Benzyne mechanism template

NaNH₂ removes the H next to Cl in chlorobenzene, chloride leaves to give benzyne, NH₂⁻ adds and NH₃ protonates the aryl anion to give aniline.

Mechanism step by step

Elimination–addition: a very strong base makes a benzyne, which the amide ion then attacks

1. Deprotonation – NH₂⁻ removes the H ortho to Cl
2. Benzyne – The carbanion pushes out Cl⁻: a strained triple bond
3. Addition – NH₂⁻ attacks either end of the triple bond
4. Protonation – NH₃ protonates the aryl anion

Key points

• The extra π bond of benzyne uses sp² orbitals in the ring plane: poor overlap, so it is weak and very reactive.
• ¹⁴C-labelled chlorobenzene gives aniline with NH₂ half on the labelled carbon, half on its neighbour (Roberts, 1953).
• Substituted aryl halides can give mixtures: the nucleophile ends up on the carbon that carried the halogen (ipso) or on its neighbour (cine substitution).
• No H ortho to the halogen, no benzyne: such halides do not react.

Drawn with 76 atoms, 13 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 22 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 13 Textbook
• Roberts, Simmons, Carlsmith & Vaughan, J. Am. Chem. Soc. 1953, 75, 3290 Original paper