Haloalkanes and Haloarenes notes: CBSE Class 12 Chemistry

1. Classification, nomenclature and the C–X bond

Classification: by the number of halogens (mono-, di-, poly-) and by the carbon bearing X — sp³: primary, secondary, tertiary alkyl, allylic, benzylic; sp²: vinylic, aryl. Dihalides: geminal (same C, e.g. 1,1-dichloroethane) and vicinal (adjacent C, e.g. 1,2-dibromoethane).

Naming: halogen as a prefix with a locant — bromoethane, 2-chloropropane, 2-bromo-2-methylpropane, 3-chloroprop-1-ene (allyl chloride), (chloromethyl)benzene (benzyl chloride), 1,4-dichlorobenzene.

C–X bond: polar (C δ+, X δ−); bond length C–F 139 < C–Cl 178 < C–Br 193 < C–I 214 pm; bond enthalpy decreases F (452) > Cl (351) > Br (293) > I (234 kJ mol⁻¹), so reactivity is R–I > R–Br > R–Cl > R–F.

C–X bond lengths and enthalpies
Bond	Length / pm	Enthalpy / kJ mol⁻¹
C–F	139	452
C–Cl	178	351
C–Br	193	293
C–I	214	234

Key terms

Allylic halide: Halogen on an sp³ carbon next to a C=C double bond.
Vinylic halide: Halogen on an sp² carbon of a C=C double bond.
Geminal dihalide: Two halogen atoms on the same carbon.
Vicinal dihalide: Two halogen atoms on adjacent carbons.

2. Preparation

From alcohols: $\ce{ROH + HCl ->[ZnCl2] RCl + H2O}$ (3° alcohols react with conc. HCl alone); $\ce{3ROH + PCl3 -> 3RCl + H3PO3}$ ; $\ce{ROH + PCl5 -> RCl + POCl3 + HCl}$ ; $\ce{ROH + SOCl2 -> RCl + SO2 + HCl}$ (best, gaseous by-products). PBr₃ and PI₃ are made in situ from red P and Br₂ or I₂.

From hydrocarbons: free-radical halogenation of alkanes (mixtures); HX addition to alkenes (Markovnikov; HBr with peroxide gives anti-Markovnikov); X₂ addition gives vic-dihalides; arenes + X₂ with FeX₃ (dark) give o- and p-haloarenes.

Sandmeyer reaction: $\ce{C6H5N2Cl ->[CuCl/HCl] C6H5Cl + N2}$ (also CuBr/HBr).

Halogen exchange: Finkelstein $\ce{RCl + NaI ->[acetone] RI + NaCl}$ (NaCl precipitates); Swarts $\ce{CH3Br + AgF -> CH3F + AgBr}$ (also Hg₂F₂, CoF₂, SbF₃).

Key terms

Sandmeyer reaction: Replacement of the diazonium group by Cl or Br using copper(I) halide.
Finkelstein reaction: Halogen exchange with NaI in dry acetone to form iodoalkanes.
Swarts reaction: Preparation of fluoroalkanes by heating chloro- or bromoalkanes with metallic fluorides.

3. Physical properties

Higher boiling points than alkanes of similar mass (polarity, larger size). For the same alkyl group: RF < RCl < RBr < RI. For isomers, branching lowers bp: 2-bromo-2-methylpropane (346 K) < 2-bromobutane (364 K) < 1-bromobutane (375 K).
Density increases with the number and mass of halogen atoms; bromo- and iodo-compounds are denser than water.
Only slightly soluble in water (energy to break H-bonds in water is not compensated); soluble in organic solvents.
p-Dichlorobenzene (mp 323 K) melts higher than o- (256 K) and m- (249 K) isomers because its symmetrical molecules pack better.

Boiling points of halomethanes
Halomethane	bp / °C
CH₃F	−78
CH₃Cl	−24
CH₃Br	4
CH₃I	42

Key terms

Van der Waals forces: Weak attractions between molecules that increase with molecular size and surface area.

4. Nucleophilic substitution: SN1 and SN2

Products: aq. KOH → alcohol; NaOR → ether; KCN → nitrile (via C); AgCN → isocyanide (via N); KNO₂ → alkyl nitrite (via O); AgNO₂ → nitroalkane (via N); NH₃ → amine; LiAlH₄ → alkane; R′COOAg → ester.

SN2: one step; backside attack; rate $= k[\ce{RX}][\ce{Nu-}]$ ; inversion (Walden inversion); reactivity CH₃X > 1° > 2° > 3° (steric hindrance).

SN1: two steps via a carbocation; rate $= k[\ce{RX}]$ ; mostly racemisation; reactivity 3° > 2° > 1° > CH₃X; favoured by polar protic solvents; allylic and benzylic halides react fast (resonance-stabilised carbocations).

Stereochemistry: a carbon with four different groups is chiral; enantiomers rotate plane-polarised light equally and oppositely (d/+, l/−); a racemic mixture is optically inactive. Retention: same configuration; inversion: opposite configuration.

Schematic; the inversion is like an umbrella turning inside out in the wind.

SN1 and SN2 compared
Feature	SN1	SN2
Steps	2 (carbocation)	1
Order	First	Second
Reactivity	3° > 2° > 1°	CH₃ > 1° > 2° > 3°
Stereochemistry	Racemisation	Inversion

Formulas

SN2 rate law
$\text{SN2: rate} = k[\ce{RX}][\ce{Nu-}]$
SN1 rate law
$\text{SN1: rate} = k[\ce{RX}]$

Key terms

Nucleophile: An electron-rich species that attacks an electron-deficient carbon.
Ambident nucleophile: A nucleophile that can attack through two different atoms, e.g. CN⁻ (C or N).
Racemisation: Formation of an equal mixture of enantiomers from one enantiomer.
Walden inversion: Inversion of configuration at carbon during an SN2 reaction.

5. Elimination, reactions with metals and reactions of haloarenes

Elimination: alcoholic KOH removes HX (β-elimination); Saytzeff rule — the more substituted alkene is the major product (2-bromobutane → but-2-ene major). Strong bulky bases and 3° halides favour elimination.

Metals: $\ce{RX + Mg ->[dry ether] RMgX}$ (Grignard; water gives RH); Wurtz $\ce{2RX + 2Na -> R-R + 2NaX}$ ; Wurtz–Fittig ArX + RX + 2Na → Ar–R; Fittig 2ArX + 2Na → Ar–Ar.

Haloarenes are much less reactive to nucleophilic substitution: resonance gives C–X partial double-bond character; sp² carbon; unstable phenyl cation; repulsion by the π cloud. Severe conditions are needed ( $\ce{C6H5Cl + 2NaOH ->[623 K, 300 atm] C6H5ONa + NaCl + H2O}$ ) unless –NO₂ groups occupy o/p positions. In electrophilic substitution the halogen is deactivating (–I) but o/p-directing (+R): nitration, halogenation, sulphonation and Friedel–Crafts reactions give o- and p- products (p major).

Key terms

Saytzeff rule: In dehydrohalogenation, the alkene with more alkyl groups on the double-bond carbons is the major product.
Grignard reagent: An alkyl- or arylmagnesium halide, RMgX, made in dry ether.

6. Polyhalogen compounds

Dichloromethane: solvent, paint remover, propellant; affects the central nervous system.
Trichloromethane (chloroform): solvent; once an anaesthetic. Oxidised by air in light to poisonous phosgene, $\ce{2CHCl3 + O2 -> 2COCl2 + 2HCl}$ , so stored in dark, completely filled bottles.
Triiodomethane (iodoform): yellow solid, antiseptic due to liberated iodine; objectionable smell.
Tetrachloromethane: formerly a solvent and in refrigerant manufacture; causes liver damage; ozone depleting.
Freons (CFCs): refrigerants and propellants; stable, but in the stratosphere release Cl atoms that destroy ozone.
DDT: insecticide used against malaria-carrying mosquitoes; not biodegradable, accumulates in fatty tissue and food chains; banned or restricted in many countries.

Uses and hazards of polyhalogen compounds
Compound	Main use	Main hazard
CH₂Cl₂	Solvent	Nervous system
CHCl₃	Solvent	Phosgene
CHI₃	Antiseptic	Smell
CCl₄	Formerly solvent	Liver; ozone
Freons	Refrigerants	Ozone depletion
DDT	Insecticide	Persistence

Key terms

Phosgene: Carbonyl chloride, COCl₂, a very poisonous gas formed when chloroform is oxidised by air.
Freons: Chlorofluorocarbons used as refrigerants and propellants; they deplete stratospheric ozone.

Exam tips

Common mistakes

Using aqueous KOH when elimination is asked (use alcoholic KOH) or vice versa.
Writing isocyanide for KCN (it gives a nitrile; AgCN gives an isocyanide).
Giving the SN1 order of reactivity for an SN2 question.
Saying SN1 gives inversion (it gives mostly racemisation).
Calling a halogen on benzene meta-directing (it is ortho/para-directing though deactivating).
Forgetting "dry ether" for Grignard and Wurtz reactions.

How marks are usually awarded

1-mark items: product, reagent, name or order.
Conversions (2–3 marks): reagent and condition for each step (½–1 each) + correct intermediate.
Mechanism answers: rate law (½), steps or transition state (1), stereochemistry (½–1).
Reasoning (2 marks): fact + reason (resonance, steric hindrance or carbocation stability).

1. Classification, nomenclature and the C–X bond

Key terms

2. Preparation

Key terms

3. Physical properties

Key terms

4. Nucleophilic substitution: SN1 and SN2

Formulas

Key terms

5. Elimination, reactions with metals and reactions of haloarenes

Key terms

6. Polyhalogen compounds

Key terms

Exam tips

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