Revision: Organic Compounds Containing Halogens JEE Main Organic Compounds Containing Halogens

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.

C_nH_2n+1OH

C_nH_2n+2

Common Names:

• Alkyl halide or aryl halide
• e.g., CH₃Cl → Methyl chloride; CH₂=CHCl → Vinyl chloride

IUPAC Names:

• Haloalkane or arylhalide
• Rule 1: Find the longest carbon chain containing the halogen. If a double/triple bond is present, give it priority.
• Rule 2: Number from the end nearer the first substituent. Assign each substituent a position number.
• Multiple same halogens → di-, tri-, tetra- prefix.
• Different halogens → list alphabetically and number to give the alphabetically first halogen the lowest possible number.

Examples:

• CH₃Cl → Chloromethane
• CH₂=CHCl → Chloroethene
• (CH₃)₃CCl → 2-Chloro-2-methylpropane (common: tert-butyl chloride)
• 2-Chloro-1-methylbenzene → o-Chlorotoluene → IUPAC: 1-Chloro-2-methylbenzene

On the Basis of Number of Halogen Atoms-

Gem dihalide: Both halogens on the same carbon (e.g., 1,1-dichloroethane).
Vic dihalide: Halogens on adjacent carbons (e.g., 1,2-dichloroethane).

On the Basis of Type of Carbon Bearing the Halogen:

Alkyl halide carbon (with X) is sp³ hybridised; aryl halide carbon is sp² hybridised — this is why aryl C–X bond is shorter and stronger.

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.

With magnesium: \[\ce{RX + Mg ->[Dry][Ether] RMgX (Grignard reagent)}\]

With sodium (Wurtz Reaction) \[\ce{-> RX + 2Na + XR ->[Dry ether] R - R + 2NaX}\]

Reduction: \[\ce{RX + 2H ->[Zn/HCl (conc)][or Zn-Cu/C2H5OH]RH + HX}\]

Aryl halides are less reactive than alkyl halides in nucleophilic substitution. Due to resonance effect, lone pair on halogen is delocalized into benzene ring.

This gives partial double bond character to C–X bond → bond becomes shorter & stronger.
Strong electron-withdrawing groups (EWGs) like –NO₂ increase reactivity.
EWGs must be at ortho or para positions for effective substitution.

Example reaction:
p-chloronitrobenzene + OH⁻ → p-nitrophenol (Cl replaced by OH).

Mechanism is SNAr (Addition–Elimination).

• Step 1: Nucleophile attacks carbon bearing halogen → forms intermediate.
• Step 2: Leaving group (Cl⁻) departs → aromaticity restored.
• Reactivity order:
  More –NO₂ groups = higher reactivity
  (Tri-NO₂ > Di-NO₂ > Mono-NO₂ > no EWG)

• Haloarenes undergo electrophilic substitution at a slower rate than benzene (halogen is deactivating due to –I effect).
• However, halogen is an ortho/para director (due to +M/resonance effect — lone pair donation to ring at ortho and para positions increases electron density there).
• Chlorine's single electron pair engages in resonance with the ring → electron density rises at ortho and para positions → electrophile attacks there.