Conjugation with π electrons or lone pairs affects IR C=O stretches
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص411-412
2025-05-31
559
We need to see how conjugation works when it is with a π 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.
The key differences are the frequency of the C=O stretch (lowered by 40 cm−1 by conjugation) and the strength (that is, the intensity) of the C=C stretch (increased by conjugation) in the IR. In the 13C NMR, C3 in the conjugated enal is moved out of the alkene region just into the carbonyl region, showing how electron-deficient this carbon atom must be. In the proton NMR there are many effects but the downfi eld shift of the protons on the alkene, especially C3 (again!), is probably the most helpful.
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-rais ing). The third is the effect of small rings and this we next need to consider in a broader context.
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