Overview of Haloalkanes and Haloarenes

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 primary carbon atom is called primary alkyl halide.

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

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.

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:
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.

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

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:
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.