Elimination Reactions of Halogenoalkanes (Oxford AQA International A Level Chemistry): Revision Note

Elimination Mechanism

• In an elimination reaction, an organic molecule loses a small molecule

  • In the case of halogenoalkanes this small molecule is a hydrogen halide (e.g. HCl)

• The halogenoalkanes are heated with ethanolic sodium hydroxide (no water present) causing the C-X bond to break heterolytically, forming an X^- ion and leaving an alkene as an organic product

• Under these conditions the OH^- ion can act as a base removing an H⁺ ion from the halogenoalkane

Elimination of a halogenoalkane

Elimination with OH^-

• There are three curly arrows in this mechanism

  • Curly arrow from lone pair on :OH^- ion to H atom bonded to the adjacent carbon atom

  • Curly arrow from the C–H bond adjacent to the appropriate C–C bond

  • Curly arrow from C-X bond to Br (X) atom

Which product will form?

• Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions

Isomeric alkenes

• It is possible for elimination of halogenoalkanes to produce a mixture of alkenes which can also exhibit E / Z isomerism