Question

Although the carbonyl absorption of cyclic ketones generally shifts to higher wavenumber with decreasing ring size, C=O of cyclopropenone absorbs at lower wavenumber in its roman IR spectrum than the C=O of cyclohex-2-enone. Explain this observation by using the principles of aromaticity learned in Chapter 17.

Short answer

Lack of aromaticity leads to absorption at a lower wavenumber.

Step-by-step solution

Aromaticity

The property by which cyclic planar conjugated compounds with (4n+2) pi-electrons in resonance would have better stability than aliphatic compounds is known as ‘aromaticity.’

Absorption and Aromaticity

Molecules with longer conjugated system would absorb light of longer wavelength due to the presence of pi-bonds.

In cyclopropenone, the pi-electrons are localized and are not allowed to delocalize freely within the ring. The polarity of the compound does not dominate allowing the movement of electrons.

Therefore, cyclopropane would absorb at a lower wavenumber than cyclohex-2-enone.