Question

Which occurs at a larger wavenumber:

a. the C=O stretch of phenol or the C-O stretch of cyclohexanol?

b. the C=O stretch of a ketone or the C=O stretch of an amide?

c. the CN stretch of cyclohexylamine or the CN stretch of aniline?

Short answer

1. The C-O bond of phenol stretches at higher than cyclohexanol.
2. The C-O bond of ketone stretches at higher than amide.
3. The C-N bond of cyclohexylamine stretches at higher than aniline.

Step-by-step solution

Delocalization Energy (a and b)

The delocalization energy is the difference in energy between the different resonating structures of a molecule.Because of the delocalization of the bond the wavenumber for stretching decreases. Phenol has partial double character as a result of delocalization:

Partial bond in phenol

Similarly, aniline also has high stretch because of partial bond character

Partial bond in aniline

Double bond character (c)

if a molecule has more double bond character then it stretches at a higher wavenumber, while a molecule has less double bond character than it stretches at a lower wavenumber.

Double bond character (c)