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Ortho-aminobenzoyl chloride is treated sequentially with (i) NaN3 and (ii) heat (delta). The major product Q of this reaction is:

1. A fused benzolactam with one carbonyl and one NH in a five-membered ring fused to benzene (2H-3,1-benzoxazin-2-one type)
2. 2-benzimidazolone: a five-membered ring fused to benzene containing one C=O and two NH groups at positions 1 and 3
3. A fused bicyclic structure containing a carbonyl and two NH groups arranged as an imide-like ring (six-membered)
4. Ortho-aminobenzamide (the COCl simply hydrolyzes to CONH2)

Correct answer: 2-benzimidazolone: a five-membered ring fused to benzene containing one C=O and two NH groups at positions 1 and 3

Solution

Step 1: o-H2N-C6H4-COCl + NaN3 -> o-H2N-C6H4-CO-N3 (acyl azide). Step 2: Heating causes Curtius rearrangement: acyl azide loses N2 and rearranges to an isocyanate: o-H2N-C6H4-N=C=O. Step 3: The ortho-amino group (-NH2) reacts intramolecularly with the isocyanate (-N=C=O) to form a five-membered ring. The product is 2-benzimidazolone (1,3-dihydro-2H-benzimidazol-2-one) which has two NH groups and one C=O in a 5-membered ring fused to benzene.

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