The best way is to relate all our carbonyl frequencies to those for saturated ketones (1715 cm⁻¹). 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⁻¹’ appears quite a lot (–30 cm⁻¹ for both alkene and aryl, for example), that the increment for small rings is 35 cm⁻¹ each time (30 to 65 cm⁻¹ and then 65 to 100 cm⁻¹), and that the extreme effects of Cl and NH₂ are +85 and –85 cm⁻¹, respectively. These effects are additive. If you want to estimate the C=O frequency of a proposed structure, just add or subtract all the adjustments to 1715 cm⁻¹ and you will get a reasonable result.

Try this out with the five-membered unsaturated (and conjugated) lactone (cyclic ester) in the margin. We must add 30 cm⁻¹ for the ester, subtract 30 cm⁻¹ for the double bond, and add 30 cm⁻¹ for the five-membered ring. Two of those cancel out, leaving just 1715 + 30 = 1745 cm⁻¹. These compounds absorb at 1740 – 1760 cm⁻¹. Not bad!

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