Halogenoalkanes

Full Notes

Halogenoalkanes and their reactions have been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

Halogenoalkanes contain a halogen (F, Cl, Br, I) bonded to a carbon. They are useful organic compounds for use synthesis processes and there are several ways they can be formed, including:

From Alkanes – Free Radical Substitution

Reagents: Cl₂ or Br₂, UV light

Example Chloroethane formation

CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl (plus further substitution)

From Alkenes – Electrophilic Addition

Reagents: HX(g) or X₂
Conditions: Room temperature

Example Bromoethane from ethene

CH₂=CH₂ + HBr → CH₃CH₂Br

From Alcohols – Substitution

Reagents:

• HX(g) or
• KCl + conc. H₂SO₄ / H₃PO₄ or
• PCl₃ + heat, PCl₅, or SOCl₂

Example Chloroethane from ethanol

CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O

Classification of Halogenoalkanes

TABLE 1

The classification of a halogenoalkane is important and can be used to help explain (and predict) its reaction mechanism with a nucleophile.

Nucleophilic Substitution Reactions

Halogenoalkanes undergo substitution where a nucleophile replaces the halogen. The C–X bond is polar, with the carbon having a partial positive charge. This means nucleophiles are attracted to the carbon atom in the C–X bond and can replace the halogen.

Reaction with NaOH (aq)

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

Example Ethanol from bromoethane

CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr

Reaction with KCN

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

Example Propanitrile from bromoethane

CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

Reaction with NH₃

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

Example Ethanamine from bromoethane

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

Reaction with AgNO₃ in Ethanol (Test for Halide)

• Reagents: Aqueous AgNO₃
• Conditions:Ethanol co-solvent (water and ethanol), Warm gently
• Observation:
  • AgCl → white ppt
  • AgBr → cream ppt
  • AgI → yellow ppt

Example Alcohol formation and precipitate

CH₃CH₂Br + AgNO₃ → CH₃CH₂OH + AgBr↓ + HNO₃

This tests which halogen is present based on ppt colour.

Elimination Reaction

Halogenoalkanes can undergo elimination reactions form alkenes.

• Reagent: NaOH in ethanol
• Conditions: Heat under reflux
• Competes with substitution
• Forms alkene + water + halide salt

Example Ethene from bromoethane

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 from the alkene (forming H₂O). This is different to the substitution reaction of a halogenalkane and OH⁻ ions in which the OH⁻ ions act as a nucleophile (donating a lone pair of electrons and forming a bond to the carbon in the C–X bond).

Mechanisms – SN1 and SN2

Nucleophilic substitution reactions occur when a nucleophile (electron pair donor) replaces a leaving group (a halide ion) in a halogenoalkane.

There are two possible mechanisms that can occur (SN1 and SN2) – primary halogenoalkanes tend to undergo SN2, while tertiary halogenoalkanes tend to follow SN1. Secondary halogenoalkanes often follow both SN1 and SN2 pathways.

SN2 Mechanism – Substitution Nucleophilic Bimolecular

• Occurs in one step (both reactants are involved in the same step).
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.
• The nucleophile attacks the carbon at the same time as the leaving group (halide) departs.
• A transition state is formed with partial bonds — both the nucleophile and the leaving group are briefly attached.

Favoured by primary halogenoalkanes, where the central carbon is less hindered (‘blocked’) by other carbon atoms.

Example SN2 with hydroxide

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

Key point: Steric hindrance is low in primary halogenoalkanes, allowing the nucleophile to attack easily from the back.

SN1 Mechanism – Substitution Nucleophilic Unimolecular

• Occurs in two steps.
• 1. The halide leaves first, forming a carbocation (slow step – rate-determining).
• 2. The nucleophile then attacks the positively charged carbon (carbocation intermediate)
• 3. New bond formed between the C and OH.

Favoured by tertiary halogenoalkanes, where the carbocation is stabilised by alkyl groups via the positive inductive effect (electron-donating effect of surrounding methyl groups).

Example Tert-butyl carbocation then alcohol

(CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻
Then: (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH

Key point: Tertiary carbocations are stabilised by three alkyl groups, making SN1 favourable for tertiary halogenoalkanes.

Matt’s exam tip

Primary halogenoalkanes can’t follow SN1 mechanisms as the positive inductive effect isn’t strong enough to stabilise a carbocation intermediate long enough for it to form and react with a nucleophile, meaning it has to react by SN2. Tertiary halogenoalkanes can’t follow SN2 mechanisms because the bulky carbon groups bonded to the C in the C–X group ‘block’ the incoming nucleophile (this is called steric hinderance).

SN1/SN2 Pathway Summary

Reactivity of Halogenoalkanes

The reactivity of a halogenoalkane is dependent on the strength of the carbon-halogen bond as the bond has to break at the start of the reaction.

The weaker the bond, the more reactive the halogenoalkane and 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

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 halogenoalkae and silver ions from the silver nitrate. The faster the forming of a precipitate, the faster the rate of reaction.

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 can be made from alkanes (free radical substitution), alkenes (electrophilic addition), and alcohols (substitution with suitable reagents).
• They undergo nucleophilic substitution (NaOH(aq), KCN/ethanol, NH₃/ethanol) and can be tested with AgNO₃/ethanol for halide ions.
• Elimination with ethanolic NaOH under reflux forms alkenes and competes with substitution.
• SN2 is one-step (favoured by primary), SN1 is two-step via a carbocation (favoured by tertiary); secondary can do both.
• Reactivity trend follows C–X bond strength: I > Br > Cl > F.