Revision: Halogen Derivatives of Alkanes (And Arenes) >> Haloalkanes Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

Compounds in which one or more hydrogen atom(s) of an alkane is (are) replaced by halogen.

General formula: R–X.

Compounds in which one or more hydrogen atom(s) directly bonded to an aromatic ring is (are) replaced by halogen.

General formula: Ar–X.

A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is called a racemic mixture or a racemic modification.

The C–X bond in alkyl halides is polarised (Cδ+–Xδ–), making alkyl halides reactive towards nucleophiles.

Two Types of S_N Reactions

S_N1 (Unimolecular Nucleophilic Substitution):

• First-order kinetics: Rate = k[RX] (depends only on substrate concentration)
• Two-step mechanism: Step 1 (slow) — ionisation to form carbocation; Step 2 (fast) — attack by nucleophile.
• Intermediate: Trigonal planar carbocation.
• More substituted alkyl halides react faster (more stable carbocation).
• Reactivity order: R₃CX > R₂CHX > RCH₂X (3° > 2° > 1°)
• Gives a racemic mixture (optically inactive product) because the nucleophile can attack from both faces.
• For aryl/vinyl halides: Ar₂CX > Ar₂CHX > ArCH₂X = CH₂=CHX > CH₂=CHCH₂X

S_N2 (Bimolecular Nucleophilic Substitution):

• Second-order kinetics: Rate = k[RX][Nu] (depends on both substrate and nucleophile concentration)
• One-step mechanism (concerted): Nucleophile attacks from the back side as leaving group departs simultaneously → Transition State is formed.
• Results in Walden Inversion (inversion of configuration at the carbon — stereochemistry inverted).
• Reactivity order: Methyl halide > Primary > Secondary > Tertiary (CH₃X > 1° > 2° > 3°)
• The S_N2 reaction rate depends on the concentration of both alkyl halide and nucleophile.

β-Elimination Reaction:

• When alkyl halides are heated with alcoholic KOH or KNH₂, they undergo β-elimination of HX to form an alkene (new π bond).
• The carbon directly attached to X = α-carbon; the carbon adjacent to it = β-carbon.
• Order of reactivity in elimination: R–Cl < R–Br < R–I

Saytzeff's Rule (Zaitsev's Rule):

• In unsymmetrical alkyl halides, hydrogen is preferentially eliminated from the β-carbon with fewer hydrogen atoms → forms the more highly substituted alkene (major product).
• e.g., 2-bromopentane → pent-2-ene (81%) [major] + pent-1-ene (19%) [minor]

Types of Elimination:

1. α-elimination: Atom or group lost from the same carbon (gives carbene intermediates).
2. β-elimination: H from β-carbon, X from α-carbon → alkene.

• E₁ reaction: Two steps (similar mechanism to S_N1)
• E₂ reaction: One step (concerted, anti-periplanar geometry required — similar to S_N2 but gives alkene)

Dehydrohalogenation:

• Loss of HX from alkyl halide with alc. KOH → alkene.

\[\ce{\underset{Alky halide}{C_{n}H_{2n + 1}X} ->[Alcholic KOH] \underset{Alkene}{C_{n}H_{2n}} + KX + H2O}\]

• With NaOH, Con. NH₃, t-BuONa, KNH₂, NaNH₂: elimination also occurs.