Organic Chemistry

Section 13.10 The Position of Absorption Bands 505

2.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

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Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

C O

O

Figure 13.16
The IR spectrum of 2-cyclohexenone. Electron delocalization gives its carbonyl group less
double-bond character, so it absorbs at a lower frequency than does a
carbonyl group with localized electrons (' 1720 cm-^1 ).

(' 1680 cm-^1 )

Putting an atom other than carbon next to the carbonyl group also causes the position
of the carbonyl absorption band to shift. Whether it shifts to a lower or to a higher
frequency depends on whether the predominant effect of the atom is to donate electrons
by resonance or to withdraw electrons inductively.

The predominant effect of the nitrogen of an amide (Figure 13.17) is electron dona-
tion by resonance. Therefore, the carbonyl group of an amide has less double-bond
character than does the carbonyl group of a ketone, so it is weaker and stretches more
easily In contrast, the predominant effect of the oxygen of an ester is in-
ductive electron withdrawal, so the resonance contributor with the single bond
contributes less to the hybrid. The carbonyl group of an ester, therefore, has more dou-
ble-bond character than does the carbonyl group of a ketone, so the former is stronger
and harder to stretch 11740 cm-^12 (Figure 13.18).

C¬O

11660 cm-^12.

O O−

C

R Z

C

RZ+

resonance electron donation

O

C

R Z

inductive electron withdrawal

2.5 2.6 2.72.82.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 16

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

CH 3 CH 2 CN(CH 3 ) 2

O

C O

Figure 13.17
The IR spectrum of N,N-dimethylpropanamide. The carbonyl group of an amide has less
double-bond character than does the carbonyl group of a ketone, so the former stretches
more easily (' 1660 cm-^1 ).