Halogenoalkanes

Full Notes:

Halogenoalkanes (or haloalkanes) are organic compounds where a 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.

They can be classified as:

• 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

Halogenoalkane Substitution Reactions

Halogenoalkanes commonly undergo substitution reactions, where the halogen is replaced by another atom or group.

Examples you need to know include:

Reaction with NaOH (aq)

• Reagent: NaOH(aq)
• Conditions: Heat
• Forms: Alcohol

Example CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr

Reaction with KCN

• Reagent: KCN in ethanol
• Conditions: Heat
• Forms: Nitrile

Example CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

Matt’s exam tip

Notice how the substitution of a CN into a halogenoalkane increases the carbon chain length of the molecule. This is very useful and is a unique feature of this reaction.

Reaction with NH₃

• Reagent: NH₃ in ethanol
• Conditions: Heated under pressure
• Forms: Primary amine

Example CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

Nucleophilic Substitution Mechanism

Halogenoalkanes can undergo nucleophilic substitution.

The halogen is replaced by a nucleophile (electron pair donor). The reaction proceeds via a curly arrow mechanism.

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.

You need to know the mechanism for halogenoalkane substitution with OH⁻ ions and NH₃.

Reaction with OH⁻ (Hydrolysis to Alcohols)

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

Reaction with NH₃ (Formation of Amines)

Note that here there is a middle step in the mechanism as well — this is because another NH₃ molecule will take a H⁺ ion from the NH₃⁺ group in the intermediate, forming an NH₄⁺ ion.

• Equation: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
• Reagent: Excess NH₃ in ethanol.
• Conditions: Sealed tube, pressure.

Elimination Reactions

Halogenoalkanes can also undergo elimination reactions to form alkenes.

Substitution tends to dominate in aqueous solution, while elimination dominates in ethanol.

Here, the hydroxide ion acts as a base (not a nucleophile), removing a hydrogen atom from a carbon adjacent to the one bonded to the halogen.

• Reagent: NaOH in ethanol
• Conditions: Heat under reflux

Example CH₃CH₂Br + NaOH (ethanol) → CH₂=CH₂ + NaBr + H₂O

Matt’s exam tip

Remember in the elimination reaction, OH⁻ ions are acting as a base. They accept a H⁺ ion (forming H₂O). This is different to the substitution reaction of a halogenoalkane and OH⁻ ions in which the OH⁻ ions act as a nucleophile, donating a lone pair to the carbon in the C–X bond.

Reactivity and Hydrolysis of Halogenoalkanes

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 is the strongest → least reactive, slowest rate.
• C–I bond is the weakest → most reactive, fastest rate.

Order of reactivity: Iodoalkanes > Bromoalkanes > Chloroalkanes > Fluoroalkanes

The rate of hydrolysis of halogenoalkanes is also dependent on the classification of halogenoalkane:

Primary halogenoalkanes react slower than Secondary halogenoalkanes and Tertiary halogenoalkanes react fastest.

This is because tertiary halogenoalkanes form a more stable carbocation intermediate during the reaction, making them more reactive.

Primary halogenoalkanes typically react via what’s called an SN2 mechanism which is slower because it involves a single-step attack on a less accessible carbon atom.

In summary:

• Tertiary halogenoalkanes react fastest
• Secondary are intermediate
• Primary are the slowest

This trend is often observed by comparing the speed of hydrolysis with aqueous silver nitrate.

Comparing Rates of Hydrolysis

It is possible to compare the rates of hydrolysis for different halogenoalkanes 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.

Observations:

• AgCl = white precipitate (forms slowly)
• AgBr = cream precipitate (forms faster)
• AgI = yellow precipitate (forms quickest)

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 (like water and Ag⁺ ions), while its ethyl group (CH₃CH₂) helps it dissolve non-polar substances, such as the hydrocarbon chain of a halogenoalkane.

Summary

• Halogenoalkanes undergo nucleophilic substitution to give alcohols, nitriles and amines, and elimination to give alkenes.
• Mechanisms involve nucleophile attack on δ⁺ carbon with loss of halide; NH₃ substitution includes a deprotonation step.
• Bond enthalpy controls reactivity: C–I > C–Br > C–Cl > C–F for rate.
• Hydrolysis is fastest for tertiary, then secondary, then primary halogenoalkanes.
• AgNO₃ in ethanol allows comparison of hydrolysis rates via precipitate formation.