First slide

Alkynes

Question

Which of the following will give alkyne on reaction of phosphorous-penta -chloride followed by dehydrohalogenation with Sodamide ?

Moderate

A

Benzoic acid
B

Acetophenone
C

Benzaldehyde
D

Benzyl alcohol

Solution

aldehydes or ketones can form gem dihalides on reaction with $P C l_{5}$ . The gem dihalides having hydrogens will undergo double dehydrohalogenation to form Alkynes.
Only acetophenone will form gem dihalide with $P C l_{5}$ . Further, it will undergo dehydrohalogenation with $N a N H_{2}$ and forms phenyl-Acetylene.
Other compounds will form halogen derivative but will not participate in dehydrohalogenation due to lack of alpha hydrogens.

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