Why ArX is unreactive: (1) C-X bond has partial double bond character (X lone pair → ring), (2) sp2 carbon — Nu can't approach from backside, (3) phenyl cation too unstable for SN1.

SNAr (addition-elimination):

• Requires strong EWG at ortho/para to X (e.g., 2,4-dinitro)
• Mechanism: Nu:- adds to ipso carbon → Meisenheimer complex (anionic sigma complex) → X- leaves
• F is the best leaving group (stabilizes Meisenheimer best)
• Example: 2,4-Dinitrochlorobenzene + NaOH → 2,4-dinitrophenol

Benzyne (elimination-addition):

• Strong base (NaNH2) at high T → eliminates HX → benzyne (triple bond-like strained intermediate) → nucleophile adds → product
• Gives mixture of substitution positions (ortho and meta to original X)
• Evidence: 14C labeling shows product has label at TWO positions