91Ó°ÊÓ

For simple aldehydes and ketones, a strong IR absorption is seen between the range $\mathbf{1710}\mathbf{-}\mathbf{1740}\text{ }{\mathbf{cm}}^{-1}$, but due to the presence of conjugation of C=O by phenyl ring or double bond, the stretching frequency gets lowered.

(a) In 4-Androstene-3,17-dione, only carbonyl absorptions are noted. A 5-membered ring ketone has IR absorption at ${\text{1750 c³¾}}^{\text{-1}}$and an $\text{α,β}$-unsaturated ketone has IR absorption at ${\text{1685 c³¾}}^{\text{-1}}$.

IR absorption in (a)

(b) In 1-Indanone, there is a 5-membered ring and aromatic ketone. Both these have IR absorptions at ${\text{1720 c³¾}}^{\text{-1}}$.

IR absorption in (b)

(c) In the given compound, there is a 5-membered ring ketone. It has IR absorption at ${\text{1750 c³¾}}^{\text{-1}}$.

IR absorption in (c)

(d) There is one aromatic aldehyde and one aliphatic ketone in the given compound. Aromatic aldehyde has IR absorption at ${\text{1705 c³¾}}^{\text{-1}}$, ${\text{2720 c³¾}}^{\text{-1}}$, and ${\text{2820 c³¾}}^{\text{-1}}$. Aliphatic ketone has IR absorption at ${\text{1715 c³¾}}^{\text{-1}}$.

IR absorption in (d)