Elimination

Overview

Base-promoted elimination removes HBr to form an alkene.

Transformation

Bromoalkanes → Alkenes

Equation

R-CH2-CH2-Br + OH- -> R-CH=CH2 + H2O + Br-

Reagents

ethanolic KOH

Environment

heat

Reaction class

elimination

Mechanism

elimination

Evidence level

textbook core

Scope and limitations

Scope

Elimination of hydrogen bromide using ethanolic hydroxide.

Limitations

Conditions are chosen to favour elimination over substitution. The mechanism diagram uses a bromoethane E2-style exemplar; regioselectivity, stereochemical requirements, and alkene mixtures from more substituted bromoalkanes are not generalized.

Related reactions

• Bromination: Alcohols → Bromoalkanes

  Bromide and acid conditions convert alcohols into bromoalkanes.

• Hydrochlorination: Alkenes → Chloroalkanes

  Addition of hydrogen chloride across an alkene can form a chloroalkane.

• Hydrobromination: Alkenes → Bromoalkanes

  Addition of hydrogen bromide across an alkene can form a bromoalkane.

• Hydroiodination: Alkenes → Iodoalkanes

  Addition of hydrogen iodide across an alkene can form an iodoalkane.

• Cyanation: Bromoalkanes → Nitriles

  Cyanide substitution forms a nitrile and extends the carbon chain by one carbon.

• Amination: Bromoalkanes → Amines

  Ammonia substitutes bromide to form a primary amine; excess ammonia limits further alkylation.

• Elimination: Chloroalkanes → Alkenes

  Base-promoted elimination removes HCl to form an alkene.

• Hydrolysis: Bromoalkanes → Alcohols

  The C-Br bond can be displaced by hydroxide to form an alcohol.

• Elimination: Iodoalkanes → Alkenes

  Base-promoted elimination removes HI to form an alkene.

• Free radical bromination: Alkanes → Bromoalkanes

  Photochemical bromination can convert alkanes into bromoalkanes.

• Cracking: Alkanes → Alkenes

  Thermal or catalytic cracking converts long-chain alkanes into shorter molecules including alkenes.

• Hydrogenation: Alkenes → Alkanes

  Hydrogenation reduces alkenes to alkanes.

• Halogen addition: Alkenes → Dihalogenoalkanes

  Bromine addition converts alkenes into vicinal dibromoalkanes.

• Dihydroxylation: Alkenes → Diols

  Cold, dilute manganate(VII) oxidises an alkene to a vicinal diol without the oxidative cleavage associated with stronger conditions.

• Addition polymerisation: Alkenes → Addition polymers

  Alkenes can form addition polymers by chain-growth addition.

• Hydration: Alkenes → Alcohols

  Catalytic hydration converts alkenes into alcohols.

References

1. Pearson Edexcel Level 3 Advanced GCE in Chemistry specificationPearson Education Limited · Pearson qualifications · 2024

   Supports the textbook-core organic reaction routes used for displayed route and mechanism content.