Question

why the carbonyl stretch in crotonalehyde is at a significantly lower energy than in the carbonyl in butanal?

Answer 1

When single bond character of C=O (carbonyl double bond) is increased, the stretching frequency decreases as energy decreases. When carbonyl is in conjugation with C=C or any other conjugation, carbonyl single bond character increases due to the resonance. -bond electrons are moved to C=O and it becomes C-O^- hence it's IR frequency decreases. Thus in crotonaldehyde carbonyl is in conjugation with C=C, in comparison butanal does not have such conjugation.

So crotonaldehyde C=O stretching frequency is lower then that of butanal.