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.

مواضيع ذات صلة

Alkenes react with bromine

Effects of substituents on CH2 groups

Effects of functional groups

Identifying compounds from nature

Squares and cubes: molecules with unusual structures

Reactive intermediates can be detected by spectroscopy

Identifying products spectroscopically An ambiguous reaction product

Why is there no coupling to protons in normal 13C NMR spectra

Coupling in carbon NMR spectra

Interactions between different nuclei can give enormous coupling constants

Proton NMR distinguishes axial and equatorial protons in cyclohexanes

NMR spectra of alkynes and small rings