Effects of Conjugation in a,3-Unsaturated Aldehydes and Ketones

This electron delocalization stabihzes a conjugated system. Under conditions chosen to bring about their interconversion, the equilibrium between a 3,7-unsaturated ketone and an a,p-unsaturated analog favors the conjugated isomer. 

FIGURE 18.5 Acrolein (H2C = CHCH = 0) is a planar molecule. Oxygen and each carbon are sp -hybridized, and each contributes one electron to a conjugated -rr electron system analogous to that of 1,3-butadlene. 

PROBLEM 18.14 Commercial mesityl oxide, (CH3)2C=CHCCH3, is often contaminated with about 10% of an isomer having the same carbon skeleton. What is a likely structure for this compound  

In resonance terms, electron delocalization in a,(3-unsaturated carbonyl compounds is represented by contributions from three principal resonance structures  

The carbonyl group withdraws tt electron density from the double bond, and both the carbonyl carbon and the (3 carbon are positively polarized. Their greater degree of charge separation makes the dipole moments of a,p-unsaturated carbonyl compounds significantly larger than those of comparable aldehydes and ketones. 

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