1. Classification and Nature of C–X Bond

Replacement of hydrogen atom(s) in a hydrocarbon by halogen atom(s) results in formation of alkyl halide (haloalkane) and aryl halide (haloarene).

1.1 Classification Based on Carbon Hybridization

• sp³ C–X Bond:
  • Alkyl halides (R–X): Halogen is bonded to an alkyl group. Can be primary (1°), secondary (2°), or tertiary (3°).
  • Allylic halides: Halogen is bonded to an sp³ hybridized carbon adjacent to a carbon-carbon double bond (allylic carbon). Example: CH₂=CH–CH₂X
  • Benzylic halides: Halogen is bonded to an sp³ hybridized carbon attached to an aromatic ring. Example: C₆H₅CH₂X
• sp² C–X Bond:
  • Vinylic halides: Halogen is bonded directly to an sp² hybridized carbon of a carbon-carbon double bond. Example: CH₂=CHX
  • Aryl halides (Haloarenes): Halogen is bonded directly to the sp² hybridized carbon of an aromatic ring. Example: C₆H₅X

1.2 Nature of C–X Bond

Halogens are more electronegative than carbon. Thus, the carbon-halogen bond is polarized: the carbon atom bears a partial positive charge (δ+) and the halogen atom bears a partial negative charge (δ−).

• Bond length: Increases down the group (C–F < C–Cl < C–Br < C–I) as atomic size increases.
• Bond enthalpy: Decreases down the group (C–F > C–Cl > C–Br > C–I) because longer bonds are weaker.
• Dipole moment: Decreases in the order CH₃Cl > CH₃F > CH₃Br > CH₃I. (CH₃Cl has a higher dipole moment than CH₃F because the bond length d for C-Cl is much larger, compensating for the smaller charge difference q in μ = q × d).

2. Methods of Preparation of Haloalkanes

2.1 From Alcohols (Best Methods)

The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides, or thionyl chloride.

2.2 From Hydrocarbons

• Free Radical Halogenation: Cl₂/UV light gives a complex mixture of isomeric mono- and polyhaloalkanes which is difficult to separate. (Not a good lab method).
• Electrophilic Addition to Alkenes:
  CH₂=CH–CH₃ + HBr → CH₃–CH(Br)–CH₃ (Major, Markovnikov)

  Peroxide effect (Anti-Markovnikov): Occurs ONLY with HBr in presence of peroxide. Radical mechanism.

• Addition of Halogens: Br₂ in CCl₄ adds to alkenes to form vicinal dibromides (Reddish-brown color of Br₂ disappears – test for unsaturation).

2.3 Halogen Exchange Reactions

• Finkelstein Reaction (for R-I):
  R–X + NaI → (Dry Acetone) → R–I + NaX↓ (X = Cl, Br). NaX precipitates out in acetone, driving the reaction forward (Le Chatelier's principle).
• Swarts Reaction (for R-F):
  Heating alkyl chloride/bromide in presence of metallic fluorides like AgF, Hg₂F₂, CoF₂ or SbF₃.
  CH₃Br + AgF → CH₃F + AgBr

3. Preparation of Haloarenes

3.1 Electrophilic Substitution

Aryl chlorides and bromides can be prepared by electrophilic substitution of arenes with Cl₂/Br₂ in presence of Lewis acid catalysts (iron or iron(III) chloride).

Reaction with iodine requires an oxidizing agent (HNO₃, HIO₄) to oxidize the HI formed, as the reaction is reversible.

3.2 Sandmeyer's Reaction

When primary aromatic amine (aniline) is treated with NaNO₂ + HCl at cold temp (0-5°C), a benzene diazonium salt is formed. Treating this with cuprous chloride (Cu₂Cl₂) or cuprous bromide (Cu₂Br₂) replaces the diazo group with -Cl or -Br.

Note: Replacement by iodine does not require cuprous halide; just warming with KI is enough.

4. Chemical Reactions of Haloalkanes

4.1 Nucleophilic Substitution Reactions

The highly electronegative halogen atom is replaced by a nucleophile (Nu⁻).

S_N2 Mechanism (Substitution Nucleophilic Bimolecular)

• Nucleophile approaches the carbon atom from the side opposite to the leaving group (halogen).
• It is a one-step concerted mechanism. A transition state is formed where both incoming and outgoing groups are partially attached.
• Inversion of configuration: If the reactant is optically active, the product has inverted stereochemistry (Walden inversion).
• Reactivity order: CH₃X > 1° > 2° > 3° (Due to Steric hindrance; bulky groups block the attack of nucleophile).

S_N1 Mechanism (Substitution Nucleophilic Unimolecular)

• It is a two-step process.
  Step 1: Slow cleavage of C-X bond to form a carbocation (Rate determining step).
  Step 2: Fast attack of nucleophile on the carbocation.
• Racemization: The carbocation is planar, so the nucleophile can attack from either side, resulting in a 50:50 mixture of enantiomers (racemic mixture) if the chiral carbon is reacting.
• Reactivity order: 3° > 2° > 1° > CH₃X (Due to stability of the carbocation intermediate: 3° > 2° > 1°).
• Note: Allylic and benzylic halides show high reactivity towards S_N1 because the corresponding carbocations are resonance stabilized.

4.2 Elimination Reactions (β-Elimination)

When haloalkane is heated with alcoholic KOH, a hydrogen from the β-carbon and the halogen from the α-carbon are eliminated to form an alkene.

• Zaitsev (Saytzeff) Rule: If more than one alkene can be formed, the more highly substituted alkene (alkene with more alkyl groups attached to the double bonded carbons) is the major product.
• Example: 2-Bromobutane + alc. KOH → But-2-ene (Major, 81%) + But-1-ene (Minor, 19%).

4.3 Reaction with Metals

• Grignard Reagent: RX + Mg → (dry ether) → RMgX. The C-Mg bond is highly polar. Grignard reagents are highly reactive and react with any source of proton (water, alcohols) to give hydrocarbons: RMgX + H₂O → RH + Mg(OH)X.
• Wurtz Reaction: 2RX + 2Na → (dry ether) → R-R + 2NaX. Used for preparing symmetrical alkanes with double the number of carbon atoms.

5. Chemical Reactions of Haloarenes

Haloarenes are extremely less reactive towards nucleophilic substitution compared to haloalkanes. Why?

1. Resonance effect: Lone pairs on halogen are in conjugation with the benzene ring. The C-X bond gains partial double bond character, making it shorter and stronger.
2. Difference in hybridization: The carbon atom in C-X of haloarenes is sp² hybridized (more electronegative = holds electron pair more tightly), compared to sp³ in haloalkanes.
3. Instability of phenyl cation: Phenyl cation formed in an S_N1 mechanism is extremely unstable because the positive charge is on an sp² carbon.

5.1 Replacement by Hydroxyl Group (Dow's Process)

Chlorobenzene can be converted to phenol but requires extreme conditions: aqueous NaOH at 623 K and 300 atm pressure. Presence of electron-withdrawing groups (-NO₂) at ortho and para positions heavily increases reactivity.

5.2 Electrophilic Substitution Reactions

Halogen atom on benzene ring is ortho/para directing but deactivating. Reagents include Nitration (HNO₃/H₂SO₄), Sulfonation (conc. H₂SO₄), Halogenation (Cl₂/FeCl₃), and Friedel-Crafts alkylation/acylation.

5.3 Reaction with Metals

• Wurtz-Fittig Reaction: 1 Aryl halide + 1 Alkyl halide + 2Na → (dry ether) → Alkylbenzene.
• Fittig Reaction: 2 Aryl halides + 2Na → (dry ether) → Diphenyl (biphenyl).

6. Stereochemistry Foundations

• Chiral Carbon: A carbon atom attached to 4 different groups. The molecule lacks a plane of symmetry.
• Enantiomers: Non-superimposable mirror images of each other. They rotate plane-polarized light in opposite directions.
• Racemic Mixture: An equimolar mixture of two enantiomers. It is optically inactive due to external compensation. Racemization occurs in S_N1 mechanism.
• Walden Inversion: The turning inside out of an umbrella shape. Perfect inversion of configuration occurs in S_N2 mechanism.

🎓 NEET Previous Year Questions

📝 S_N1 vs S_N2 and Reactivity (Q1 – Q7)

💡 Reactions and Mechanisms (Q8 – Q14)

📈 Organic Conversions (Q15 – Q22)