Section 17.17 Spectroscopic Analysis of Aldehydes and Ketones

Infrared Spectroscopy Presence of a C=O group is readily apparent in infrared spectrum C=O stretching gives an intense absorption at 1710-1750 cm-1 In addition to peak for C=O, aldehydes give two weak peaks near 2720 and 2820 nm for H—C=O 6

Figure 17.13 Infrared Spectrum of Butanal H—C=O CH3CH2CH2CH=O C=O 2000 3500 3000 2500 1000 1500 500 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

Aldehydes: H—C=O proton is at very low field (d 9-10 ppm). 1H NMR Aldehydes: H—C=O proton is at very low field (d 9-10 ppm). Methyl ketones: CH3 singlet near d 2 ppm. 6

Figure 17.14 H C O CH(CH3)2 1 Chemical shift (d, ppm) 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (d, ppm) 1

Figure 17.15 CH3 C O CH3CH2 1 Chemical shift (d, ppm) 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Chemical shift (d, ppm) 1

13C NMR Carbonyl carbon is at extremely low field-near d 200 ppm Intensity of carbonyl carbon is usually weak

Figure 17.16 O CH3CH2CCH2CH2CH2CH3 Chemical shift (d, ppm) 1 20 40 60 20 40 60 80 100 120 140 160 180 200 Chemical shift (d, ppm) 1

UV-VIS Aldehydes and ketones have two bands in the UV region: p Æ p* and n Æ p* p Æ p*: excitation of a bonding p electron to an antibonding p * orbital p Æ p*: excitation of a nonbonding electron on oxygen to an antibonding p * orbital

UV-VIS H3C C O •• • • p Æ p* lmax 187 nm n Æ p* lmax 270 nm

Molecular ion fragments to give an acyl cation Mass Spectrometry Molecular ion fragments to give an acyl cation CH3CH2CCH2CH3 •+ O • • m/z 86 CH3CH2C O • • + + CH2CH3 • m/z 57