ALKANES AND HALOALKANES.

What are alkanes?

Alkanes are saturated hydrocarbons containing only single C–C and C–H bonds. General formula: CₙH₂ₙ₊₂.

What is meant by 'saturated' in alkanes?

Each carbon atom forms four single bonds, meaning no double or triple bonds are present.

Describe the bonding in alkanes.

Alkanes contain sigma (σ) bonds formed by overlap of orbitals, allowing free rotation around C–C bonds.

What is the shape around each carbon atom in alkanes?

Tetrahedral with a bond angle of 109.5

What is the trend in boiling points of straight-chain alkanes?

Boiling point increases with chain length due to stronger London forces (more surface area for contact).

How does branching affect boiling points of alkanes?

Branched isomers have lower boiling points as they have fewer points of contact and less surface area.

How do alkanes react with oxygen?

Alkanes undergo complete combustion in excess oxygen to form CO₂ + H₂O; in limited oxygen, incomplete combustion forms CO or C.

What are the three steps of free radical substitution?

• Initiation (e.g. Cl₂ → 2Cl•)

• Propagation (e.g. Cl• + CH₄ → CH₃• + HCl)

• Termination (e.g. CH₃• + CH₃• → C₂H₆)

What are the limitations of free radical substitution?

It leads to multiple substitution and mixtures of products, making it not useful for synthesising pure compounds.

What is the general formula of a haloalkane?

For a mono-haloalkane: CₙH₂ₙ₊₁X (where X = halogen).

What is the bond polarity in C–X bonds?

The C–X bond is polar due to electronegativity difference. C is δ+, X is δ−.

What is nucleophilic substitution?

A nucleophile replaces the halogen atom in a haloalkane. The halogen leaves as a halide ion.

What is formed when a haloalkane reacts with aqueous NaOH?

An alcohol is formed.
E.g. CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

What affects the rate of hydrolysis of haloalkanes?

The C–X bond strength:
C–I < C–Br < C–Cl (weaker bond = faster hydrolysis).

How is the rate of hydrolysis of haloalkanes measured?

By reacting with aqueous AgNO₃ in ethanol; AgX precipitate forms:

• AgCl: white

• AgBr: cream

• AgI: yellow