Simple calculations of C=O stretching frequencies in IR spectra
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص413-414
2025-05-31
519
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!
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