Revision: Class 12 >> Organic Compounds Containing Halogens NEET (UG) 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.

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.

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

• The C–X bond is polar because halogens are more electronegative than carbon. Carbon gets a partial positive charge (δ+), and halogen gets a partial negative charge (δ–).
• Bond strength increases as halogen size decreases: C–F > C–Cl > C–Br > C–I
• Bond length increases as halogen size increases: C–F < C–Cl < C–Br < C–I
• Reactivity in substitution reactions increases with longer/weaker bond: R–I > R–Br > R–Cl > R–F

Remember: Bond strength ↑ = reactivity ↓ = bond length ↓

Weakest C–X bond: Benzyl bromide (C₆H₅CH₂Br) — C–X bond in benzyl halides is much weaker than in vinyl bromide and bromobenzene because the benzyl cation is stabilised by resonance.

Formation of Alkyl Halide from Alcohols:

From Alkenes

(a) Addition of halogen acids:

(b) Allylic halogenation:

\[\ce{\underset{Propene}{CH3 - CH} = CH2 + Cl2 ->[775K] \underset{\underset{(Allyl chloride)}{3-Chloro-1-propene}}{ClCH2 - CH = CH2 + HCl}}\]

From alkanes (Swarts reaction)

Alkyl chloride or bromide react with AgF, SbF₃, or Hg₂F₂ give alkyl fluoride. This reaction is known Swarts reaction. Antimony trifluoride (SbF₂) is commonly used in this reaction.

2CH₃CH₃CI + Hg₂F₂→ 2CH₃CH₂F+ Hg₂Cl₂

From the halide exchange method (Finkelstein reaction)

Alkyl chloride or bromide react with Nal or KI in presence of acetone give alkyl iodide. It is halide exchange reaction or Finkelstein reaction. It is a S_N2 reaction.

\[\ce{R - Cl + Nal ->[Acetone or Methanol] R - I + NaCl}\]

\[\begin{array}{cc}
\phantom{..........}\ce{Cl}\phantom{................................................}\ce{I}\phantom{..}\\
\phantom{..........}|\phantom{..................................................}|\phantom{.}\\
\phantom{}\ce{CH3 - CH2 - CH - CH3 ->[Nal][Acetone] CH3 - CH2 - CH - CH3}\phantom{}
\end{array}\]

Borodine-Hunsdiecker reaction

Silver salt of carboxylic acid on heating with Br₂ + carbon tetrachloride give alkyl bromide

\[\ce{\underset{Silver acetate}{CH3COOAg} + Br2 ->[CCl4/Reflux] \underset{Methyl bromide}{CH3 - Br} + AgBr + CO2}\]

Note: If silver salt of carboxylic acid is heated with I₂ in presence of CCI₄ ester is obtained. This reaction is known as simonini reaction.

\[\ce{2RCOOAg + I2 ->[CCl4] \underset{Ester}{RCOOR} + 2CO2 + 2AgI}\]

If silver salt of carboxylic acid is heated with I₂ in presence of HgO or lead tetra acetate than alkyl iodide is obtained

\[\ce{2RCOOAg ->[I2/HgO][Δ] 2R - X + 2CO2 + HgI2 + H2O}\]

Direct halogenations

Chlorobenzene is prepared by direct chlorination of benzene in the presence of Lewis acid catalysts such as FeCl₃.

From benzene diazonium chloride

Chlorobenzene is prepared by the Sandmeyer reaction or the Gattermann reaction using benzene diazonium chloride.

Sandmeyer reaction

When an aqueous solution of benzene diazonium chloride is warmed with Cu₂Cl₂ in HCl, chlorobenzene is formed.

Preparation of iodobenzene

Iodobenzene is prepared by warming benzene diazonium chloride with aqueous KI solution.

\[\ce{\underset{(\underset{\underset{chloride}{benzene diazonium}}{Sandmeyer rection)}}{C6H5N2Cl + Kl} ->[warm] \underset{Iodo benzene}{C6H5I + N2 + KCl}}\]

Preparation of fluorobenzene

Aryl fluorides are prepared by

When a diazonium salt is treated with fluoroboric acid (HBF), the diazonium fluoro borate precipitates out of solution. If this precipitated salt is filtered and then heated, it decomposes to give the aryl fluoride.

Commercial preparation of chloro benzene (Raschig process)

Chlorobenzene is commercially prepared by passing a mixture of benzene vapour, air and HCl over heated cupric chloride. This reaction is called the Raschig process.

Melting and Boiling Points

• Depend on Van der Waals dispersion forces and dipole–dipole interactions.
• Boiling point ∝ size of halogen atom and number of electrons: R–I > R–Br > R–Cl > R–F (for the same carbon chain)
• Boiling point ∝ surface area ∝ no. of carbons in chain (longer chain → higher B.P.)
• Branching reduces B.P.: B.P. ∝ 1/branching (isomers go from primary → tertiary, B.P. falls)
• Para-isomers of dihalobenzenes have higher melting points than ortho- and meta-isomers due to symmetry fitting better in the crystal lattice.

Density

• Density ∝ no. of halogen atoms / molecular mass.
• Bromo, iodo, and polychloro derivatives are heavier than water: Density: R–I > R–Br > R–Cl > R–F
• For isomers of chlorobenzene: density ∝ molecular mass → benzene < chlorobenzene < dichlorobenzene < bromochlorobenzene.

Solubility

• Haloalkanes are very slightly soluble in water (attraction between alkyl halide molecules is stronger than attraction between alkyl halide and water, and they fail to form H-bonds with water).
• Solubility order in water: R–F > R–Cl > R–Br > R–I
• Haloalkanes dissolve readily in organic solvents (due to similar intermolecular forces).

Alkyl iodide is so unstable that it decomposes in sunlight: 2R–I → 2R + I₂ (violet vapours)

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.