Identify the structure of the final product (D) in the following sequence of the reactions : [figure] Total number of sp2 hybridised carbon atoms in product D is .
Answer
7
Solution
Step 1: PCl5 replaces the carbonyl oxygen by two Cl atoms (gem-dichloride):
A = Ph-CCl2-CH3
Step 2: Two equivalents of NaNH2 remove two molecules of HCl (double dehydrohalogenation) giving phenylacetylene; the third equivalent removes the acidic terminal H:
B = Ph-C C Na (sodium phenylacetylide)
Step 3: Acidification gives back the terminal alkyne:
C = Ph-C CH (phenylethyne)
Step 4: Hydroboration–oxidation of a terminal alkyne adds H and OH anti-Markovnikov; the enol formed tautomerises to an aldehyde:
D = Ph-CH2-CHO (phenylacetaldehyde) [figure]
sp2 carbons in D: 6 (benzene ring) + 1 (CHO carbon) = 7; the CH2 carbon is sp3.
Answer: 7