Q 20.17: In IR spectroscopy, a conjugated ketone will produce a C=O signal at a wavenumber, compared with a non-conjugated ketone of similar structure.

See explanation

Explanation:

Infrared spectroscopy is concerned with transitions between vibrational energy levels. Each infrared active vibration leads to a change in dipole moment.

A ketone compound contains the -C=O group whose signal normally occurs at about 1710 cm-1.

A conjugated ketone is one in which the -C=O group is attached to the -C=C  group. This leads to delocalization and reduces the electron density of the C=O  double bond, which weakens this bond hence lowering the stretching frequency of the  -C=O bond from  1710 to about 1680cm-1.