What conditions form a Grignard reagent?

Haloalkane and magnesium turnings in dry ethoxyethane (ether), heated under reflux.

Why must Grignard preparations be dry?

Water reacts with the reagent to give an alkane, destroying it and lowering the yield.

Write the equation for the Grignard reagent reacting with water.

C₂H₅MgBr + H₂O → C₂H₆ + Mg(OH)Br

Why is the carbon in RMgX nucleophilic?

The C–Mg bond is polar with carbon δ−, so carbon can donate an electron pair.

What does a Grignard reagent give with carbon dioxide, then dilute acid?

A carboxylic acid with one more carbon atom than the haloalkane.

Why is dilute acid added after the carbonyl reaction?

To protonate the alkoxide intermediate and release the alcohol.

What safety precaution is needed when heating ether?

No naked flames: use a water bath or electric heater, as ether is highly flammable.

Grignard reagents and carbon chain extensionEdexcel International A Level Chemistry: Flashcards

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What is a Grignard reagent?

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What is a Grignard reagent?: An organomagnesium compound RMgX, made from a haloalkane and magnesium.
What conditions form a Grignard reagent?: Haloalkane and magnesium turnings in dry ethoxyethane (ether), heated under reflux.
Why must Grignard preparations be dry?: Water reacts with the reagent to give an alkane, destroying it and lowering the yield.
Write the equation for the Grignard reagent reacting with water.: C₂H₅MgBr + H₂O → C₂H₆ + Mg(OH)Br
Why is the carbon in RMgX nucleophilic?: The C–Mg bond is polar with carbon δ−, so carbon can donate an electron pair.
What happens to magnesium when a Grignard reagent forms?: It is oxidised from 0 to +2.
What does a Grignard reagent give with carbon dioxide, then dilute acid?: A carboxylic acid with one more carbon atom than the haloalkane.
Name the product from CH₃CH₂CH₂MgBr, CO₂, then H⁺.: Butanoic acid.
What type of alcohol does methanal give with a Grignard reagent?: A primary alcohol.
What type of alcohol does an aldehyde other than methanal give?: A secondary alcohol.
What type of alcohol does a ketone give?: A tertiary alcohol.
What is the type of reaction between RMgX and a carbonyl compound?: Nucleophilic addition.
Why is dilute acid added after the carbonyl reaction?: To protonate the alkoxide intermediate and release the alcohol.
What safety precaution is needed when heating ether?: No naked flames: use a water bath or electric heater, as ether is highly flammable.

Exam questions on Grignard reagents and carbon chain extension

A research student prepares ethylmagnesium bromide, C₂H₅MgBr, from bromoethane and magnesium. The first attempt is carried out in apparatus that has been washed and left to drain, and the student is told that this is a poor choice for this reaction.
Explain why the apparatus and the ether must be completely dry for this preparation.2 marks
A student plans to convert 1-bromopropane into butanoic acid in two stages. In stage 1 the haloalkane is turned into a Grignard reagent. In stage 2 the Grignard reagent is added to an excess of crushed solid carbon dioxide, and the mixture is then treated with dilute hydrochloric acid.
Explain why the Grignard reagent attacks the carbon atom of carbon dioxide.2 marks
A chemist has a bottle of methylmagnesium bromide, CH₃MgBr, in dry ethoxyethane. She wants to make three different alcohols by reacting separate portions of the reagent with three different carbonyl compounds, each followed by dilute acid: methanal, ethanal and propanone.
Deduce the name of the alcohol formed from each carbonyl compound, and state whether it is a primary, secondary or tertiary alcohol.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).