Question

Which of the following statements about phenytoin is true?

• A. The phenyl rings must be in the same plane
• B. The hydantoin ring is not necessary
• C. Aromatic substitution at position 5 is necessary
• D. The structure contains a barbiturate acid ring

Hint:

Phenytoin is the anticonvulsant 5,5-diphenylhydantoin. Its activity depends on a few key parts of the structure (this is its structure-activity relationship, or SAR).

Answer 1

The Correct Option is C

Concept:
Phenytoin is the anticonvulsant 5,5-diphenylhydantoin. Its activity depends on a few key parts of the structure (this is its structure-activity relationship, or SAR). To answer, we check each statement against what is actually needed for the drug to work.

Step 1:
The core of phenytoin is the hydantoin ring (a five-membered ring with two nitrogen atoms and two C=O groups). This ring is essential for the anticonvulsant action, so saying it is "not necessary" is wrong. That rules out option 2.

Step 2:
At carbon-5 of the hydantoin ring, phenytoin carries two aromatic (phenyl) groups. Having an aromatic substitution at position 5 is what gives strong activity against generalised tonic-clonic seizures. So an aromatic group at position 5 is genuinely required for activity. This makes option 3 the correct statement.

Step 3:
The two phenyl rings do not have to lie in the same plane, so option 1 is not a true requirement. And phenytoin contains a hydantoin ring, not a barbituric-acid ring, so option 4 is also wrong.

Answer: Option (3) — Aromatic substitution at position 5 is necessary.