Question

For carbonyls that are part of a conjugated π-network, the C=O stretch:

A. has a higher frequency than the analogous C=O in a non-conjugated system
B. is always located at 1715 cm-1
C. has a lower frequency than the analogous C=O in a non-conjugated system
D. exhibits a much lower magnitude of peak intensity
E. is identical to the analogous C=O stretch in a non-conjugated system

Answer 1

Carbonyls that are part of a conjugated π-network have a lower frequency than the analogous C=O in a non-conjugated system.

Step-by-step explanation:

A carbonyl that is part of a conjugated π-network typically has a lower frequency than the analogous C=O in a non-conjugated system. This is because the presence of conjugation affects the electron density distribution in the molecule, leading to a decrease in the C=O bond strength and subsequently a decrease in the stretching frequency. An example of this can be seen with the carbonyl stretch in conjugated unsaturated ketones, which tends to absorb on the longer wavelength end (1650-1700 cm-1) compared to non-conjugated carbonyls.