Dehydration

Overview

Dehydration converts alcohols into alkenes.

Transformation

Alcohols → Alkenes

Equation

RCH2CH2OH -> RCH=CH2 + H2O

Reagents

concentrated H3PO4

Environment

heat

Reaction class

elimination

Mechanism

acid-catalysed elimination

Evidence level

textbook core

Scope and limitations

Scope

Alcohols can be dehydrated to alkenes under strongly acidic heated conditions.

Limitations

Acid-catalysed dehydration can give positional isomers from unsymmetrical alcohols.

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.

• Bromination: Alcohols → Bromoalkanes

  Bromide and acid conditions convert alcohols into bromoalkanes.

• 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.

• 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.

• 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.

• 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.

• Oxidation: Alcohols → Aldehydes

  Controlled oxidation of a primary alcohol gives an aldehyde.

• Oxidation: Alcohols → Ketones

  Oxidation of a secondary alcohol gives a ketone.

• Oxidation: Alcohols → Carboxylic acids

  Full oxidation of a primary alcohol gives a carboxylic acid.

References

1. Organic Chemistry: Reactions of AlcoholsJohn McMurry · OpenStax Organic Chemistry · 2023

   Supports acid-catalysed alcohol dehydration through alcohol protonation, loss of water, and alkene-forming deprotonation; used with a narrow primary-alcohol dehydration scope boundary.