Haloalkanes

Full Notes

What Are Haloalkanes?

Haloalkanes are organic compounds where at least one hydrogen in an alkane is replaced by a halogen (Cl, Br, or I).

They are polar molecules due to the difference in electronegativity between carbon and the halogen.

Classification:

• Primary (1°): halogen attached to a carbon bonded to one other carbon
• Secondary (2°): halogen on a carbon bonded to two other carbons
• Tertiary (3°): halogen on a carbon bonded to three other carbons

Hydrolysis of Haloalkanes

Haloalkanes can undergo hydrolysis and form alcohols in nucleophilic substitution reactions, where the halogen atom is replaced by an OH group.

• General Reaction: R–X + OH⁻ → R–OH + X⁻
• Heat under reflux is required

Comparing Rates of Hydrolysis

Rates of hydrolysis can be compared by adding aqueous silver nitrate and ethanol to the reaction mixture and timing how long it takes for a silver halide precipitate to form.

The precipitate is formed by the halide ion released from the halogenoalkane and silver ions from the silver nitrate. The faster the forming of a precipitate, the faster the rate of reaction.

• AgCl = white precipitate (slowest)
• AgBr = cream precipitate (faster)
• AgI = yellow precipitate (fastest)

Matt’s exam tip

Ethanol is added to help the halogenoalkane dissolve in the aqueous mixture. Its OH group allows it to mix with polar substances, while its ethyl group helps it dissolve non-polar hydrocarbon chains.

What is a Nucleophile?

A nucleophile is an electron pair donor that attacks a δ⁺ carbon atom.

• Common nucleophiles include OH⁻, CN⁻, NH₃, and H₂O.
• These species are attracted to electron-deficient carbons, such as those in polar C–X bonds in haloalkanes.

Nucleophilic Substitution Mechanism

Halogenoalkanes undergo nucleophilic substitution, where the halogen is replaced by a nucleophile.

Primary and secondary halogenoalkanes follow the following mechanism when they react with nucleophiles:

1. Curly arrow from nucleophile to δ⁺ carbon
2. Curly arrow from C–X bond to halogen (X⁻ leaves)
3. New bond forms between nucleophile and carbon

For example:Reaction with OH⁻ (Hydrolysis to Alcohols)

• Reagent: Aqueous NaOH/KOH
• Conditions: Warm, reflux

Trends in Hydrolysis Rate

The rate of substitution is dependent on the strength of the carbon-halogen bond (bond enthalpy) as the bond has to break at the start of the reaction.

The weaker the bond, the faster the rate of reaction as less energy is needed to break the bond (lower activation energy).

• C–F bond: strongest = least reactive
• C–Cl bond: stronger = slower
• C–Br bond: weaker = faster
• C–I bond: weakest = fastest

Order of reactivity: Iodoalkanes > Bromoalkanes > Chloroalkanes > Fluoroalkanes

CFCs and Ozone Depletion

CFCs (chlorofluorocarbons) are stable compounds once widely used as aerosols and refrigerant gases.

In the upper atmosphere, UV light causes homolytic fission of C–Cl bonds:

Chlorine radicals then catalyse breakdown of ozone:

One chlorine radical can destroy thousands of ozone molecules. This environmental impact led to international agreements such as the Montreal Protocol.

Summary

• Haloalkanes undergo nucleophilic substitution to form alcohols.
• Hydrolysis rate depends on C–X bond strength: C–I fastest, C–F slowest.
• Hydrolysis rates can be compared using AgNO₃/ethanol test.
• Nucleophiles donate electron pairs to δ⁺ carbons in haloalkanes.
• CFCs produce chlorine radicals under UV, catalysing ozone depletion.