Q66E - Organic Chemistry (Mcmurry)

Question

At what position would you expect to observe IR absorptions for the following molecules?

a. 

b.

c. 

d.

Step-by-Step Solution

Answer

1. A 5-membered ring ketone has IR absorption at ${\text{1750\hspace{0.17em}cm}}^{\text{-1}}$ , and an $\text{α,β}$ -unsaturated ketone has IR absorption at ${\text{1685\hspace{0.17em}cm}}^{\text{-1}}$ .
2. A 5-membered ring and aromatic ketone both have IR absorption at ${\text{1720\hspace{0.17em}cm}}^{\text{-1}}$.
3. A 5-membered ring ketone has IR absorption at ${\text{1750\hspace{0.17em}cm}}^{\text{-1}}$.
4. Aromatic aldehyde has IR absorption at ${\text{1705\hspace{0.17em}cm}}^{\text{-1}}$,${\text{2720\hspace{0.17em}cm}}^{\text{-1}}$ and ${\text{2820\hspace{0.17em}cm}}^{\text{-1}}$ . Aliphatic ketone has IR absorption at ${\text{1715\hspace{0.17em}cm}}^{\text{-1}}$.

Step 1: IR absorptions

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

Step 2: The IR absorptions in 4-Androstene-3,17-dione

(a)  In 4-Androstene-3,17-dione, only carbonyl absorptions are noted. A 5-membered ring ketone has IR absorption at ${\text{1750\hspace{0.17em}cm}}^{\text{-1}}$ and an $\text{α,β}$-unsaturated ketone has IR absorption at ${\text{1685\hspace{0.17em}cm}}^{\text{-1}}$.

                                              IR absorption in (a)

Step 3: The IR absorptions in 1-Indanone

(b) In 1-Indanone, there is a 5-membered ring and aromatic ketone. Both these have IR absorptions at ${\text{1720\hspace{0.17em}cm}}^{\text{-1}}$.

                                        IR absorption in (b)

Step 4: The IR absorptions in compound (c)

(c) In the given compound, there is a 5-membered ring ketone. It has IR absorption at ${\text{1750\hspace{0.17em}cm}}^{\text{-1}}$.

IR absorption in (c)

Step 5: The IR absorptions in compound (d)

(d)  There is one aromatic aldehyde and one aliphatic ketone in the given compound. Aromatic aldehyde has IR absorption at ${\text{1705\hspace{0.17em}cm}}^{\text{-1}}$, ${\text{2720\hspace{0.17em}cm}}^{\text{-1}}$, and ${\text{2820\hspace{0.17em}cm}}^{\text{-1}}$ . Aliphatic ketone has IR absorption at ${\text{1715\hspace{0.17em}cm}}^{\text{-1}}$.

IR absorption in (d)