Final answer:
Conjugation affects the IR spectra of aldehydes and ketones by shifting the absorption peak to a longer wavelength compared to non-conjugated carbonyl compounds. This is due to the delocalization of electrons in conjugated systems, which reduces the overall polarity of the carbonyl bond.
Step-by-step explanation:
Conjugation affects the IR spectra of aldehydes and ketones by shifting the absorption peak to a longer wavelength compared to non-conjugated carbonyl compounds. In the IR spectra, the carbonyl group in aldehydes and ketones typically absorbs in the range of 1650-1750 cm-1, which corresponds to a wavelength of 5.86 mm. However, when the carbonyl group is part of a conjugated system, such as in conjugated unsaturated ketones, the absorption peak is shifted to a lower frequency, around 1650-1700 cm-1.
This shift in absorption peak is due to the delocalization of electrons in conjugated systems. Conjugation allows for the overlap of p-orbitals, creating a system of alternating single and double bonds. This delocalization of electrons reduces the overall polarity of the carbonyl bond, resulting in a weaker absorption of infrared light. As a result, the absorption peak is shifted to a longer wavelength compared to non-conjugated carbonyl compounds.
For example, a non-conjugated aldehyde like butanal would have a strong absorption peak at around 1716 cm-1, while a conjugated unsaturated ketone like pentadienone would have a weaker absorption peak at around 1680 cm-1.