Question 21.60: A compound with molecular formula C₄H₅N has a strong absorpt......
A compound with molecular formula C₄H₅N has a strong absorption at 2250 cm^{−1}. Suggest two possible structures and explain how they could be distinguished by other infrared absorptions.
The absorption of compound II occurs at lower wavenumber than compound I because conjugation of the carbon–carbon double bond with the carbonyl group increases the single bond character of the carbonyl group. The absorption of compound III occurs at higher wavenumber than compound I because the electron withdrawal of the carbon–carbon double bond decreases the electron density of the oxygen atom, and hence the availability of electrons to stabilize the dipolar resonance form of the carbonyl group. Thus, the order for compounds I, II, and III is 1775, 1741, and 1806 cm^{−1}, respectively.