2- Cyclohexenone carbonyl absorption

Cyclohexenone is a cyclic a,B-unsaturated ketone whose carbonyl IR absorption occurs at 1685 cm-1. If direct addition product A is formed, the carbonyl absorption will vanish and a hydroxyl absorption will appear at 3300 cm"1. If conjugate addition produces B, the carbonyl absorption will shift to 1715 cm-1, where 6-membered-ring saturated ketones absorb. 

Explain why a carbonyl absorption shifts to lower frequency in an a,p-unsaturated carbonyl compound—a compound having a carbonyl group bonded directly to a carbon-carbon double bond. For example, the carbonyl absorption occurs at 1720 cm" for cyclohexanone, and at 1685 cm" for 2-cyclohexenone. 

Although the carbonyl absorption of cyclic ketones generally shifts to higher wavenumber with decreasing ring size, the C = 0 of cyclopropenone absorbs at lower wavenumber in its IR spectrum than the C = 0 of cyclohexenone. Explain this observation by using the principles of aromaticity learned in Chapter 17. 

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