Question

Total number of $sp^2$ hybridised carbon atoms in product D is _______.

Answer 1

7

Answer 2

Solution:

1. Step 1 – Formation of Intermediate A:
   Acetophenone, $\text{PhCOCH}_3$, reacts with $\text{PCl}_5$ under heating to convert the carbonyl group into a geminal dichloride. Thus,

   $$\text{PhCOCH}_3 \xrightarrow[\Delta]{\text{PCl}_5} \text{PhCCl}_2\text{CH}_3.$$

2. Step 2 – Formation of Intermediate B:
   Treatment of $\text{PhCCl}_2\text{CH}_3$ with 3 equivalents of $\text{NaNH}_2$ in $\text{NH}_3$ causes double dehydrohalogenation. This elimination removes two molecules of HCl to generate a triple bond, forming phenylacetylene (which, in its anionic form, is later finally protonated).

   $$\text{PhCCl}_2\text{CH}_3 \xrightarrow{3\ \text{eq. NaNH}_2/\text{NH}_3} \text{PhC}\equiv\text{CH}.$$

3. Step 3 – Formation of Intermediate C:
   Acidification of the reaction mixture protonates the acetylide ion (if present) to give phenylacetylene, $\text{PhC}\equiv\text{CH}$.

4. Step 4 – Formation of Final Product D:
   Phenylacetylene is then subjected to hydroboration–oxidation:

   • First, reaction with $\text{B}_2\text{H}_6$, and then
   • Oxidation with $\text{H}_2\text{O}_2$ in the presence of $\text{OH}^-$ converts the alkyne to an enol, which tautomerizes to give the methyl ketone, acetophenone ($\text{PhCOCH}_3$).

Thus, the final product $D$ is acetophenone.

Counting sp² Hybridized Carbon Atoms in Product D (Acetophenone):

• Benzene ring: 6 sp² hybridized carbons
• Carbonyl carbon: 1 sp² hybridized carbon

$$\text{Total} = 6 + 1 = 7.$$