Revision: Haloalkanes and Haloarenes Chemistry Science (English Medium) Class 12 CBSE

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

General formula: Ar–X.

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

General formula: R–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 reaction in which diazonium salt is treated with cuprous halide to form haloarene is called Sandmeyer reaction.

The reaction in which halogen is added across a carbon-carbon double bond is called halogenation reaction.

The reaction in which alkyl chloride or bromide is converted into alkyl fluoride using metallic fluorides is called Swarts reaction.

The organic compounds in which one or more hydrogen atoms of hydrocarbons are replaced by halogen atoms are called halogen derivatives.

The compounds in which halogen atom is bonded to an sp³ hybridised carbon atom of an alkyl group are called haloalkanes.

The compounds in which halogen atom is directly bonded to an sp² hybridised carbon atom of an aromatic ring are called haloarenes.

The alkyl halide in which halogen is attached to a secondary carbon atom is called secondary alkyl halide.

The alkyl halide in which halogen is attached to a tertiary carbon atom is called tertiary alkyl halide.

The compounds in which halogen is bonded to an sp³ carbon adjacent to a carbon-carbon double bond are called allylic halides.

The compounds in which halogen is bonded to an sp³ carbon attached to an aromatic ring are called benzylic halides.

The compounds in which halogen is bonded to an sp² carbon of a carbon-carbon double bond are called vinylic halides.

The reaction in which a halogen atom in an alkyl halide is replaced by a nucleophile is called nucleophilic substitution reaction.

The reaction in which a halogen atom is removed along with a hydrogen atom forming a double bond is called elimination reaction.

The reaction in which alkyl chloride or bromide reacts with sodium iodide in acetone to form alkyl iodide is called Finkelstein reaction.

The formation of equal amounts of two enantiomers resulting in optically inactive mixture is called racemisation.

The alkyl halide in which halogen is attached to a primary carbon atom is called primary alkyl halide.

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

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.

Statement:
Alkyl chlorides or bromides react with metallic fluorides (AgF, Hg₂F₂, CoF₂) to form alkyl fluorides.

Equation:
R–Cl + AgF → R–F + AgCl

Importance:
Used for preparation of alkyl fluorides.

Statement:
Aryl diazonium salts react with cuprous halide (CuCl/CuBr) to give haloarenes.

Equation:
Ar–N₂⁺Cl⁻ + CuCl → Ar–Cl + N₂

Importance:
Used to introduce halogen into aromatic ring.

• Halogen is more electronegative than carbon.
• Bond becomes polar.
• Carbon carries partial positive charge.
• Reactivity depends on bond strength.

• Generally colourless liquids or gases.
• Boiling point increases with molecular mass.
• Slightly soluble in water.
• Density increases with number of halogen atoms.

A bimolecular nucleophilic substitution reaction in which nucleophile attacks carbon from backside and displaces leaving group in a single step.

Characteristics:

• One-step reaction
• No intermediate
• Transition state formed
• Inversion of configuration
• Rate depends on both reactants

Order:
Methyl > 1° > 2° > 3°

Reason:
Steric hindrance affects reaction.

A unimolecular nucleophilic substitution reaction in which leaving group departs first forming carbocation intermediate.

Characteristics:

• Two-step reaction
• Carbocation intermediate
• Rate depends only on substrate
• Racemisation possible
• Favoured by 3° halides

Order:
3° > 2° > 1°

Reason:
Stability of carbocation.

Statement:
In elimination reactions, the preferred product is the alkene having greater number of alkyl groups attached to the doubly bonded carbon atoms.

Example:
2-Bromopentane → Pent-2-ene (major)

Reason:
More substituted alkene is more stable.

Statement:
Alkyl halides react with sodium metal in dry ether to form higher alkanes.

Reaction:
2R–X + 2Na → R–R + 2NaX

Limitation:
Best for primary alkyl halides.

Statement:
Alkyl halides react with magnesium in dry ether to form Grignard reagents (R–MgX).

Nature:
Highly reactive, reacts with water.

Important:
Reaction must be moisture free.

Statement:
Alkyl chlorides or bromides react with sodium iodide in dry acetone to form alkyl iodides.

Equation:
R–Cl + NaI → R–I + NaCl↓

Reason:
NaCl precipitates in acetone driving the reaction forward.

Statement:
Aryl halide reacts with alkyl halide in presence of sodium in dry ether forming alkyl arene.

Mechanism:
Free radical coupling reaction.

Limitation:
Mixture of products may form.