Organic Chemistry 2 · Acyl substitution and derivative choice
Benzoyl azide from benzoyl chloride and sodium azide
Conditions: NaN3, acetone/H2O, 0 °C
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The mechanism, step by step
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Azide adds to the carbonyl carbon
Azide ion is a small, strongly nucleophilic anion that reacts with benzoyl chloride faster than the water cosolvent does. A lone pair on its terminal nitrogen attacks the carbonyl carbon and the C=O π pair moves onto oxygen, giving an anionic tetrahedral intermediate.
The curved arrows
- A lone pair on the negatively charged terminal nitrogen of azide forms the new N–C bond.
- The carbonyl π electrons move onto the electronegative oxygen, which becomes an alkoxide.
Collapse expels chloride
The alkoxide lone pair re-forms the C=O bond and chloride, the best leaving group on the tetrahedral carbon, departs. Keeping the mixture at 0 °C matters here: warming the acyl azide would start the Curtius rearrangement, losing N2 to give phenyl isocyanate.
The curved arrows
- The oxygen lone pair returns to rebuild the carbonyl π bond.
- The C–Cl bonding pair leaves with chlorine as chloride ion, a very weak base.
Benzoyl azide
Benzoyl azide forms, with sodium chloride as the by-product. The acyl azide is itself an acyl donor and, on heating, the precursor of an isocyanate.
What you'll practice
Draw azide ion substituting for chloride on benzoyl chloride by addition–elimination, stopping at benzoyl azide.
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