What effects does conjugation (alternating single and double or alternating single and triple bonds) have on the location of C=O absorptions? Explain, using...

Question

What effects does conjugation (alternating single and double or alternating single and triple bonds) have on the location of C=O absorptions? Explain, using specific examples. (First find all examples of the C=O functional groups and then determine which absorption(s) they share. Then compare those absorptions within a functional group between the conjugated and unconjugated examples to determine the effect of conjugation. Each C=O containing functional group has at least two conjugated examples and at least two unconjugated examples.)

Answer 1

Conjugated carbonyl compounds tend to absorb at lower IR wavenumbers, or lower frequency/energy, than the non-conjugated analogs. The reason for this are the contributions of the charged resonance forms that reduce the bond order of the carbonyl. The greater these contributions, the lower the IR absorption. This is why the carbonyl stretch for para-amino benzaldehyde is considerably shifted to the right of the carbonyl for para-methyl benzaldehyde.