Haloalkanes and HaloarenesNEET Chemistry · Class 12 · NCERT Chapter 6

Introduction and Classification

Haloalkanes (alkyl halides) are compounds formed by replacing one or more hydrogen atoms of an alkane with a halogen atom (F, Cl, Br, or I). They have the general formula R-X, where R is an alkyl group and X is a halogen.

Haloarenes (aryl halides) are compounds where the halogen is directly attached to an aromatic ring. The most common example is chlorobenzene (C6H5Cl).

Classification of Haloalkanes

Based on the number of halogen atoms, haloalkanes are classified as mono, di, tri, or polyhalogen compounds. Based on the carbon bearing the halogen:

Classification of Haloarenes

• Aryl halide: halogen directly on benzene ring — C6H5Cl (chlorobenzene), C6H5Br (bromobenzene)
• Aralkyl halide: halogen on the side chain — C6H5CH2Cl (benzyl chloride). This behaves more like a haloalkane than a haloarene.

IUPAC Nomenclature

Rules for Haloalkanes

1. Choose the longest carbon chain containing the halogen-bearing carbon as the parent chain.
2. Number the chain from the end that gives the halogen the lowest possible locant.
3. Halogens are named as prefixes: fluoro-, chloro-, bromo-, iodo- in alphabetical order if more than one.
4. If there is a functional group of higher priority (e.g., -OH, -COOH), number from that end instead.

Rules for Haloarenes

For monosubstituted benzene, simply prefix the halogen to benzene: chlorobenzene, bromobenzene. For disubstituted, use ortho (1,2-), meta (1,3-), para (1,4-) prefixes or IUPAC locants. When other substituents are present, the ring is numbered to give the lowest set of locants.

• C6H5Cl: chlorobenzene (or 1-chlorobenzene)
• 1,2-C6H4Cl2: 1,2-dichlorobenzene (o-dichlorobenzene)
• 4-ClC6H4CH3: 1-chloro-4-methylbenzene (p-chlorotoluene)
• C6H5CH2Cl: (chloromethyl)benzene (benzyl chloride)

Nature of the C-X Bond

The carbon-halogen bond is polar covalent. Because halogens are more electronegative than carbon, the bonding electrons are pulled towards the halogen, giving carbon a partial positive charge ($\delta +$) and halogen a partial negative charge ($\delta -$). This polarity makes carbon electrophilic and susceptible to nucleophilic attack.

Key trends: Bond length increases C-F to C-I (as atomic radius of halogen increases). Bond strength (enthalpy) decreases C-F to C-I. Therefore, C-I is the weakest and most reactive bond; C-F is the strongest and least reactive.

Dipole moment order: C-F > C-Cl > C-Br > C-I (because both electronegativity difference and bond length matter; electronegativity dominates here). However, CHF3 has lower dipole moment than CHCl3 because F is small and bond length is short.

Preparation of Haloalkanes

1. From Alcohols

• With HX (Lucas reagent): R-OH + HX → R-X + H2O. Reactivity: HI > HBr > HCl. Lucas test: ZnCl2/HCl. 3° alcohol reacts immediately (turbidity); 2° reacts in 5 min; 1° does not react at room temperature.
• With SOCl2 (thionyl chloride): R-OH + SOCl2 → R-Cl + SO2 + HCl. Best method for making alkyl chlorides because SO2 and HCl are gaseous byproducts, giving pure product. Reaction proceeds with retention of configuration in pyridine (SNi mechanism).
• With PCl5: R-OH + PCl5 → R-Cl + POCl3 + HCl.
• With PCl3: 3 R-OH + PCl3 → 3 R-Cl + H3PO3.
• With PBr3 or PI3 (in situ): for alkyl bromides and iodides, red P + Br2 or P + I2 generate PBr3 and PI3 in situ.

2. From Alkenes

• Addition of HX: follows Markovnikov's rule (H adds to the C with more H). CH2=CH2 + HBr → CH3CH2Br.
• Anti-Markovnikov addition (peroxide effect): in presence of peroxide (ROOR), HBr adds anti-Markovnikov via free radical mechanism. Only HBr shows this; HCl and HI do not.
• Halogenation: alkene + Cl2/Br2 → vicinal dihalide. Cyclic halonium intermediate gives anti addition.

3. Halogen Exchange (Finkelstein and Swartz Reactions)

• Finkelstein reaction: R-Cl + NaI (in dry acetone) → R-I + NaCl. NaCl precipitates out, driving equilibrium forward. Also: R-Br + NaI → R-I + NaBr.
• Swartz reaction: R-Cl + AgF → R-F + AgCl. Uses silver fluoride to make fluoroalkanes. (Also: HgF2 or SbF3.)

4. From Hydrocarbons

• Free radical halogenation of alkanes: CH4 + Cl2 (hv) → CH3Cl + HCl. Selectivity: F2 is too reactive; I2 is too unreactive; Cl2 and Br2 are practical.
• Electrophilic aromatic substitution (for haloarenes): benzene + Cl2 (FeCl3/AlCl3) → chlorobenzene + HCl.

SN1 and SN2 Mechanisms

Nucleophilic substitution reactions are the most important reactions of haloalkanes. A nucleophile (Nu:) attacks the electrophilic carbon bearing the halogen (the leaving group) and replaces it.

SN2 Mechanism (Substitution Nucleophilic Bimolecular)

SN2 is a one-step concerted mechanism. The nucleophile attacks the back side of the carbon bearing the leaving group (180° to the C-X bond) while the leaving group departs simultaneously. No intermediate is formed.

• Rate: rate = k [RX][Nu]. Depends on both reactant concentrations (bimolecular, second order overall).
• Stereochemistry: Walden inversion (complete inversion of configuration, like an umbrella turning inside-out). A pure R enantiomer gives a pure S product.
• Substrate order: CH3X > 1° > 2° > 3°. Tertiary haloalkanes cannot undergo SN2 because the three bulky groups block back-side attack (steric hindrance).
• Solvent: polar aprotic solvents (acetone, DMSO, DMF) favour SN2. These solvents solvate the cation but not the anion, leaving the nucleophile free and powerful.
• Nucleophile: strong nucleophiles (CN-, I-, OH-, RS-) favour SN2.

SN1 Mechanism (Substitution Nucleophilic Unimolecular)

SN1 is a two-step stepwise mechanism. First, the C-X bond breaks heterolytically to form a carbocation intermediate. Then, the nucleophile attacks the planar carbocation from either face.

• Rate: rate = k [RX]. Only depends on substrate concentration (unimolecular, first order). Rate-determining step is carbocation formation.
• Stereochemistry: racemisation (mixture of R and S products) because the planar carbocation can be attacked from both faces equally.
• Substrate order: 3° > 2° > 1° > CH3X. Tertiary carbocations are most stable (hyperconjugation and inductive effect), so 3° undergoes SN1 fastest.
• Solvent: polar protic solvents (water, alcohols, acetic acid) favour SN1. They stabilise the carbocation intermediate and the leaving group anion by solvation.
• Rearrangements: carbocation intermediates in SN1 can rearrange via hydride shift or methyl shift to form a more stable carbocation before the nucleophile attacks.

Common Nucleophilic Substitution Reactions

Grignard Reagent (RMgX)

Grignard reagent is formed by reacting a haloalkane with magnesium metal in dry diethyl ether: R-X + Mg (dry ether) → R-MgX. The carbon-magnesium bond is highly polar (C has partial negative charge), making it a powerful nucleophile and a strong base. It reacts with water, CO2, aldehydes, ketones, and esters. It must be kept absolutely dry because water destroys it: R-MgX + H2O → R-H + Mg(OH)X.

Elimination Reactions (E1 and E2)

When a haloalkane is treated with a strong base, elimination (loss of HX) can compete with substitution. The product is an alkene.

E2 Mechanism (Elimination Bimolecular)

E2 is a one-step concerted mechanism. The base abstracts a proton from the beta carbon at the same time the leaving group departs. The base and substrate both appear in the rate law: rate = k[RX][Base].

• Stereochemistry: requires anti-periplanar arrangement (180°) of H and leaving group. This is the anti elimination rule.
• Regioselectivity: Zaitsev's rule — the major product is the more substituted (more stable) alkene. E.g., 2-bromobutane + KOH/alc → mainly but-2-ene (more substituted), not but-1-ene.
• Favoured by: 3° substrates, strong bulky base (KOtBu), high temperature, polar aprotic solvent.

E1 Mechanism (Elimination Unimolecular)

E1 is a two-step stepwise mechanism. A carbocation forms first (same first step as SN1), then a base removes a beta proton. Rate = k[RX] (unimolecular). Favoured by: 3° substrate, weak base, polar protic solvent, high temperature. Gives Zaitsev product.

Substitution vs Elimination: How to Choose

• Strong, concentrated base + high temperature: elimination favoured
• Weak base or neutral nucleophile + low temperature: substitution favoured
• Bulky base (KOtBu): strongly favours E2 even for 1° substrates
• KOH in aqueous ethanol: favours substitution (SN2)
• KOH in alcoholic solution (alc. KOH): favours E2 elimination

Optical Isomerism in Haloalkanes

A haloalkane with a chiral centre (an sp3 carbon bearing four different groups) shows optical isomerism. The two non-superimposable mirror-image forms are called enantiomers.

• Dextrorotatory (+): rotates plane-polarised light clockwise.
• Laevorotatory (-): rotates plane-polarised light anticlockwise.
• Racemic mixture (±): equal mixture of both enantiomers. Optically inactive (rotations cancel out).
• R and S configuration: assigned by CIP (Cahn-Ingold-Prelog) rules. Arrange substituents in decreasing priority. If the arrangement (highest to lowest) is clockwise with the lowest priority group pointing away, the centre is R (rectus). If anticlockwise, it is S (sinister).

SN2 gives inversion: R substrate + Nu → S product (Walden inversion).

SN1 gives racemisation: chiral substrate → racemic product (planar carbocation attacked from both faces).

Example: 2-bromobutane (CH3CHBrC2H5) has a chiral centre. It exists as two enantiomers: (R)-2-bromobutane and (S)-2-bromobutane.

Polyhalogen Compounds and Their Uses

Haloarenes: Structure and Reactivity

Why Haloarenes Are Less Reactive Than Haloalkanes

In haloarenes, the halogen atom is directly bonded to the aromatic ring. The lone pairs on the halogen interact with the pi electron system of the ring by resonance (mesomeric effect). This delocalisation gives the C-X bond partial double-bond character, making it shorter and stronger than a typical C-X single bond in haloalkanes.

As a result, nucleophilic substitution is much more difficult in haloarenes. The C-X bond energy is higher and the carbon is less electrophilic because of electron donation from the ring via resonance.

Resonance Structures of Chlorobenzene

Chlorobenzene shows five resonance structures. In two of them, negative charge appears on the ortho and para carbons of the ring, and there is a C=Cl double bond. This confirms the partial double-bond character of C-Cl and explains why the C-Cl bond in chlorobenzene (169 pm) is shorter than that in cyclohexyl chloride (179 pm).

Electrophilic Aromatic Substitution of Haloarenes

Even though halogens are electron-withdrawing by induction (sigma framework), they areortho-para directing by resonance in electrophilic aromatic substitution. The lone pair donation from halogen to the ring activates the ortho and para positions towards electrophilic attack, but deactivates the ring overall (reaction is slower than benzene itself).

• Overall effect: halogen = deactivating (ring is less reactive than benzene) but ortho-para directing.
• Chlorobenzene + Cl2 (FeCl3) → mixture of 1,2-dichlorobenzene and 1,4-dichlorobenzene (ortho and para products). Very little 1,3-dichlorobenzene (meta product).

Nucleophilic Aromatic Substitution (NAS)

Haloarenes can undergo nucleophilic substitution under extreme conditions or when strongly electron-withdrawing groups are present at ortho/para positions.

• Dow process (industrial): chlorobenzene + NaOH (steam, 300°C, 200 atm) → sodium phenoxide → phenol + NaCl. Very harsh conditions needed.
• Meisenheimer complex mechanism: nucleophile adds first (addition step) to form a negatively charged cyclohexadienyl anion (Meisenheimer complex), then the leaving group departs. The rate-determining step is the addition, not the departure of halide.
• Activation by electron-withdrawing groups: NO2 groups at ortho/para positions stabilise the Meisenheimer complex by delocalising its negative charge. 2,4,6- trinitrochlorobenzene (picryl chloride) reacts even with weak nucleophiles.
• Benzyne intermediate mechanism: at moderate temperatures, some haloarene reactions proceed via a benzyne (dehydrobenzene) intermediate, where HX is eliminated first and then the nucleophile adds across the triple bond.

Wurtz-Fittig Reaction

Aryl halide + alkyl halide + 2Na → aryl-alkyl compound + 2NaX. Example: C6H5Br + CH3Br + 2Na → C6H5CH3 (toluene) + 2NaBr.

Fittig Reaction

Two molecules of aryl halide + 2Na → diaryl compound + 2NaX. Example: 2 C6H5Br + 2Na → C6H5-C6H5 (biphenyl) + 2NaBr.

Worked NEET Problems

Summary Cheat Sheet

Reactivity of Halogens

• C-X bond strength: C-F > C-Cl > C-Br > C-I
• C-X reactivity (ease of breaking): C-I > C-Br > C-Cl > C-F
• Leaving group ability: I- > Br- > Cl- > F-

SN1 vs SN2 One-Line Rules

• 3° + weak Nu + polar protic solvent = SN1 (racemisation)
• 1°/CH3 + strong Nu + polar aprotic solvent = SN2 (inversion)
• Benzyl and allyl substrates: both SN1 and SN2 fast (resonance-stabilised carbocation for SN1, good orbital overlap for SN2)

Key Name Reactions

• Finkelstein: R-Cl/R-Br + NaI (dry acetone) → R-I (halogen exchange)
• Swartz: R-Cl + AgF → R-F (fluoroalkane synthesis)
• Grignard: R-X + Mg (dry ether) → R-MgX (powerful nucleophile/base)
• Wurtz: 2 R-X + 2Na → R-R + 2NaX (coupling, symmetrical alkane)
• Wurtz-Fittig: ArX + RX + 2Na → Ar-R + 2NaX
• Fittig: 2 ArX + 2Na → Ar-Ar + 2NaX (biaryl synthesis)

Haloarenes vs Haloalkanes

• Haloarenes: less reactive (C-X has partial double bond character due to resonance)
• Haloarenes in EAS: ortho-para directing but ring-deactivating
• Haloarenes in NAS: need electron-withdrawing groups (NO2) at ortho/para OR extreme conditions
• Benzyl chloride (C6H5CH2Cl): behaves like haloalkane (allylic/benzylic resonance)