Organic reaction

Grignard Reaction

A Grignard reaction uses an organomagnesium reagent to add a carbon group to electrophiles such as aldehydes, ketones, esters, and carbon dioxide.

Grignard reaction scheme: benzaldehyde with phenylmagnesium bromide, then acid workup, gives diphenylmethanol, with the new C–C bond highlighted
Grignard reaction: PhMgBr adds to benzaldehyde to give diphenylmethanol; the new C–C bond is highlighted.

Reaction Scheme

R-MgX + carbonyl -> alcohol after workupRequires dry ether or THF; water and acids quench the reagent.

Primary reaction SMILES: C=O.C[Mg]Br>>CCO

Two Common Examples

Methylmagnesium bromide + formaldehyde

Forms ethanol after protonation.

Phenylmagnesium bromide + benzaldehyde

Forms diphenylmethanol after acidic workup.

MolDraw Use

Use MolDraw to draw the organomagnesium reagent, carbonyl partner, workup arrow, and product.

How It Works

A Grignard reagent, R–MgX, is made by inserting magnesium metal into a carbon–halogen bond in a dry ether solvent such as diethyl ether or THF. The carbon bonded to magnesium is strongly nucleophilic and basic. It adds to the electrophilic carbon of a carbonyl group, forming a magnesium alkoxide, which is protonated on acidic workup to give an alcohol.

Conditions and Scope

  • Formaldehyde gives a primary alcohol; other aldehydes give secondary alcohols; ketones give tertiary alcohols.
  • Esters react twice, giving tertiary alcohols with two identical R groups from the Grignard.
  • CO2 gives a carboxylic acid after workup; epoxides give alcohols two carbons from the new bond.
  • Water, alcohols, and acids destroy the reagent by protonation, so glassware and solvents must be dry.

FAQ

Why must the conditions be anhydrous? R–MgX is a strong base; water converts it to R–H. How should I draw the scheme? Show two steps over the arrow: "1. RMgBr, Et2O" and "2. H3O+", which makes clear that the acid is added only after the addition is complete.

WittigAldolFriedel-CraftsAll reactions