Which of the following reagents will give a primary alcohol on reacting with a Grignard reagent? (a) Methanal

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Q. Which of the following reagents will give a primary alcohol on reacting with a Grignard reagent?

(a) Methanal

(b) Ethanal

(d) Benzaldehyde

Meghalaya MboseMBOSE Meghalaya Intermediate Board 2020MCQ· 1mImportance★★★★★

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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 ( $\ce{C=O}$ ). After a simple workup with water or acid, you get an alcohol.

What is a Grignard Reagent?

A Grignard reagent has the general formula $\ce{R-Mg-X}$ , where $\ce{R}$ is an alkyl or aryl group and $\ce{X}$ is a halogen (usually $\ce{Cl}$ , $\ce{Br}$ , or $\ce{I}$ ). The carbon-magnesium bond is highly polarised: carbon carries a significant partial negative charge ( $\delta^-$ ) and magnesium carries a partial positive charge ( $\delta^+$ ). 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 $\pi$ bond of $\ce{C=O}$ breaks, and the electrons move to oxygen, forming a magnesium alkoxide intermediate.

\ce{R-MgX + R'-C(=O)-R'' -> R'-C(R)(O^-MgX)-R''}

Step 2 — Hydrolysis: The alkoxide intermediate is treated with dilute acid or water. This protonates the negatively charged oxygen, giving the final alcohol.

\ce{R'-C(R)(O^-MgX)-R'' + H2O -> R'-C(R)(OH)-R'' + Mg(OH)X}

\ce{R-MgX + R'-C(=O)-R'' ->[1. dry ether][2. H3O+] R'-C(R)(OH)-R''}

The Three Types of Alcohols You Can Make

The beauty of this method is that by choosing the right carbonyl compound, you can selectively produce primary, secondary, or tertiary alcohols.

1. Primary Alcohols (from Formaldehyde)

Formaldehyde ( $\ce{HCHO}$ ) has two hydrogens on the carbonyl carbon. After Grignard addition and hydrolysis, you get a primary alcohol.

\ce{R-MgX + HCHO ->[1. ether][2. H3O+] R-CH2OH}

Note

The product has one more carbon than the Grignard reagent's $\ce{R}$ group. The new carbon comes from formaldehyde.

2. Secondary Alcohols (from Aldehydes other than Formaldehyde)

Any aldehyde other than formaldehyde ( $\ce{R'-CHO}$ ) gives a secondary alcohol. The carbonyl carbon now has one $\ce{R'}$ group and one $\ce{H}$ .

\ce{R-MgX + R'-CHO ->[1. ether][2. H3O+] R'-CH(OH)-R}

3. Tertiary Alcohols (from Ketones)

Ketones ( $\ce{R'-C(=O)-R''}$ ) have two alkyl/aryl groups on the carbonyl carbon. After addition, you get a tertiary alcohol.

\ce{R-MgX + R'-C(=O)-R'' ->[1. ether][2. H3O+] R'-C(OH)(R)-R''}

A Quick Summary Table

Carbonyl Compound	Type of Alcohol Produced	General Product
Formaldehyde ( $\ce{HCHO}$ )	Primary	$\ce{R-CH2OH}$
Other aldehydes ( $\ce{R'-CHO}$ )	Secondary	$\ce{R'-CH(OH)-R}$
Ketones ( $\ce{R'-C(=O)-R''}$ )	Tertiary	$\ce{R'-C(OH)(R)-R''}$

A Common Mistake to Avoid …

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$RMgX$ 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:

$R'_2C=O + RMgX \longrightarrow R'_2C(OMgX)R \xrightarrow{H_3O^{+}} R'_2C(OH)R$

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): $HCHO + RMgX \rightarrow R\text{–}CH_2\text{–}OH$ → primary alcohol.
Ethanal (any other aldehyde), $R'CH=O$ (one carbon substituent): gives $R'CH(OH)R$ → secondary alcohol.
Propanone (a ketone), $R'_2C=O$ (two carbon substituents): gives $R'_2C(OH)R$ → tertiary alcohol. …

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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. …
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CBSE 2020Set NC1 markMCQ
$Q. Which of the following reagents will give a primary alcohol on reacting with a Grignard reagent?$
(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:

$R'_2C=O + RMgX \longrightarrow R'_2C(OMgX)R \xrightarrow{H_3O^{+}} R'_2C(OH)R$

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): $HCHO + RMgX \rightarrow R\text{–}CH_2\text{–}OH$ → primary alcohol.

Ethanal (any other aldehyde), $R'CH=O$ (one carbon substituent): gives $R'CH(OH)R$ → secondary alcohol.

Propanone (a ketone), $R'_2C=O$ (two carbon substituents): gives $R'_2C(OH)R$ → tertiary alcohol. …
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CBSE 2016Set ANNUAL1 mark
$Q. Show how benzaldehyde can be converted into benzophenone?$
›Reveal solutionSolution

Benzaldehyde is first converted to a secondary alcohol via a Grignard reaction, which is then oxidised to the ketone benzophenone.

Benzaldehyde ( $C_6H_5CHO$ ) reacts with the Grignard reagent phenylmagnesium bromide ( $C_6H_5MgBr$ ) at the carbonyl carbon; aqueous acid work-up gives the secondary alcohol diphenylmethanol (benzhydrol):

$C_6H_5CHO + C_6H_5MgBr \rightarrow C_6H_5CH(OMgBr)C_6H_5 \xrightarrow{H_3O^+} (C_6H_5)_2CHOH$ …

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Q.Which of the following reagents will give a primary alcohol on reacting with a Grignard reagent?

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