Board exam focus — Haloalkanes and Haloarenes (CBSE & HBSE)

Classification, preparation of haloalkanes and haloarenes, nucleophilic substitution (SN1 and SN2), elimination reactions, stereochemistry, and polyhalogen compounds (DDT, BHC, freon).

Classification and Preparation

Haloalkanes and Haloarenes

Classification

Haloalkanes: Halogen attached to sp3 carbon (alkyl halides) Haloarenes: Halogen attached to aromatic ring (aryl halides)

Based on number of halogens:

• Monohaloalkanes: CH3Cl (chloromethane)
• Dihaloalkanes: CH2Cl2, CHCl3 (chloroform), CCl4 (carbon tetrachloride)
• Trihaloalkanes: CHCl3
• Tetrahaloalkanes: CCl4

Based on type of C bearing halogen:

• Primary (1°): -CH2X (e.g., CH3CH2Cl)
• Secondary (2°): >CHX (e.g., (CH3)2CHCl)
• Tertiary (3°): >CX (e.g., (CH3)3CCl)

Preparation of Haloalkanes

1. From Alcohols: ROH + HX → RX + H2O ROH + SOCl2 → RCl + SO2 + HCl (preferred; retains configuration) ROH + PCl5 → RCl + POCl3 + HCl ROH + PCl3 → RCl + H3PO3 Reactivity: HI > HBr > HCl (for HX method) Lucas reagent (ZnCl2/HCl): distinguishes 1°, 2°, 3° alcohols by speed of turbidity.

2. From Alkenes: Hydrohalogenation: RCH=CH2 + HX → RCHXMe (Markovnikov) Halogenation: RCH=CH2 + X2 → RCHX-CH2X (vicinal dihalide)

3. Halogen Exchange (Finkelstein and Swarts): Finkelstein: RCl + NaI (acetone) → RI + NaCl (equilibrium shifted by insolubility of NaCl) Swarts: RCl + AgF → RF + AgCl (for alkyl fluorides)

4. Free Radical Halogenation: RH + Cl2 (hv) → RCl + HCl Reactivity: F2 > Cl2 > Br2 > I2

Preparation of Haloarenes

Electrophilic Aromatic Substitution: Direct halogenation with Lewis acid catalyst: ArH + X2 → ArX + HX (X2 + FeBr3 → Br+, electrophile)

Balz-Schiemann reaction (ArF): Ar-NH2 → Ar-N2+BF4- → ArF + BF3 + N2

Sandmeyer Reaction: Ar-N2+Cl- + CuCl/HCl → ArCl + N2 Ar-N2+Br- + CuBr/HBr → ArBr + N2 Ar-N2+CN- + CuCN/KCN → ArCN + N2

Physical Properties

• Higher bp than alkanes due to dipole-dipole interactions
• Insoluble in water; soluble in organic solvents
• Density increases with number of C or halogen atoms
• CHCl3 (chloroform) less dense than CCl4; both denser than water

Nucleophilic Substitution - SN1 and SN2 Mechanisms

Nucleophilic Substitution

SN2 Mechanism (Bimolecular, Backside Attack)

Rate = k[R-X][Nu:] (second order kinetics)

Mechanism:

1. Nucleophile attacks carbon from the BACK (180° from leaving group)
2. Transition state: pentacoordinate carbon (Nu partially bonded, X partially bonded)
3. Inversion of configuration (Walden inversion) - like umbrella turning inside out
4. One-step, concerted mechanism (no intermediate)

Favored by:

• Primary alkyl halides (less steric hindrance)
• Strong nucleophile (CN-, OH-, I-, RS-)
• Polar aprotic solvent (acetone, DMSO, DMF - don't solvate nucleophile)
• Low temperature

Not favored by: Tertiary halides (bulky groups block back attack)

Stereochemistry: Always gives inversion (100% inversion)

SN1 Mechanism (Unimolecular, Carbocation)

Rate = k[R-X] (first order kinetics; depends only on R-X concentration)

Mechanism:

1. Step 1 (slow, rate-determining): R-X → R+ (carbocation) + X-
2. Step 2 (fast): R+ + Nu: → R-Nu

Favored by:

• Tertiary alkyl halides (stable carbocation: 3° > 2° > 1° > CH3+)
• Weak nucleophile or high concentration of good ionizing solvent
• Polar protic solvents (ethanol, water - stabilize carbocation and X- by solvation)
• Heat

Stereochemistry: Carbocation is planar → attack from both sides → racemization (mixture of R and S)

Comparison: SN1 vs SN2

Elimination Reactions

E2 (bimolecular): Base removes H from carbon adjacent to C-X; double bond forms; leaving group X leaves simultaneously. Anti-periplanar arrangement required. One step. Favored by 3° halides + strong base + heat.

E1: Carbocation intermediate (same as SN1 first step); base removes H from carbocation. Follows E-Zaitsev rule (H removed from C with fewer H, more substituted alkene formed).

Zaitsev's rule: More substituted (more stable) alkene is the major product of elimination.

Polyhalogen Compounds

Polyhalogen Compounds

Dichloromethane (CH2Cl2, Methylene Chloride)

Colourless liquid; bp 40 C. Uses: Solvent for extraction, paint remover, aerosol propellant, decaffeination of coffee.

Chloroform (CHCl3, Trichloromethane)

Sweet-smelling liquid; bp 61 C. Preparation: Ethanol/acetone + bleaching powder (NaOCl). CHCl3 + NaOH → HCOONa + 3HCl (reaction with NaOH) Chloroform in light + air: CCl3CHO + HCl → slowly oxidized to phosgene (COCl2, toxic) Stored in amber bottles with ethanol (to convert phosgene to diethyl carbonate). Uses: Formerly used as anesthetic (replaced by safer agents), solvent in labs, freon production.

Carbon Tetrachloride (CCl4, Tetrachloromethane)

CCl4: Colorless liquid; bp 77 C. Preparation: CS2 + 3Cl2 (sunlight) → CCl4 + S2Cl2 Uses: Formerly as dry-cleaning, fire extinguisher (Pyrene), solvent. Environmental: ozone depleter (CFC-related). Now largely phased out.

Iodoform (CHI3, Triiodomethane)

Pale yellow, crystalline; antiseptic properties. Iodoform reaction (Iodoform test): CH3CHO (or CH3CO-R) + I2 + NaOH → CHI3 (yellow ppt) + RCOONa Test: Yellow precipitate with characteristic smell confirms methyl ketone (or ethanol, acetaldehyde).

Chloroform (Freons)

CFC = chlorofluorocarbons. Freon-12: CCl2F2 (dichlorodifluoromethane) Freon-11: CCl3F (trichlorofluoromethane) Uses: Formerly refrigerants, aerosol propellants, air conditioning. Environmental problem: CFCs decompose in upper atmosphere (UV radiation → Cl• radicals). Cl• + O3 → ClO• + O2; ClO• + O → Cl• + O2 (ozone catalytically destroyed) Montreal Protocol (1987): Phasing out of CFCs; replaced by HFCs (hydrofluorocarbons).

DDT (Dichlorodiphenyltrichloroethane)

Formula: Cl-C6H4-CHCl-CCl3 (simplified: (ClC6H4)2CHCCl3) Synthesis: Chlorobenzene + CCl3CHO → DDT + H2SO4 (Cl-Baeyer's condensation) Insecticide: was highly effective against malaria-carrying mosquitoes (Anopheles). Problems:

• Non-biodegradable; accumulates in food chain (biomagnification)
• Toxic to wildlife, especially birds (eggshell thinning)
• Now banned in most countries (Stockholm Convention, 2004)

BHC/Lindane (Benzene Hexachloride)

Formula: C6H6Cl6 (1,2,3,4,5,6-hexachlorocyclohexane) Gamma-BHC (Lindane) is the active insecticidal isomer. Preparation: Benzene + 3Cl2 (hv) → BHC (free radical addition to aromatic ring) Uses: Pesticide against soil insects; soil fumigant. Also non-biodegradable and largely phased out due to environmental concerns.

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