The C-X bond is polar (C^δ+—X^δ-) with bond length increasing (C-F < C-Cl < C-Br < C-I) and bond energy decreasing in the same order (C-F: 485 > C-I: 213 kJ/mol); iodoalkanes are most reactive in nucleophilic substitution (weakest C-I bond + best $I^{-}$ leaving group).

SN1 (two steps, via planar carbocation, Rate = k[RX]) is favored by tertiary substrates, polar protic solvents, and weak nucleophiles, and gives racemization as its stereochemical outcome.

SN2 (one-step concerted, Rate = k[RX][$Nu^{-}$]) is favored by primary substrates, polar aprotic solvents, and strong nucleophiles, and gives complete Walden inversion of configuration — never racemization.

Carbocation rearrangements (1,2-hydride or alkyl shifts) are possible in SN1 but impossible in SN2 because the latter has no carbocation intermediate.

E1 (two steps, competing with SN1) and E2 (concerted, competing with SN2) both give alkenes following Saytzeff's rule: the more substituted (more stable) alkene is the major product.

E2 with a strong bulky base (tert-butoxide) gives the Hofmann (less substituted) alkene because the bulky base cannot access the hindered β-hydrogen.

In haloarenes (Ar-X), p-π resonance causes the halogen lone pair to delocalize into the ring, giving the C-X bond partial double bond character: shorter (169 pm in $C_{6}H_{5}Cl$ vs 177 pm in $C_{2}H_{5}Cl$), stronger, and far less reactive toward nucleophilic substitution.

The Dow process ($C_{6}H_{5}Cl$ + NaOH → $C_{6}H_{5}OH$, 623 K, 300 atm) requires extreme conditions precisely because of this resonance-strengthened Ar-C-Cl bond; the Finkelstein reaction (NaI/acetone → RI) and Swarts reaction (AgF → RF) use precipitation as the driving force.

Chloroform oxidizes to toxic phosgene ($COCl_{2}$) in light and air; CFCs deplete stratospheric ozone through Cl• radical chain reactions (UV cleaves C-Cl → Cl• → $O_{3}$ destruction, catalytic cycle).

DDT is an environmental hazard not due to ozone depletion (that is CFCs) but due to its non-biodegradable nature and lipophilicity, which cause biomagnification through food chains with highest concentrations in apex predators.