JEE Mains Chemistry · Aldehydes, Ketones and Carboxylic Acids

Grignard Reagents with Carbonyls and Nitriles

A Grignard reagent adds an alkyl or aryl group to a carbonyl carbon: methanal gives a 1° alcohol, other aldehydes a 2° alcohol, ketones and esters a 3° alcohol, a nitrile gives a ketone and carbon dioxide an acid.

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Why this matters

Seventeen PYQs, two numerical, one from 2026. Eleven add a Grignard reagent to an aldehyde, a ketone or an ester, or ask how many equivalents are used up; six add it to a nitrile, to carbon dioxide or to water.

Concept 1 of 2: Grignard addition to aldehydes, ketones and esters

In RMgX the carbon bonded to magnesium carries a negative charge, so the reagent behaves like R⁻. It is a strong nucleophile, and it adds once to a C=O; water then turns the magnesium alkoxide into the alcohol. It is also a strong base, so any acidic hydrogen in the molecule destroys it first.

Definition

Methanal gives a 1° alcohol, any other aldehyde a 2° alcohol, and a ketone a 3° alcohol.
An ester uses two equivalents. The first adds and pushes out the alkoxide, which leaves a ketone; the ketone adds a second R. The 3° alcohol carries two identical R groups from the reagent.
Each acidic hydrogen (OH, NH, COOH, or the H of a terminal alkyne) uses up one more equivalent, giving RH.
Working backwards: a 3° alcohol has three groups on the carbinol carbon. Take any one of them as the R of the Grignard; the other two, with the carbinol carbon, form the ketone.
A Grignard reagent cannot be made from a halide that also carries a C=O or an OH: it would react with that group in its own or a neighbouring molecule.
With a copper(I) salt present, the reagent adds to the C=C end of an α,β-unsaturated ketone (1,4-addition) instead of to the C=O.

Grignard addition, then hydrolysis

\mathrm{R'COR'' \xrightarrow{(i)\ RMgX,\ (ii)\ H_3O^+} R'R''C(OH)R} \qquad \mathrm{HCHO} \to 1^\circ,\ \mathrm{R'CHO} \to 2^\circ,\ \text{ketone or ester} \to 3^\circ

Worked example

Give two combinations of a Grignard reagent and a ketone that make 3-methylpentan-3-ol,

\mathrm{CH_3CH_2C(OH)(CH_3)CH_2CH_3}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 4 April 2025 · Q41Moderate

Example 1 · Aldehydes, Ketones and Carboxylic Acids · Grignard Reagents with Carbonyls and Nitriles

An organic compound

(X)

with molecular formula

C_{3}H_{6}O

is not readily oxidised. On reduction it gives

\left( C_{3}H_{8}O(Y) \right.

which reacts with HBr to give a bromide (Z) which is converted to Grignard reagent. This Grinard reagent on reaction with (X) followed by hydrolysis give 2,3-dimethylbutan-2-ol. Compounds

(X),(Y)

and

(Z)

respectively are :

Esters take two equivalents, plus one for each acidic H

Count the ester first (two), then add one for every OH, NH, COOH or terminal alkyne C–H in the molecule. The acidic H reacts first, before any addition.

A terminal alkyne is an acid to a Grignard

\mathrm{RC{\equiv}CH + CH_3MgBr \to RC{\equiv}CMgBr + CH_4}

. The reagent is consumed and methane is given off, even though no C=O has reacted.

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Concept 2 of 2: Grignard reagents with nitriles, carbon dioxide and water

A nitrile and carbon dioxide each take only one R group. The nitrile stops at an imine salt, which is not attacked again; water later turns it into a ketone. Carbon dioxide gives a carboxylate. Water simply protonates R to give the alkane.

Definition

Nitrile: $\mathrm{R'C{\equiv}N + RMgX \to R'C(R){=}NMgX}$ ; hydrolysis gives the ketone $\mathrm{R'COR}$ .
Carbon dioxide (dry ice): $\mathrm{RMgX + CO_2 \to RCOOMgX}$ ; acid gives $\mathrm{RCOOH}$ , which has one carbon more than R.
Water: $\mathrm{RMgX + H_2O \to RH + Mg(OH)X}$ . The gas RH identifies R: its molar mass is its mass × 22.4 L divided by its volume at STP.
A methyl ketone made from a nitrile and $\mathrm{CH_3MgBr}$ gives the iodoform test; it does not give Tollens' or Fehling's test.

One R group only: nitrile to ketone, CO₂ to acid

\mathrm{R'C{\equiv}N \xrightarrow{(i)\ RMgX,\ (ii)\ H_3O^+} R'COR} \qquad \mathrm{RMgX \xrightarrow{(i)\ CO_2,\ (ii)\ H_3O^+} RCOOH}

Worked example

A Grignard reagent RMgBr gives a gas with water; 7.5 g of the gas occupies 5.6 L at STP. Identify R, and give the products of RMgBr with dry ice (then acid) and with propanenitrile (then acid).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 24 Jan 2026 Shift 2 · Q50Moderate

Example 2 · Aldehydes, Ketones and Carboxylic Acids · Grignard Reagents with Carbonyls and Nitriles

Grignard reagent

RMgBr(P)

reacts with water and forms a gas (Q). One gram of Q occupies

1.4{dm}^{3}

at STP. (P) on reaction with dry ice in dry ether followed by

H_{3}O^{+}

forms a compound (Z). 0.1 mole of (Z) will weigh

\_\_\_\_

g. (Nearest integer)

The ketone appears only after water

Before hydrolysis the product is the imine salt, so the ketone never meets the Grignard reagent. That is why a nitrile, unlike an ester, gives a ketone and not a 3° alcohol.

Carbon dioxide adds a carbon

The acid from

\mathrm{RMgX}

and

\mathrm{CO_2}

has one more carbon than R. Propylmagnesium bromide gives butanoic acid, not propanoic acid.

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Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Grignard addition to aldehydes, ketones and esters
Grignard addition, then hydrolysis
$\mathrm{R'COR'' \xrightarrow{(i)\ RMgX,\ (ii)\ H_3O^+} R'R''C(OH)R} \qquad \mathrm{HCHO} \to 1^\circ,\ \mathrm{R'CHO} \to 2^\circ,\ \text{ketone or ester} \to 3^\circ$
Grignard reagents with nitriles, carbon dioxide and water
One R group only: nitrile to ketone, CO₂ to acid
$\mathrm{R'C{\equiv}N \xrightarrow{(i)\ RMgX,\ (ii)\ H_3O^+} R'COR} \qquad \mathrm{RMgX \xrightarrow{(i)\ CO_2,\ (ii)\ H_3O^+} RCOOH}$

Watch out for (4)

Esters take two equivalents, plus one for each acidic H
→ Grignard addition to aldehydes, ketones and esters
A terminal alkyne is an acid to a Grignard
→ Grignard addition to aldehydes, ketones and esters
The ketone appears only after water
→ Grignard reagents with nitriles, carbon dioxide and water
Carbon dioxide adds a carbon
→ Grignard reagents with nitriles, carbon dioxide and water

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