Acid derivatives are best distinguished by infrared

Conjugation with tc eiectrons or ione pairs affects IR C=0 stretches 

We need to see how conjugation works when it is with a n bond rather than with a lone pair. This will make the concept more general as it will apply to aldehydes and ketones as well as to acid groups. How can we detect whether an unsaturated carbonyl compound is conjugated or not Well, compare these two unsaturated aldehydes. 

Because the infrared carbonyl frequencies follow such a predictable pattern, it is possible to make a simple list of correlations using just three factors. Two are the ones we have heen discussing—conjugation (frequency-lowering) and the inductive effect (frequency-raising). The third is the effect of small rings and this we next need to consider in a broader 

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