Concept understanding — Grignard Synthesis of Alcohols
The Core Idea: Building a Carbon Skeleton
Imagine you have a molecule and you want to attach a brand new carbon chain to it — specifically, to turn a carbon-oxygen double bond into an alcohol. That's the job of a Grignard reagent. It's one of the most powerful tools in organic chemistry for making carbon-carbon bonds.
The intuition is simple: a Grignard reagent acts like a carbon-based nucleophile (a carbon atom with a strong negative character) that attacks the electrophilic carbon of a carbonyl group (C=O). After a simple workup with water or acid, you get an alcohol.
What is a Grignard Reagent?
A Grignard reagent has the general formula R−Mg−X, where R is an alkyl or aryl group and X is a halogen (usually Cl, Br, or I). The carbon-magnesium bond is highly polarised: carbon carries a significant partial negative charge (δ−) and magnesium carries a partial positive charge (δ+). This makes the carbon atom strongly nucleophilic and basic.
Watch out
Grignard reagents are extremely sensitive to moisture and air. They react violently with water, so the entire reaction must be carried out in anhydrous (dry) conditions, typically in dry ether as the solvent.
The Reaction: Step by Step
The reaction proceeds in two distinct stages:
Step 1 — Nucleophilic Addition: The Grignard reagent attacks the electrophilic carbonyl carbon. The π bond of C=O breaks, and the electrons move to oxygen, forming a magnesium alkoxide intermediate.
R−MgX+RX′−C(=O)−RX′′RX′−C(R)(OX−MgX)−RX′′
Step 2 — Hydrolysis: The alkoxide intermediate is treated with dilute acid or water. This protonates the negatively charged oxygen, giving the final alcohol.
Building a secondary alcohol via a Grignard reagent means adding the reagent across an aldehyde that already carries one carbon substituent on its carbonyl carbon. …
A methyl Grignard reagent adds to acetaldehyde's carbonyl carbon, and acidic hydrolysis of the resulting magnesium alkoxide releases the secondary alcohol.
Ethanal (acetaldehyde, CH₃CHO) already carries one carbon substituent (CH₃) on its carbonyl carbon, so addition of a Grignard reagent gives a secondary alcohol. Methylmagnesium bromide's nucleophilic methyl carbon attacks the electrophilic carbonyl carbon of ethanal in dry ether:
CH₃CHO + CH₃MgBr --(dry ether)--> CH₃–CH(OMgBr)–CH₃ (a magnesium alkoxide intermediate).
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set A1 markMCQ
Q.Which of the following reagents reacts easily with both Acetaldehyde and Acetone ?
(a) Fehling solution
(b) Grignard reagent
(c) Schiff's reagent
(d) Tollen's reagent
›Reveal solutionSolution
Grignard reagent adds to any carbonyl group, so it reacts with both aldehydes and ketones.
Acetaldehyde is an aldehyde and acetone is a ketone. A reagent that reacts with both must attack the common C=O group:
Fehling's solution, Tollen's reagent and Schiff's reagent are tests specific to aldehydes (they detect the -CHO group) and do not react with ketones. …
Q.Which of the following reagents will give a primary alcohol on reacting with a Grignard reagent?
(a) Methanal
(b) Ethanal
(c) Propanone
(d) Benzaldehyde
›Reveal solutionSolution
RMgX adds across C=O; the class of alcohol formed (1°, 2° or 3°) depends on how many carbon groups were already on the carbonyl carbon. Formaldehyde (methanal) has none, so it alone gives a primary alcohol.
General Grignard addition to a carbonyl compound, followed by aqueous acid work-up:
R2′C=O+RMgX⟶R2′C(OMgX)RH3O+R2′C(OH)R
The resulting alcohol's class is set by how many carbon groups flank the original carbonyl carbon:
Methanal, HCH=O (no carbon substituent on the carbonyl carbon): HCHO+RMgX→R–CH2–OH → primary alcohol.
Q.Show how benzaldehyde can be converted into benzophenone?
›Reveal solutionSolution
Benzaldehyde is first converted to a secondary alcohol via a Grignard reaction, which is then oxidised to the ketone benzophenone.
Benzaldehyde (C6H5CHO) reacts with the Grignard reagent phenylmagnesium bromide (C6H5MgBr) at the carbonyl carbon; aqueous acid work-up gives the secondary alcohol diphenylmethanol (benzhydrol):