Haloalkanes & Haloarenes

Haloalkanes (R-X) and haloarenes (Ar-X) feature the polar C-X bond (C^$\delta$+ — X^$\delta$-). The C-X bond length increases (C-F < C-Cl < C-Br < C-I) while bond energy decreases in the same order (C-F strongest, C-I weakest). This inverse relationship means that despite C-I being the longest and weakest bond, iodoalkanes are the most reactive in nucleophilic substitution because bond breaking is easier.

SN₁ Mechanism (Substitution, Nucleophilic, Unimolecular): Two steps — (1) slow ionization of C-X to form a carbocation (rate-determining, Rate = k[RX]), followed by (2) fast nucleophilic attack on the planar carbocation from either face. SN₁ is favored by: 3 degree substrate (stable carbocation, e.g., tert-butyl chloride ), polar protic solvent (H₂O, ROH — stabilizes ions through solvation), and weak nucleophile. Stereochemistry: racemization — the planar carbocation is attacked equally from both sides, producing a mixture of R and S enantiomers.

SN₂ Mechanism (Bimolecular): One step — concerted backside attack by the nucleophile on the carbon bearing the leaving group (Rate = k[RX][Nu-]). SN₂ is favored by: 1 degree substrate (minimal steric hindrance, e.g., bromoethane CCBr), polar aprotic solvent (DMSO, DMF, acetone — does not solvate nucleophile, keeping it reactive), and strong nucleophile (OH-, CN-, I-). Stereochemistry: Walden inversion — complete inversion of configuration at the carbon center, like an umbrella flipping in wind.

Elimination Reactions: E₁ (2 steps via carbocation, unimolecular, favors 3 degree substrate — competes with SN₁) and E₂ (1 step, bimolecular, concerted, strong bulky base like t-BuO-). Saytzeff's rule states the more substituted alkene is the major product (more stable due to hyperconjugation). Decision rules: SN₂ vs E₂ — strong non-bulky nucleophile favors SN₂, strong bulky base favors E₂. SN₁ vs E₁ — both compete in polar protic solvents with 3 degree substrates; higher temperature shifts toward elimination.

Haloarenes are less reactive toward nucleophilic substitution than haloalkanes because the C-X bond in Ar-X has partial double bond character. The lone pair on the halogen delocalizes into the benzene ring through resonance, making the bond shorter and stronger. The Dow process converts chlorobenzene to phenol: C₆H₅Cl + NaOH →(623 K, 300 atm) C₆H₅OH + NaCl.

Named preparations include Finkelstein reaction (RCl + NaI →(acetone) RI + NaCl, driven by NaCl precipitating in acetone) and Swarts reaction (RBr + AgF → RF + AgBr, for fluoroalkane synthesis).

Environmental impact: chloroform (CHCl₃, ) oxidizes to toxic phosgene; freons (CFCs) deplete the ozone layer; DDT () is non-biodegradable and bioaccumulates through food chains.

The key testable concept is the SN₁ vs SN₂ mechanism comparison — specifically that SN₂ gives Walden inversion (not racemization), and that haloarenes resist nucleophilic substitution due to resonance-assisted partial double bond character of C-X.