All subjects · Chemistry · WCH15 (Unit 5)

Grignard reagents and lengthening carbon chains

Making new carbon-carbon bonds is the heart of organic synthesis. Grignard reagents add an alkyl group to a carbonyl carbon or to carbon dioxide, building bigger molecules from small ones.

Making a Grignard reagent

React a halogenoalkane with magnesium in dry ether (ethoxyethane), under reflux:

CH3CH2Br + Mg -> CH3CH2MgBr (ethylmagnesium bromide)

Everything must be dry: Grignard reagents react with water to give an alkane (CH3CH2MgBr + H2O -> CH3CH3 + Mg(OH)Br), destroying the reagent. The carbon bonded to Mg behaves as if it carries a negative charge, so it is a strong nucleophile.

Reactions

The alkyl group attacks the δ+ carbon. Then dilute acid is added to give the product.

Carbon dioxide (dry ice) -> carboxylic acid with one more carbon: CH3CH2MgBr + CO2, then H+ -> CH3CH2COOH
Methanal -> primary alcohol: CH3MgBr + HCHO, then H+ -> CH3CH2OH
Other aldehydes -> secondary alcohols: CH3MgI + CH3CHO, then H+ -> CH3CH(OH)CH3
Ketones -> tertiary alcohols: CH3MgI + CH3COCH3, then H+ -> (CH3)3COH

Other ways to add a carbon

Halogenoalkane + KCN in ethanol -> nitrile (one more carbon), which can be hydrolysed to a carboxylic acid or reduced to an amine.
Carbonyl + HCN/KCN -> hydroxynitrile (one more carbon).
Friedel-Crafts alkylation or acylation adds a carbon chain to a benzene ring.

Worked example

Show how to make 2-methylbutan-2-ol, CH3CH2C(OH)(CH3)2, using a Grignard reagent.

The product is a tertiary alcohol, so use a ketone.
Ethylmagnesium bromide (from bromoethane and Mg in dry ether) adds to propanone.
Then add dilute acid.

Answer: CH3CH2MgBr + CH3COCH3, then H+

Key idea: RX + Mg in dry ether gives RMgX. With CO2: carboxylic acid; methanal: primary alcohol; other aldehydes: secondary; ketones: tertiary; then dilute acid. Keep everything dry.

Check yourself

How is ethylmagnesium bromide made?
- Ethene with magnesium bromide in water
- Bromoethane with magnesium in dry ether
- Ethanol with magnesium and HBr
- Bromoethane with magnesium oxide in water
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Bromoethane with magnesium in dry ether. Halogenoalkane + Mg in dry ether.
Why must Grignard reactions be carried out in dry conditions?
- Water oxidises magnesium to Mg2+ ions in solution only
- Ether reacts with water explosively
- Water is the nucleophile
- Grignard reagents react with water to form alkanes
Show answer
Grignard reagents react with water to form alkanes. Water destroys the reagent: RMgX + H2O -> RH + Mg(OH)X.
What is formed when CH3CH2MgBr reacts with carbon dioxide, followed by dilute acid?
- Ethanoic acid
- Propanoic acid
- Propan-1-ol
- Butanoic acid
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Propanoic acid. CO2 adds one carbon: CH3CH2COOH.
What type of alcohol forms when a Grignard reagent reacts with a ketone, followed by acid?
- Primary
- Secondary
- Tertiary
- No alcohol forms
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Tertiary. The carbonyl carbon ends up bonded to three carbon groups.
What is the product of CH3MgI with ethanal, followed by dilute acid?
- Propan-2-ol
- Propan-1-ol
- Ethanol
- 2-methylpropan-2-ol
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Propan-2-ol. CH3 adds to the carbonyl carbon of CH3CHO, giving CH3CH(OH)CH3.
Which carbonyl compound reacts with a Grignard reagent to give a primary alcohol?
- Ethanal
- Propanone
- Methanal
- Butanone
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Methanal. HCHO has two H atoms on the carbonyl carbon.
How many carbon atoms are in the alcohol formed from propylmagnesium bromide, CH3CH2CH2MgBr, and propanone, after adding acid?
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6. 3 from the Grignard reagent plus 3 from propanone: (CH3)2C(OH)CH2CH2CH3.

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Last updated 7 November 2026. Spotted a mistake? Tell us.