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2.8.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Reactions of Halogenoalkanes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 47 of 53 Unit quiz

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Part 1 of 5Why Halogenoalkanes React

2.8.2 Reactions of Halogenoalkanes

The carbon–halogen bond is polar, so the carbon atom is open to attack by electron-rich species.

Polar bond
  • The halogen is more electronegative than carbon, so carbon carries a partial positive charge (δ+) and attracts nucleophiles.
Nucleophile
  • A nucleophile is a species that donates a lone pair of electrons to an electron-deficient carbon atom to form a new covalent bond.
Substitution
  • In substitution the halogen leaves as a halide ion and the nucleophile takes its place on the same carbon atom.
Elimination
  • In elimination the halogen and a hydrogen atom from the next carbon are removed, so a C=C double bond forms instead.
Leaving group
  • The halide ion departs with the bonding pair of electrons, and weaker C–X bonds let it leave more easily.

Part 2 of 5Substitution Reactions

Four nucleophiles are required; each needs its own solvent and conditions.

Substitution Reactions
Hydroxide ion
  • Warm aqueous NaOH or KOH converts RX into an alcohol ROH;
    • water must be the main solvent to prevent elimination.
Water
  • Water is a weak nucleophile, so hydrolysis is slow and gives the alcohol together with H⁺ and X⁻ ions.
Cyanide ion
  • Heating under reflux with KCN dissolved in ethanol gives a nitrile RCN and lengthens the carbon chain by one carbon.
Ammonia
  • Heating under pressure with a large excess of ethanolic ammonia gives a primary amine RNH₂ rather than a mixture of products.
NucleophileConditionsOrganic productExample equation
OH⁻Aqueous NaOH, warmAlcoholCH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
CN⁻KCN in ethanol, refluxNitrileCH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻
NH₃Excess ethanolic NH₃, heat under pressurePrimary amineCH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

Part 3 of 5Elimination Reaction

Changing the solvent changes the type of reaction that happens.

1
Dissolve the halogenoalkane in ethanol and add sodium hydroxide that is also dissolved in ethanol, not water.
2
Heat the mixture under reflux so that the volatile reactants and products are not lost from the flask.
3
The hydroxide ion now acts as a base and removes a hydrogen atom from the carbon next to the C–X carbon.
4
The C–X bond breaks heterolytically and the halide ion leaves, which creates a C=C double bond.
5
The overall change is CH₃CH₂Br → CH₂=CH₂ + HBr, and the HBr is neutralised by the alkali to give a salt and water.

Part 4 of 5Choosing Conditions

Examiners often ask which reagent and solvent give a stated product.

Aqueous alkali
  • Aqueous conditions with a good nucleophile favour substitution, so the product is an alcohol.
Ethanolic alkali
  • Ethanol as the solvent favours elimination, so the product is an alkene.
Reflux
  • Heating under reflux is used for slow reactions so that no volatile material escapes while the mixture is held hot.
Excess ammonia
  • A large excess of ammonia stops the amine product from reacting again and forming secondary amines.

Part 5 of 5Exam Checklist & Tips

2.8.2 – Key Points for the Exam

  • Nucleophile A species with a lone pair of electrons that attacks an electron-deficient carbon atom.
  • Hydrolysis A reaction in which water or hydroxide breaks a bond and replaces the halogen with an –OH group.
  • Nitrile An organic compound containing the –C≡N group, formed when cyanide replaces a halogen.
  • Elimination Loss of a small molecule such as HBr from adjacent atoms to form a C=C double bond.
  • Reflux Heating with a condenser fitted vertically so vapours condense and return to the flask.

Common Mistakes

  • Writing the nucleophile as the metal compound, for example NaOH, in a mechanism instead of the hydroxide ion OH⁻ that actually attacks.
  • Forgetting that ethanolic hydroxide gives elimination to an alkene, while aqueous hydroxide gives substitution to an alcohol.
  • Omitting the extra carbon atom in the nitrile product, so naming it as the starting chain length.
  • Using too little ammonia, which allows the amine to react again and form a mixture of products.

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