: 230
Aryl halides (PhX) resist both SN1 and SN2 under normal conditions. SN2 is blocked: sp2 carbon has no accessible backside, and the pi electron cloud repels approaching nucleophiles. SN1 is blocked: the phenyl cation (sp hybridised, charge in sp orbital) is extremely unstable.
The C(sp2)-X bond has partial double bond character from resonance (X lone pair donates into the ring). This makes it shorter, stronger, and less reactive than C(sp3)-X.
SNAr (nucleophilic aromatic substitution): Requires strong EWG (NO2) at ortho/para positions. Mechanism: addition-elimination via Meisenheimer complex (anionic sigma complex). The EWG stabilises the negative charge on the intermediate. Counter-intuitively, F is the best leaving group (addition is RDS, not elimination; C-F most polar → fastest addition).
Benzyne mechanism: PhCl + NaNH2 → benzyne (elimination) → aniline (addition). Proved by ^14C isotope labelling (equal amounts of C1 and C2 labelled product — benzyne is symmetrical).
Industrial reactions: Dow process: PhCl + NaOH (300 C, 300 atm) → PhOH. Ullmann coupling: 2ArI + Cu → Ar-Ar.