Haloalkanes and Haloarenes

1. Classification, Naming & the C-X Bond

Replace H of a hydrocarbon by halogen: haloalkane (R–X) if X is on an sp³ carbon, haloarene (Ar–X) if X is on an aromatic ring carbon. The type of carbon decides the reactivity, so classify first.

IUPAC naming. Halo groups are prefixes (fluoro, chloro, bromo, iodo) and are alphabetised with other substituents. Number the chain for the lowest locants. In benzene derivatives the o-, m-, p- of common names become 1,2 / 1,3 / 1,4.

• CH₃CH(Cl)CH₃ : 2-chloropropane (isopropyl chloride); (CH₃)₃CBr : 2-bromo-2-methylpropane (tert-butyl bromide)
• CH₂=CHCl : chloroethene (vinyl chloride); CH₂=CHCH₂Br : 3-bromopropene (allyl bromide)
• C₆H₅CH₂Cl : chlorophenylmethane (benzyl chloride); CHCl₃ : trichloromethane; CCl₄ : tetrachloromethane
• gem-dihalide: both X on the same carbon (1,1-dichloroethane); vic-dihalide: X on adjacent carbons (1,2-dichloroethane).
• Isomer counts: C₄H₉Br has 4 isomers; C₅H₁₁Br has 8.

The C–X bond is polar: carbon is δ⁺, halogen δ⁻. This polarity is why haloalkanes undergo nucleophilic substitution.

Down the group, bond length increases and bond enthalpy decreases, so C–I breaks most easily (best leaving group). The dipole moment does not follow a simple trend: CH₃Cl is highest, because dipole depends on charge and bond length.

2. Methods of Preparation

A. Haloalkanes from alcohols (best method)

Aryl halides cannot be made from phenol this way: the C–O bond in phenol has partial double bond character and is hard to break.

B. Haloalkanes from hydrocarbons

• Free radical halogenation of alkanes (Cl₂, UV): gives a mixture of isomers, so poor yield of any one compound.
• Alkene + HX follows Markovnikov's rule: CH₃CH=CH₂ + HI → CH₃CHICH₃ (major). HBr with peroxide gives the anti-Markovnikov product CH₃CH₂CH₂Br (peroxide effect works for HBr only).
• Alkene + Br₂ in CCl₄ gives a colourless vic-dibromide; loss of the red-brown colour tests for a double bond.
• Side chain vs ring: toluene + Br₂ with heat/UV → C₆H₅CH₂Br (free radical, benzylic). Toluene + Br₂ with Fe in the dark → o- and p-bromotoluene (electrophilic). Ethylbenzene + Br₂/heat → C₆H₅CH(Br)CH₃.

C. Halogen exchange

• Finkelstein: R–Cl/R–Br + NaI in dry acetone → R–I + NaCl/NaBr↓. The precipitate leaves the solution, so the reaction goes forward (Le Chatelier).
• Swarts: R–Cl/R–Br + AgF (or Hg₂F₂, CoF₂, SbF₃) → R–F. Best route to alkyl fluorides.

D. Haloarenes

• Electrophilic substitution: benzene/arene + Cl₂ or Br₂ with Fe or FeCl₃ (Lewis acid). Iodination is reversible and needs an oxidising agent (HNO₃/HIO₃); fluorination is too violent.
• Sandmeyer: Ar–NH₂ → (NaNO₂/HX, 273–278 K) → ArN₂⁺X⁻ → (CuCl/HCl or CuBr/HBr) → ArCl/ArBr + N₂. For iodine, just use KI; no copper needed.

3. Physical Properties

• Boiling point: higher than the parent hydrocarbon (dipole-dipole forces plus greater mass). For the same R: RI > RBr > RCl > RF, because a larger halogen has stronger van der Waals forces.
• Branching lowers the boiling point (smaller surface area): n-butyl bromide 375 K > sec-butyl 364 K > tert-butyl 346 K (2025 board MCQ).
• Melting point of dihalobenzenes: the para isomer is highest (p-dichlorobenzene 323 K; ortho 256 K; meta 249 K) because its symmetry packs better in the crystal. Their boiling points are almost equal.
• Density: bromo, iodo and polychloro compounds are denser than water.
• Solubility: only slightly soluble in water (the new haloalkane-water attractions are weaker than the H-bonds broken), but freely soluble in organic solvents.

4. Haloalkanes: Nucleophilic Substitution, Elimination & Metals

The nucleophile (Nu⁻) replaces X⁻: R–X + Nu⁻ → R–Nu + X⁻.

Ambident nucleophiles (CN⁻, NO₂⁻) have two nucleophilic sites. The ionic salt attacks through the more nucleophilic atom, the covalent silver salt through the lone pair on N.

SN2 and SN1 side by side

• Leaving group: I⁻ > Br⁻ > Cl⁻ >> F⁻ in both mechanisms (weaker C–X bond, better leaving ability).
• Allylic and benzylic halides are reactive in SN1 because the carbocation is resonance stabilised; primary ones are also fast in SN2.
• Worked comparisons (NCERT): isomeric bromobutanes in SN1: n-butyl < isobutyl < sec-butyl < tert-butyl; in SN2 the exact reverse. Between 1-iodobutane and 1-chlorobutane, the iodide is faster (better leaving group).

Stereochemistry in three lines

• Chiral molecule: non-superimposable on its mirror image, usually because of a carbon with four different groups (asymmetric or stereocentre). Examples: butan-2-ol, 2-chlorobutane, CHBrClI. Achiral: propan-2-ol, 2-bromopropane.
• Enantiomers are mirror images. They share every physical property (m.p., b.p., refractive index) and chemical reactivity with achiral reagents, but rotate plane-polarised light equally in opposite directions (+ dextro, − laevo).
• Racemic mixture (±): 50:50 mixture of enantiomers, optically inactive because the rotations cancel; the process is racemisation.

Inversion, retention, racemisation: if the bond at the stereocentre is broken, the outcome is inversion (SN2), racemisation (SN1) or, rarely, retention. If no bond to the stereocentre is broken, configuration is retained.

Elimination reactions

Heating R–X having a β-hydrogen with alcoholic KOH removes H (from β-carbon) and X (from α-carbon) to give an alkene (β-elimination, dehydrohalogenation).

• Zaitsev (Saytzeff) rule: the more substituted (more stable) alkene is the major product. 2-Bromopentane → pent-2-ene (81%) over pent-1-ene (19%). Likewise 1-chloropropane → propene, and 2-bromobutane → but-2-ene.
• Aqueous KOH → substitution (alcohol); alcoholic KOH → elimination (alkene). In alcohol, the alkoxide RO⁻ is a stronger base than OH⁻ and pulls out the β-proton.
• Substitution and elimination compete: bulky or strong bases and 3° halides favour elimination; 1° halides with good nucleophiles favour SN2.

Reactions with metals

• Grignard reagent: R–X + Mg (dry ether) → R–MgX. The C–Mg bond is polar with carbon δ⁻, so RMgX acts as a strong base and nucleophile. It reacts with any proton source: RMgX + H₂O → R–H + Mg(OH)X. This is why it must be made under anhydrous conditions (2024 board). Using D₂O gives R–D.
• Wurtz reaction: 2R–X + 2Na (dry ether) → R–R + 2NaX (doubles the carbon chain).

5. Haloarenes: Why Less Reactive

Reasons for low reactivity in nucleophilic substitution (any two for 2 marks, 2026 board):

1. Resonance: the halogen lone pair conjugates with the ring, giving C–Cl partial double bond character (C–Cl 169 pm in chlorobenzene vs 177 pm in a haloalkane), so it is harder to break.
2. sp² carbon (33% s-character) is more electronegative than sp³ (25%) and holds the C–X electrons more tightly, making a shorter, stronger bond.
3. Phenyl cation is unstable (no resonance stabilisation), so SN1 is ruled out.
4. The electron-rich π cloud repels the incoming nucleophile.

Forcing the reaction: chlorobenzene + NaOH at 623 K, 300 atm → sodium phenoxide → phenol (after H⁺).

Nitro groups help, but only at ortho and para. An –NO₂ group at o/p withdraws electron density and stabilises the negatively charged intermediate by resonance (the charge sits on the carbon next to –NO₂). A meta –NO₂ cannot delocalise that charge, so it has no effect.

Electrophilic substitution. Haloarenes undergo halogenation, nitration, sulphonation and Friedel-Crafts reactions. Halogens are deactivating yet ortho/para-directing (NCERT Example 10.9):

• The –I effect withdraws electrons and slows the reaction (deactivation).
• The +R effect (lone-pair donation) enriches the o/p positions, so it controls the position of attack.
• Para is the major product; ortho is minor (steric hindrance). Example: chlorobenzene + Cl₂/FeCl₃ → 1,4-dichlorobenzene (major).

Reactions with sodium (dry ether)

• Fittig: 2 C₆H₅Cl + 2Na → C₆H₅–C₆H₅ (biphenyl) + 2NaCl (2025 board).
• Wurtz-Fittig: C₆H₅Cl + RCl + 2Na → C₆H₅–R (alkylbenzene).

6. Polyhalogen Compounds

7. Quick Sheet & Last-Minute Checklist

Before the exam, check:

1. Can I classify a halide as allylic, benzylic, vinylic or aryl, and name it in IUPAC form?
2. Can I write the product and reagent for each of the eight name reactions above?
3. Can I explain SN1 versus SN2 with rate law, stereochemistry and reactivity order?
4. Can I decide whether a molecule is chiral, and predict inversion or racemisation?
5. Can I give two reasons for the low reactivity of chlorobenzene, and explain the nitro-group effect?
6. Can I explain why a Grignard reagent needs dry ether and why chloroform is kept in dark, full bottles?