Grignard reagents and carbon chain extensionEdexcel International A Level Chemistry: Revision notes
Section 1
Why chain length matters
Most reactions studied so far keep the number of carbon atoms the same. To build a bigger molecule you need a reaction that makes a new carbon–carbon bond. Two methods are in the specification: the nitrile route (haloalkane + cyanide, adding one carbon) and Grignard reagents, which can add carbon to carbonyl compounds and to carbon dioxide.
Section 2
Making a Grignard reagent
A Grignard reagent, RMgX, is made by reacting a haloalkane with magnesium turnings in dry ethoxyethane (ether), heated under reflux:
C₂H₅Br + Mg → C₂H₅MgBr
Magnesium is oxidised from 0 to +2. The carbon bonded to magnesium is δ− because magnesium is less electronegative than carbon, so the carbon behaves like a carbanion: a strong nucleophile and a strong base.
Writing the reagent as R–Mg without the halogen. It is RMgX, for example CH₃CH₂MgBr.
Section 3
Why everything must be dry
The Grignard reagent reacts with water (and with alcohols) to give an alkane, destroying the reagent:
C₂H₅MgBr + H₂O → C₂H₆ + Mg(OH)Br
So glassware, ether and the other reactant must be dry. Ether is used as the solvent because it is unreactive and its lone pairs stabilise the reagent. Ether is highly flammable: heat with a water bath or electric heater, never a naked flame.
If a question says a Grignard yield was low and an alkane was formed, the answer is water: the reagent was protonated.
Section 4
Reaction with carbon dioxide: carboxylic acids
The Grignard reagent is added to solid carbon dioxide (or CO₂ gas) in dry ether. The δ− carbon attacks the δ+ carbon of CO₂ (nucleophilic addition), giving a magnesium carboxylate. Dilute acid is then added to give the carboxylic acid:
CH₃CH₂CH₂MgBr + CO₂ → CH₃CH₂CH₂COOMgBr CH₃CH₂CH₂COOMgBr + H⁺ → CH₃CH₂CH₂COOH + Mg²⁺ + Br⁻
The acid has one more carbon than the haloalkane used: 1-bromopropane (3 C) gives butanoic acid (4 C).
Section 5
Reaction with carbonyl compounds: alcohols
Grignard reagents add to the C=O group of aldehydes and ketones in dry ether, and dilute acid is then added to release the alcohol. The type of alcohol depends on the carbonyl compound:
- methanal, HCHO gives a primary alcohol
- any other aldehyde gives a secondary alcohol
- a ketone gives a tertiary alcohol
Example: CH₃MgBr + CH₃COCH₃, then H⁺, gives (CH₃)₃COH, 2-methylpropan-2-ol. The carbonyl carbon is δ+ (oxygen is more electronegative); the Grignard carbon attacks it, the C=O π bond breaks and an alkoxide forms, which dilute acid protonates.
Saying ethanal gives a primary alcohol. Only methanal does; every other aldehyde gives a secondary alcohol.
Must know
- Grignard reagent: RMgX from RX + Mg in dry ether
- Strong nucleophile and base: carbon is δ−
- Water destroys it, giving the alkane
- CO₂ then H⁺ gives a carboxylic acid with one extra carbon
- Methanal gives primary, other aldehydes secondary, ketones tertiary alcohols
- Dry ether is flammable: no naked flames
That's the notes covered.
Carry on to the next subtopic.
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
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).