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🧪 Chemistry · Class 12 · NEET & JEE

Haloalkanes & Haloarenes

Study carbon-halogen compounds: how they are made, how they react via SN1/SN2 and elimination, their stereochemistry, and their environmental impact.

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Reading time~7 min

Revision time~2 min

Last updated2026-07-19

1 Read the chapter ~7 min

🎯 Key Points

SN2: one step, backside attack, inversion, favoured by 1° + polar aprotic solvent; SN1: two steps via carbocation, racemisation, favoured by 3° + polar protic solvent
SN1 reactivity: 3° > 2° > 1°; SN2 reactivity: 1° > 2° > 3°
Haloarenes have a stronger C-X bond (resonance, partial double-bond character) and resist nucleophilic substitution unlike haloalkanes
Halogen on a benzene ring is deactivating but ortho/para directing (resonance donates, induction withdraws — induction wins on reactivity, resonance wins on direction)
Grignard reagent (RMgX) must be prepared/used under strictly anhydrous conditions; reacts with carbonyls to give alcohols, CO₂ to give acids, water to give alkane (decomposes it)
CFCs deplete ozone via Cl• radical chain reactions; DDT causes biomagnification up the food chain

Classification and Nomenclature

Haloalkanes have a halogen attached to an sp³ carbon; haloarenes have a halogen attached directly to an aromatic ring.

Primary (1°): halogen on carbon with one alkyl group; e.g., CH₃CH₂Cl (chloroethane)
Secondary (2°): halogen on carbon with two alkyl groups; e.g., (CH₃)₂CHCl
Tertiary (3°): halogen on carbon with three alkyl groups; e.g., (CH₃)₃CCl

Methods of Preparation

Free-radical halogenation of alkanes (UV light): CH₄ + Cl₂ → CH₃Cl + HCl
Addition of HX to alkenes (Markovnikov): CH₂=CH₂ + HBr → CH₃CH₂Br
Reaction of alcohols with HX, PCl₃, PCl₅, or SOCl₂; SOCl₂ is preferred (gives pure product with SO₂ and HCl as gases)
Halogen exchange (Finkelstein reaction): RCl + NaI → RI + NaCl (in dry acetone)
Swarts reaction: RCl + AgF → RF + AgCl (preparation of fluoroalkanes)

Nucleophilic Substitution: SN1 vs SN2

SN2 proceeds in a single step with backside attack and inversion of configuration, while SN1 forms a planar carbocation intermediate first, leading to racemisation.

SN2: One step; back-side attack; inversion of configuration (Walden inversion); favoured by primary substrates and polar aprotic solvents (DMF, DMSO, acetone)
SN1: Two steps; carbocation intermediate; racemisation; favoured by tertiary substrates and polar protic solvents (water, ethanol)
Reactivity toward SN1: 3° > 2° > 1°; Reactivity toward SN2: 1° > 2° > 3°

SN2 reaction: the nucleophile attacks the carbon from the side opposite the leaving group X, passing through a five-coordinate transition state and inverting the configuration

The S_N2 mechanism: the nucleophile attacks carbon from the side opposite the leaving group in a single concerted step, passing through a trigonal-bipyramidal (sp²-like) transition state and inverting the configuration (Walden inversion). Rate depends on both substrate and nucleophile; favoured by 1° substrates. Image: Pillsmarch, CC BY 4.0, via Wikimedia Commons.

SN1 reaction: the alkyl halide first ionises to a planar carbocation intermediate, which the nucleophile then attacks from either face

The S_N1 mechanism: the leaving group departs first to give a planar carbocation, which the nucleophile then attacks from either face (giving racemisation). Rate depends only on the substrate concentration; favoured by 3° substrates and polar protic solvents. Image: Pillsmarch, CC BY 4.0, via Wikimedia Commons.

Elimination Reactions

Dehydrohalogenation with strong base (KOH/alcohol): produces alkene; follows Zaitsev's rule (more substituted alkene is major product)
E2 (one step, bimolecular) and E1 (two-step via carbocation) mechanisms

Stereochemistry

Chiral centre: carbon bonded to four different groups
R/S configuration: assign priorities by CIP rules; clockwise = R, anticlockwise = S
Enantiomers: non-superimposable mirror images; equal and opposite optical rotation
Racemic mixture: 50:50 mixture of enantiomers; optically inactive

Haloarenes

C-X bond in haloarenes is stronger (partial double-bond character due to resonance) than in haloalkanes
Less reactive than haloalkanes toward nucleophilic substitution
Nucleophilic substitution possible only under drastic conditions (high T and P) or when strongly electron-withdrawing groups are present ortho/para
Electrophilic aromatic substitution: halogen is deactivating but ortho/para directing (due to resonance)

Grignard Reagent

RX + Mg in dry ether → RMgX (Grignard reagent)
Reacts with carbonyl compounds to give alcohols; with CO₂ to give carboxylic acids; with water to give alkane
Must be prepared and used under strictly anhydrous conditions

Environmental Effects

Freons (chlorofluorocarbons, CFCs): used as refrigerants; deplete the ozone layer by releasing Cl• radicals in the stratosphere
DDT (dichlorodiphenyltrichloroethane): first synthetic insecticide; persistent organic pollutant; causes biomagnification; now banned in most countries

Reactions with Metals

Wurtz reaction: 2 R-X + 2 Na (dry ether) → R-R + 2 NaX; used to make symmetrical alkanes with an even number of carbons
Wurtz-Fittig reaction: aryl halide + alkyl halide + Na → alkylarene (couples an aryl and an alkyl group)
Fittig reaction: 2 aryl halides + Na → biaryl (e.g. two C₆H₅X → biphenyl)
Grignard formation (RX + Mg) already covered — a further metal-insertion reaction

Common Nucleophilic Substitutions and Ambident Nucleophiles

-OH (aq. KOH) → alcohol; -OR (Williamson) → ether; -CN → nitrile; -SH → thiol; -NH₂ → amine
KCN vs AgCN: KCN (ionic) attacks through C → alkyl cyanide (nitrile); AgCN (covalent) attacks through N → alkyl isocyanide
KNO₂ vs AgNO₂: KNO₂ gives alkyl nitrite (R-O-N=O); AgNO₂ gives nitroalkane (R-NO₂)
These are ambident nucleophiles — they can attack through either of two atoms, giving different products depending on the counter-ion
Reduction: R-X + LiAlH₄ (or Zn/HCl) → R-H (alkane)

Polyhalogen Compounds

Chloroform (CHCl₃): solvent and former anaesthetic; slowly oxidised by air/light to poisonous phosgene (COCl₂), so stored in dark bottles filled to the brim with a little ethanol added
Iodoform (CHI₃): yellow solid, antiseptic (action due to liberated iodine); formed in the iodoform test
Carbon tetrachloride (CCl₄): solvent and former fire extinguisher (Pyrene); harmful to the ozone layer
Freons (CFCs) and DDT: covered under environmental effects; both persistent and ozone-depleting/biomagnifying

🚀 JEE Advanced Edge

Ozone depletion chain mechanism: UV light splits a C-Cl bond in a CFC, releasing a Cl• radical. Cl• reacts with O₃ to form ClO• + O₂; ClO• then reacts with another O atom to regenerate Cl•, which goes on to destroy more ozone — each Cl atom can destroy thousands of O₃ molecules before being deactivated, explaining why even trace CFC releases cause large-scale ozone depletion.

Why retention of configuration occurs in SN1 (sometimes) vs full racemisation: If the leaving group doesn't fully depart before the nucleophile attacks (tight/contact ion pair), there can be a slight preference for attack from the same side the leaving group departed, giving partial (not full 50:50) racemisation with a slight excess of inverted product — full racemisation is the idealised case assuming a completely free, planar carbocation.

Worked problem: Predict the major product when 2-bromobutane reacts with alcoholic KOH vs aqueous KOH. Approach: Alcoholic KOH favours E2 elimination (strong base, low nucleophilicity in alcohol solvent) → major product is but-2-ene (Zaitsev, more substituted alkene). Aqueous KOH favours SN2 substitution (better nucleophile in protic solvent, weaker base character relatively) → major product is butan-2-ol. The SAME reagent (KOH) gives opposite dominant pathways depending purely on solvent choice.

2 Revise ~2 min before the exam

🔑 Key Reactions

SN1: two steps via a carbocation; rate ∝ [substrate] only; favoured by 3° halides and polar protic solvents; gives racemisation
SN2: one step, backside attack; rate ∝ [substrate][nucleophile]; favoured by 1° halides; inverts configuration (Walden inversion)
Reactivity order (SN2): 1° > 2° > 3° | (SN1): 3° > 2° > 1°
Elimination (E1/E2): gives alkenes; Saytzeff rule — the more substituted alkene predominates
Haloarenes are unreactive toward substitution (resonance + partial double-bond character of C–X)
Wurtz reaction: 2R–X + 2Na → R–R + 2NaX (makes symmetrical alkanes)
Grignard reagent: R–Mg–X — a powerful nucleophile that builds C–C bonds; destroyed by water

3 Practice apply it

✍️ Worked Examples

Example 1 — SN1 vs SN2
Q: Would tert-butyl bromide react by SN1 or SN2, and what does its rate depend on?
Step 1 — It is a tertiary halide, too crowded for backside SN2 attack.
Step 2 — It readily forms a stable 3° carbocation, favouring the two-step SN1 pathway.
Step 3 — In SN1 the slow step is carbocation formation, involving only the substrate.
Answer: SN1; the rate depends only on the substrate concentration. Note: because a planar carbocation forms, the product is racemic.

Example 2 — Why chlorobenzene resists substitution
Q: Why is chlorobenzene far less reactive toward nucleophilic substitution than chloroethane?
Step 1 — In chlorobenzene the chlorine lone pair delocalises into the ring, giving the C–Cl bond partial double-bond character.
Step 2 — This shortens and strengthens the bond, making it hard to break.
Step 3 — The ring's π electrons also repel incoming nucleophiles.
Answer: resonance strengthens the C–Cl bond, so chlorobenzene resists substitution. Note: it reacts only under forcing conditions (e.g. Dow process, high T and P).

Example 3 — Saytzeff product
Q: What is the major alkene from dehydrohalogenation of 2-bromobutane?
Step 1 — Removing HBr can give but-1-ene or but-2-ene.
Step 2 — Saytzeff's rule favours the more substituted (more stable) alkene.
Step 3 — But-2-ene has two alkyl groups on the double bond vs one for but-1-ene.
Answer: but-2-ene is the major product. Key idea: more substituted alkenes are stabilised by hyperconjugation.