 •• • R +