Elimination

Overview

Base-promoted elimination removes HCl to form an alkene.

Transformation

Chloroalkanes → Alkenes

Equation

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

Reagents

ethanolic KOH

Environment

heat

Reaction class

elimination

Mechanism

elimination

Evidence level

textbook core

Scope and limitations

Scope

Elimination of hydrogen chloride using ethanolic hydroxide.

Limitations

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

Related reactions

• Chlorination: Alcohols → Chloroalkanes

  Alcohols can be converted into chloroalkanes to create a better leaving group for downstream substitution.

• Chlorination with thionyl chloride: Alcohols → Chloroalkanes

  Thionyl chloride converts alcohols into chloroalkanes with sulfur dioxide and hydrogen chloride as by-products.

• 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.

• Hydrolysis: Chloroalkanes → Alcohols

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

• Cyanation: Chloroalkanes → Nitriles

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

• Amination: Chloroalkanes → Amines

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

• Elimination: Bromoalkanes → Alkenes

  Base-promoted elimination removes HBr to form an alkene.

• Elimination: Iodoalkanes → Alkenes

  Base-promoted elimination removes HI to form an alkene.

• Free radical chlorination: Alkanes → Chloroalkanes

  Photochemical chlorination can convert alkanes into chloroalkanes.

• 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.

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.