Small rings introduce strain inside the ring and higher s character outside it

Conjugation with k electrons or lone pairs affects IR C=0 stretches 

We need to see how conjugation works when it is with a % 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 if an unsaturated carbonyl compound is conjugated or not Well, compare these two unsaturated aldehydes. 

The two peaks for anhydrides are the symmetrical and anti symmetrica I stretches for the two C=0 groups see Chapter 3, p.71, 

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 been 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 context. 

Small rings introduce strain inside the ring and higher s character outside it 

Cyclic ketones can achieve the perfect 120° angle at the carbonyl group only if the ring is at least six-membered. The smaller rings are strained because the orbitals have to overlap at a less than ideal angle. 

For a four-membered ring, the actual angle is 90°, so there is 120°-90° = 30° of strain at the carbonyl group. The effects of this strain on five-, four-, and three-membered rings are shown here. 

The best way is to relate all our carbonyl frequencies to those for saturated ketones (1715 cm ). We can summarize what we have just learned in a table. 

Notice in this simple table (for full details you should refer as usual to a specialist book) that the adjustment 30 cm appears quite a lot (-30 cm for both alkene and aryl, for example), that the increment for small rings is 35 cm each time (30 to 65 cm and then 65 to 100 cm ), and that the extreme effects of Cl and NH2 are +85 and -85 cm , respectively. These effects are additive. If you want to estimate the C=0 frequency of a proposed structure, just add or subtract all the adjustments to 1715 cm and you will get a reasonable result. 

---