Dihydroxylation

Overview

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

Transformation

Alkenes → Diols

Equation

RCH=CHR' + [O] + H2O -> RCH(OH)-CH(OH)R'

Reagents

cold, dilute KMnO4(aq)

Environment

cold, dilute, slightly alkaline

Reaction class

dihydroxylation

Mechanism

alkene oxidation

Evidence level

reviewed chemistry

Scope and limitations

Scope

Cold, dilute manganate(VII) adds two hydroxyl groups across an alkene double bond to form a vicinal diol.

Limitations

Temperature, concentration and acidity must be controlled. Hot, concentrated acidified manganate(VII) promotes oxidative cleavage: an alkene carbon bearing no hydrogen gives a ketone, one bearing hydrogen gives a carboxylic acid, and a terminal CH2 group can be oxidised to carbon dioxide.

Related reactions

• 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.

• 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.

• 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.

• 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.

• Polyester formation co reactant: Diols → Polyesters

  Diols condense with dicarboxylic acids to form polyesters.

• Epoxidation: Alkenes → Epoxides

  A peroxyacid transfers oxygen across the alkene in a concerted step, forming two C-O bonds while breaking the C=C pi bond. Both bonds form from one face, retaining the alkene substituents’ relative stereochemistry in the epoxide.

• Epoxide hydrolysis: Epoxides → Diols

  Protonation activates the strained ring toward attack by water. Backside attack breaks one C-O bond; proton transfer leaves hydroxyl groups on adjacent carbons, with an anti relationship when ring geometry makes that distinction meaningful.

References

1. Osmium-free direct syn-dihydroxylation of alkenesC. J. R. Bataille and T. J. Donohoe · Chemical Society Reviews · 2011

   Supports permanganate-mediated alkene dihydroxylation and the need to distinguish controlled diol-forming conditions from over-oxidation and oxidative cleavage.