Hydroiodination

Overview

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

Transformation

Alkenes → Iodoalkanes

Equation

CH2=CH2 + HI -> CH3CH2I

Reagents

HI

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 iodide across a C=C bond forms an iodoalkane.

Limitations

Unsymmetrical alkenes can show regioselectivity; this route records the HI branch of alkene hydrohalogenation separately from HCl and HBr.

Related reactions

• Iodination: Alcohols → Iodoalkanes

  Iodine and red phosphorus conditions convert alcohols into iodoalkanes.

• 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.

• Cyanation: Iodoalkanes → Nitriles

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

• Amination: Iodoalkanes → Amines

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

• Elimination: Chloroalkanes → Alkenes

  Base-promoted elimination removes HCl to form an alkene.

• Hydrolysis: Iodoalkanes → Alcohols

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

• Elimination: Bromoalkanes → Alkenes

  Base-promoted elimination removes HBr to form an alkene.

• Elimination: Iodoalkanes → Alkenes

  Base-promoted elimination removes HI to form an alkene.

• 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.

• Dehydration: Alcohols → Alkenes

  Dehydration converts alcohols into alkenes.

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.