Imagine you have a carbon atom that is desperate to attack a positive centre. Normally, carbon in organic molecules is neutral or slightly negative, but it rarely has the raw power to act as a strong base or nucleophile. Now imagine you take that carbon and attach it to a magnesium atom, which is electropositive — it wants to give away electrons. The result is a bond where the carbon becomes strongly negative (carbanion-like) and the magnesium becomes positive.
That is the Grignard reagent: RMgX, where R is an alkyl or aryl group, Mg is magnesium, and X is a halogen (usually Cl, Br, or I). The C–Mg bond is highly polarised: δ−C⋯Mgδ+X. This makes the carbon a powerful nucleophile and a strong base.
Note
The name comes from Victor Grignard, who won the 1911 Nobel Prize for discovering these reagents. They are made by reacting an organic halide with magnesium metal in dry ether.
The Precise Statement
A Grignard reagent is an organomagnesium compound of the general formula RMgX, where:
R = alkyl (CH3−, C2H5−), aryl (C6H5−), vinyl, etc.
X = halogen (Cl, Br, I — Br is most common)
The C–Mg bond is covalent but highly polar, with carbon bearing a partial negative charge.
Because of this polarity, the carbon acts as a carbon nucleophile — it attacks electrophilic centres like carbonyl carbons (C=O), epoxides, carbon dioxide, and even protons (from water or acids).
RMgXwhereδ−R−Mgδ+X
How It Behaves: The Key Reactions
The Grignard reagent is famous for carbon-carbon bond formation. Here is the most important reaction:
1. Reaction with Carbonyl Compounds
The nucleophilic carbon attacks the electrophilic carbonyl carbon. For example, with formaldehyde (HCHO), you get a primary alcohol after hydrolysis:
CH3MgBr+HCHOetherCH3CH2OMgBrH3O+CH3CH2OH
With other aldehydes or ketones, you get secondary or tertiary alcohols.
2. Reaction with Carbon Dioxide
This is a classic way to make carboxylic acids:
RMgX+CO2→RCOOMgXH3O+RCOOH
3. Reaction with Water (a trap!)
Grignard reagents react violently with water, destroying the reagent:
RMgX+H2O→RH+Mg(OH)X
This is why they must be prepared and used in absolutely dry conditions — any moisture kills them.
Watch out
Never use a Grignard reagent in the presence of water, alcohols, or any compound with an acidic hydrogen (like −OH, −NH, −SH). The reagent will be quenched before it can react with your intended substrate.
Each reaction follows from what its reagent does to the given carbon skeleton: a Grignard reagent adding to CO₂ builds a one-carbon-longer acid, dilute base lets acetaldehyde undergo self-aldol condensation, and a poisoned Pd catalyst (Rosenmund conditions) stops hydrogenation of an acid chloride cleanly at the aldehyde stage. …
Grignard + CO2 builds a carboxylic acid one carbon longer; base-catalyzed self-aldol of acetaldehyde condenses to crotonaldehyde; and a poisoned-Pd (Rosenmund) hydrogenation stops an acid chloride's reduction cleanly at the aldehyde.
(i) A Grignard reagent adds to the electrophilic carbon of CO₂ exactly as it would to any carbonyl, giving (after hydrolysis) a carboxylic acid one carbon longer than the original halide: CH₃MgCl + CO₂ --(dry ether)--> CH₃COOMgCl (magnesium salt) --(H₃O⁺)--> CH₃COOH (acetic acid).
(ii) Acetaldehyde has α-hydrogens, so dilute base (NaOH) generates its enolate, which attacks the carbonyl carbon of a second acetaldehyde molecule (aldol addition) to give 3-hydroxybutanal; heating (Δ) then dehydrates this β-hydroxy-aldehyde (aldol condensation) to the conjugated enal: 2CH₃CHO --(dil. NaOH)--> CH₃CH(OH)CH₂CHO --(Δ, −H₂O)--> CH₃–CH=CH–CHO (crotonaldehyde, but-2-enal).
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set V11 markMCQ
Q.Isopropyl magnesium bromide and n-propyl magnesium bromide react separately with H2O to yield the organic compounds, respectively are
(a) Isopropyl bromide and n-propyl bromide
(b) Isopropyl alcohol and n-propyl alcohol
(c) n-propyl alcohol and Isopropyl alcohol
(d) Propane and propane
›Reveal solutionSolution
Water protonates both Grignard reagents, and since both carry a propyl group, each gives propane.
Grignard reagents (R–MgX) have a strongly nucleophilic/basic R group. When they meet a compound with an active (acidic) hydrogen such as water, they are decomposed to the corresponding alkane:
Q.Identify X and Y in the following reaction: cyclohexyl bromide + Mg --(dry ether)--> X --(H2O)--> Y + Mg(OH)Br
›Reveal solutionSolution
Cyclohexyl bromide reacts with magnesium in dry ether to form a Grignard reagent, which is then hydrolysed by water to give the parent hydrocarbon.
Step 1: Cyclohexyl bromide reacts with magnesium metal in dry ether (an aprotic solvent, essential since Grignard reagents react violently with any trace of water or protic solvent):
A Grignard reagent is formed by reacting an alkyl (or aryl) halide with magnesium metal in dry ether; CH3MgBr is a simple example.
CH3-Br + Mg --(dry ether)--> CH3-MgBr
CH3MgBr (methylmagnesium bromide) is a Grignard reagent - a general class of organomagnesium halides RMgX widely used to form new C-C bonds by reacting with carbonyl compounds, CO2, etc. Other equally valid exampl …
Q.The common name of alkyl magnesium halide is \rule{2cm}{0.4pt}.
›Reveal solutionSolution
The common name of an alkyl magnesium halide (RMgX) is Grignard reagent.
Alkyl (or aryl) magnesium halides of the general formula RMgX are known as Grignard reagents, named after Victor Grignard. They are prepared by the reaction of a haloalkane with magnesium in dry ether: …
Q.How will you carry out the following conversion? Ethylchloride to propanoic acid
›Reveal solutionSolution
Because propanoic acid has one more carbon than ethyl chloride, the chain is first extended via a nitrile (using KCN) and the nitrile is then hydrolysed to the carboxylic acid.
Step 1 — Nucleophilic substitution to form the nitrile
Ethyl chloride reacts with KCN (in ethanolic solution) via an SN2 mechanism, in which CN− displaces Cl−, attaching through carbon (since KCN is ionic and the carbon end of CN− is the more nucleophilic site):
CH3−CH2−Cl+KCNethanolCH3−CH2−C≡N+KCl
This step is essential because it is the only step here that adds a carbon atom to the chain — direct hydrolysis of the C–Cl bond alone (giving ethanol) could never yield a 3-carbon acid.
Step 2 — Hydrolysis of the nitrile to the carboxylic acid
The nitrile (propanenitrile) is then hydrolysed under acidic (or alkaline) aqueous conditions with heating; the reaction proceeds through an amide intermediate to the carboxylic acid:
Q.Identify the product formed when phenyl magnesium bromide undergoes hydrolysis.
›Reveal solutionSolution
Grignard reagents react instantly with the acidic H of water to give the parent hydrocarbon — here, PhMgBr + H₂O gives benzene.
Grignard reagents (R-MgX), prepared by reacting a haloalkane/haloarene with magnesium in dry ether, are extremely reactive towards any source of even mildly acidic hydrogen, because the carbon attached to Mg is highly nucleophilic/basic (behaves like a carbanion, Rδ−-Mgδ+X).
Water readily protonates this carbanion-like carbon:
Q.In the Grignard reagent, _______ metal forms an organometallic bond.
(a) Mg
(b) Na
(c) K
›Reveal solutionSolution
A Grignard reagent, R-Mg-X, is formed when magnesium metal inserts into the carbon-halogen bond of an alkyl/aryl halide, creating a carbon-magnesium organometallic bond.
A Grignard reagent is prepared by reacting an alkyl or aryl halide (R-X) with magnesium turnings in dry ether: