Unit 6: Haloalkanes and Haloarenes

Organic compounds containing halogen atoms — intermediates in synthesis and important in industry

6.1 Classification

Haloalkanes and haloarenes are classified based on the number of halogen atoms and the type of carbon-halogen (C-X) bond.

Classification of Haloalkanes and Haloarenes

sp³ C-X Bond

Haloalkanes: halogen bonded to sp³ carbon. Includes alkyl, allylic, and benzylic halides.

sp² C-X Bond

Vinylic (on C=C) and aryl (on aromatic ring) halides. Less reactive due to partial double bond character.

Geminal & Vicinal Dihalides

Geminal: both halogens on same carbon. Vicinal: halogens on adjacent carbons.

The nature of the C-X bond is crucial: sp³ C-X undergoes nucleophilic substitution easily, while sp² C-X bonds are much less reactive due to partial double bond character from resonance.

6.2 Preparation

From Alcohols

R-OH + HX → R-X + H₂O
3R-OH + PCl₃ → 3R-Cl + H₃PO₃
R-OH + PCl₅ → R-Cl + POCl₃ + HCl
R-OH + SOCl₂ → R-Cl + SO₂ + HCl

Reactivity of hydrogen halides: HI > HBr > HCl. The thionyl chloride method is preferred because gaseous by-products (SO₂ and HCl) escape, leaving pure product.

From Alkenes

R-CH=CH₂ + HX → R-CHX-CH₃ (Markovnikov)
R-CH=CH₂ + HBr → R-CH₂-CH₂Br (Anti-Markovnikov, peroxide effect)

Halogen Exchange

R-Cl + NaI → R-I + NaCl (Finkelstein, dry acetone)
R-Cl + AgF → R-F (Swarts reaction)

        Reactivity order of alkyl halides: R-I > R-Br > R-Cl >> R-F

        Reactivity order of alkyl groups: 3° > 2° > 1°

6.3 Nucleophilic Substitution

Nucleophilic substitution is the most characteristic reaction of alkyl halides, where a nucleophile replaces the halide leaving group.

SN2 Mechanism (Bimolecular)

Single-step concerted mechanism
Nucleophile attacks from the back side
Rate = k [substrate] [nucleophile] — second order
Causes inversion of configuration (Walden inversion)
Favoured by: primary alkyl halides, strong nucleophile, polar aprotic solvent

SN1 Mechanism (Unimolecular)

Two-step mechanism via carbocation intermediate
Rate = k [substrate] — first order
Causes racemisation (partial inversion in practice)
Favoured by: tertiary alkyl halides, weak nucleophile, polar protic solvent

Methyl and primary alkyl halides undergo SN2. Tertiary alkyl halides undergo SN1. Secondary halides can go either way depending on conditions. Allylic and benzylic halides are reactive in both because their carbocations are resonance-stabilised (SN1) and their transition states are stabilised (SN2).

6.4 Stereochemistry

Stereochemistry of Nucleophilic Substitution

Optical Activity

Chiral molecules rotate the plane of plane-polarised light
d-form (+): dextrorotatory (clockwise rotation)
l-form (−): laevorotatory (anticlockwise rotation)
Racemic mixture: 50:50 mixture of enantiomers — optically inactive

Walden Inversion

In SN2, the nucleophile attacks from the side opposite to the leaving group. This inverts the configuration at the carbon, like an umbrella turning inside out. Example: (−)-2-bromooctane gives (+)-octan-2-ol on hydrolysis with NaOH (SN2).

6.5 Elimination Reactions

When an alkyl halide is treated with alcoholic KOH, hydrogen is eliminated from the β-carbon and the halide from the α-carbon, forming an alkene.

R-CH₂-CH₂-X + alc. KOH → R-CH=CH₂ + KX + H₂O

        Zaitsev's Rule: When different β-hydrogens are available, the major product is the more substituted (more stable) alkene.

        Example: 2-Bromopentane gives pent-2-ene (major, disubstituted) rather than pent-1-ene (minor, monosubstituted).

6.6 Reactions with Metals

Grignard Reagent

R-X + Mg → RMgX (in dry ether)

Grignard reagents are extremely reactive organometallic compounds. The C-Mg bond is covalent but highly polar, making the carbon nucleophilic. They react with water, alcohols, and many electrophiles.

Wurtz Reaction

2R-X + 2Na → R-R + 2NaX (dry ether)

Used to prepare symmetrical alkanes from alkyl halides. Not useful for unsymmetrical alkanes (gives mixture of products).

6.7 Reactions of Haloarenes

Nucleophilic Substitution — Very Difficult

The C-X bond in haloarenes has partial double bond character due to resonance
The sp² carbon is more electronegative, holding the halogen more tightly
Requires extreme conditions: Dow process (623 K, 300 atm) for chlorobenzene

Electrophilic Substitution — Preferred

The halogen is deactivating but ortho/para directing in electrophilic aromatic substitution.

Halogens withdraw electrons through inductive effect (−I) — deactivating
Donate electrons through resonance (+R) — ortho/para directing
Reactions: nitration, sulfonation, Friedel-Crafts reactions

6.8 Important Polyhalogen Compounds

Compound	Formula	Use
Chloroform	CHCl₃	Formerly used as anaesthetic; solvent
Carbon tetrachloride	CCl₄	Fire extinguisher (Pyrene); solvent
Freons	CCl₂F₂ (CFC-12)	Refrigerants, aerosol propellants (ozone depleting)
DDT	(ClC₆H₄)₂CHCCl₃	Pesticide — non-biodegradable, banned in many countries

DDT (dichlorodiphenyltrichloroethane) was widely used as a pesticide but accumulates in the food chain. It is non-biodegradable and has been banned in many countries due to environmental concerns.

Summary

Haloalkanes and haloarenes are classified by the number of halogens and C-X bond type. They are prepared from alcohols, alkenes, or by halogen exchange. Nucleophilic substitution proceeds via SN2 (backside attack, inversion) or SN1 (carbocation, racemisation) mechanisms. Elimination gives alkenes following Zaitsev's rule. Grignard reagents are versatile organometallic reagents. Haloarenes resist nucleophilic substitution but undergo electrophilic substitution (ortho/para directing). Important polyhalogen compounds include chloroform, CCl₄, Freons, and DDT.