Hydrochlorination

Overview

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

Transformation

Alkenes → Chloroalkanes

Equation

CH2=CH2 + HCl -> CH3CH2Cl

Reagents

HCl

Environment

room temperature

Reaction class

electrophilic addition

Mechanism

electrophilic addition

Evidence level

textbook core

Scope and limitations

Scope

Ethene is used as the representative alkene; addition of hydrogen chloride across a C=C bond forms a chloroalkane.

Limitations

Unsymmetrical alkenes can show regioselectivity; the mechanism diagram uses ethene as the exemplar substrate and does not generalize Markovnikov or rearrangement behaviour.

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.

• 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: Chloroalkanes → Alkenes

  Base-promoted elimination removes HCl to form an alkene.

• 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. Organic Chemistry: Preparing Alkyl Halides from AlkenesJohn McMurry · OpenStax Organic Chemistry · 2023

   Supports HCl, HBr, and HI reacting with alkenes by polar electrophilic addition to form alkyl halides; this route uses the HCl branch to form chloroalkanes and keeps Markovnikov regioselectivity as a scope boundary for unsymmetrical alkenes.