SN2 = Walden inversion (complete). SN1 = racemization. Never swap these. SN2 is concerted backside attack — one face only, always inversion. SN1 has a planar carbocation attacked from both faces — always 50:50 racemic.

C-F is shortest AND strongest $\frac{485 kJ}{mol}$, least reactive. C-I is longest AND weakest $\frac{213 kJ}{mol}$, most reactive. Short = strong = hard to break = unreactive. Long = weak = easy to break = reactive. The inverse relationship is the foundation of halide reactivity.

Haloarenes (Ar-X) are less reactive than haloalkanes (R-X) because resonance gives Ar-C-X partial double bond character — making it shorter (169 vs 177 pm for C-Cl) and stronger. This is why the Dow process needs 623 K and 300 atm to make phenol from chlorobenzene.

Finkelstein = NaI in acetone → RI (NaCl precipitates). Swarts = AgF → RF (AgBr precipitates). Dow = NaOH at 623 K/300 atm → phenol. These three named reactions are the most-tested in NEET for this chapter; memorize reagent + condition + driving force for each.

CFCs → ozone depletion (Cl• radicals destroy $O_{3}$ catalytically, UV cleaves C-Cl). DDT → biomagnification (non-biodegradable, lipophilic, accumulates in fat up the food chain). Two different environmental problems, two different mechanisms — never confuse them.