The Grignard Reaction: Mechanism, Reagents, Examples and Lab Tips

What is the Grignard reaction?

The Grignard reaction is the addition of an organomagnesium halide (a Grignard reagent, written RMgX, where X is Cl, Br or I) to an electrophilic carbon, most often the carbon of a carbonyl group. Because it joins two carbon fragments, it is one of the most important carbon–carbon bond-forming reactions in organic synthesis and one of the first that students meet in Organic Chemistry 2.

R–MgX + R′₂C=O → R′₂C(R)–O⁻ ⁺MgX → (H₃O⁺ workup) → R′₂C(R)–OH

The French chemist Victor Grignard reported the reagents in 1900 while working in Philippe Barbier's laboratory in Lyon. Barbier had added magnesium, an alkyl halide and a ketone together in one pot; Grignard's improvement was to prepare the organomagnesium reagent first, in dry ether, and add the carbonyl compound afterwards, which made the reaction far more reliable. The discovery earned Grignard a share of the 1912 Nobel Prize in Chemistry, with Paul Sabatier.

Two ideas explain almost everything about the reaction. First, a Grignard carbon is nucleophilic: it attacks electron-poor carbon. Second, it is basic: given any acidic hydrogen, it takes the proton instead. The rest of this guide follows from those two facts.

How Grignard reagents are made

A Grignard reagent is made by stirring an alkyl, vinyl or aryl halide with magnesium turnings in an anhydrous ether solvent, usually diethyl ether or tetrahydrofuran (THF):

CH₃CH₂–Br + Mg → CH₃CH₂–Mg–Br (dry Et₂O or THF)

Magnesium inserts into the carbon–halogen bond. The reaction happens on the metal surface and goes through single-electron transfers and short-lived radicals rather than the two-electron arrows used for the rest of the mechanism, so textbooks usually show it as a net equation. Two consequences of the radical pathway matter in practice: some R• radicals couple with each other (Wurtz-type coupling, R–R), and with aryl halides this side reaction gives biaryls such as biphenyl.

Polarity reversal: why the carbon becomes a nucleophile

In bromoethane the carbon bonded to bromine is electron-poor (δ+), because bromine is more electronegative than carbon. Magnesium is much less electronegative than carbon (Pauling values about 1.3 versus 2.6), so after insertion the same carbon carries a partial negative charge (δ−). An electrophilic carbon has become a nucleophilic one. This reversal of polarity, sometimes called umpolung, is the whole point of making the reagent.

Why ether or THF?

• Ethers have no acidic O–H, so they do not destroy the reagent.
• The ether oxygen lone pairs coordinate to magnesium, which is a Lewis acid. In solution a Grignard reagent is better described as RMgX(OR₂)₂ than as bare RMgX, and this solvation keeps it dissolved and reactive.
• THF coordinates more strongly than diethyl ether and is used for less reactive halides such as vinyl and aryl chlorides. 2-Methyltetrahydrofuran (2-MeTHF), made from biomass, is a greener alternative that separates cleanly from water during workup.

Which halides work

Getting the reaction started

Magnesium turnings carry a thin, passivating layer of magnesium oxide, so Grignard formation often has an induction period. Common ways to start it are a small crystal of iodine, a few drops of 1,2-dibromoethane, crushing the turnings with a glass rod, or gentle warming. Once it starts the reaction is exothermic; the ether begins to reflux and the cloudy grey solution of the reagent forms.

The Schlenk equilibrium

In solution, Grignard reagents redistribute their groups: 2 RMgX ⇌ R₂Mg + MgX₂. This Schlenk equilibrium means a "Grignard solution" is really a mixture of species. For predicting products you can treat it as RMgX; the equilibrium matters mainly for kinetics and for reactions that are sensitive to the exact magnesium species.

The Grignard reaction mechanism, step by step

The classic example is ethylmagnesium bromide adding to acetone to give the tertiary alcohol 2-methylbutan-2-ol. The mechanism has two separate stages: nucleophilic addition in dry ether, then a workup with water or dilute acid. Use the controls to replay each arrow.

Stage 1: nucleophilic addition

The electrons of the C–Mg bond attack the electrophilic carbonyl carbon and form the new C–C bond. At the same time the C=O π electrons move onto oxygen, so the carbonyl carbon never exceeds eight electrons. Both arrows describe one event. Magnesium, a Lewis acid, coordinates the carbonyl oxygen and makes the carbon more electrophilic; the product is a magnesium alkoxide, drawn here as O⁻ beside Mg⁺–Br.

The trigonal carbonyl carbon becomes tetrahedral. If the two groups already on the carbonyl carbon differ from each other and from R, a new stereocenter forms. A Grignard reagent can approach either face of a flat carbonyl, so an achiral ketone gives a racemic alcohol unless something chiral in the molecule or the reaction mixture favours one face.

Stage 2: workup

Only after the addition is complete is water, aqueous ammonium chloride or dilute acid added. The alkoxide oxygen takes a proton to give the neutral alcohol, and magnesium ends up as water-soluble salts. Adding the proton source at the same time as the carbonyl compound would simply destroy the Grignard reagent (see below).

Common mistake: starting the first arrow at the carbon atom. The nucleophilic electrons are the C–Mg bonding pair, so the arrow starts at the middle of that bond and ends at the carbonyl carbon.

What Grignard reagents react with: products at a glance

Every reaction in this table is the same nucleophilic attack on an electrophilic carbon. What changes is what that carbon carries and whether a leaving group lets a second equivalent react.

Pick a reaction to see its mechanism:

Why esters react twice

The first addition gives a tetrahedral alkoxide that still carries an alkoxy leaving group. It collapses, expelling the alkoxide, and the ketone that forms is more reactive toward the Grignard reagent than the ester it came from. So the ketone is attacked as fast as it appears, and the product is a tertiary alcohol in which two of the three groups came from the Grignard reagent. To stop at the ketone, chemists use a Weinreb amide (its tetrahedral intermediate is held together by magnesium chelation until workup) or a nitrile.

Why CO₂ and nitriles react once

Addition to CO₂ gives a carboxylate, and addition to a nitrile gives an imine anion (a magnesium imine salt). Both are negatively charged and far less electrophilic than the starting material, so a second equivalent does not add. Aqueous acid then gives the carboxylic acid, or hydrolyzes the imine to a ketone.

Epoxides: an SN2 ring opening

Epoxides are not carbonyls, but their strained three-membered ring makes the C–O bonds easy to break. The Grignard carbon attacks the less hindered ring carbon from the back side, and the C–O bond electrons move onto oxygen. With ethylene oxide the chain grows by two carbons and gives a primary alcohol: a useful complement to formaldehyde, which adds one.

α,β-Unsaturated carbonyls

With enones, Grignard reagents usually add to the carbonyl carbon (1,2-addition). To add to the β carbon instead (1,4- or conjugate addition), chemists use an organocuprate (R₂CuLi) or a catalytic amount of a copper(I) salt with the Grignard reagent.

Why Grignard reactions must be dry, and other limitations

The conjugate acid of a Grignard carbon is an alkane, with a pKa of about 50. Almost any O–H, N–H or even sp C–H is far more acidic, so the reagent is protonated and destroyed before it can find the carbonyl:

Functional groups a Grignard reagent cannot tolerate

• Acidic hydrogens anywhere in either partner: OH, NH, SH, CO₂H and terminal alkynes. Protect alcohols first, for example as silyl ethers (TBS), then remove the protecting group with fluoride after the Grignard step.
• Electrophilic groups in the same molecule as the halide: you cannot make a Grignard reagent from a bromoketone or a bromoester, because the reagent would attack its own carbonyl. Protect a ketone as an acetal first.
• Nitro groups and some other easily reduced groups, which react with the reagent.

Side reactions with hindered ketones

• Enolization: when the carbonyl carbon is crowded, the Grignard reagent acts as a base and removes an α hydrogen instead. After workup the starting ketone comes back.
• Reduction: a Grignard reagent with a β hydrogen (such as isopropylmagnesium bromide) can transfer that hydride to the carbonyl carbon through a six-membered transition state, giving a secondary alcohol and an alkene instead of the addition product.
• Wurtz coupling during reagent formation gives R–R, for example biphenyl from bromobenzene.

Planning a synthesis with Grignard reagents (retrosynthesis)

To plan a Grignard synthesis of an alcohol, find the carbon that carries the OH group (the carbinol carbon) and mentally break one of its bonds to carbon. One fragment becomes the carbonyl compound (the carbinol carbon keeps its oxygen as C=O); the other becomes the Grignard reagent.

Worked example: 2-methylbutan-2-ol

The carbinol carbon of (CH₃)₂C(OH)CH₂CH₃ carries two methyl groups and one ethyl group, so there are two one-step routes and one ester route:

1. Break the C–ethyl bond: acetone + CH₃CH₂MgBr (the route animated above).
2. Break a C–methyl bond: butan-2-one + CH₃MgBr.
3. Use an ester for the two identical methyls: methyl propanoate + 2 CH₃MgBr.

Choose between routes by availability and cost of the starting materials, and by side reactions: a small Grignard reagent adding to an unhindered ketone is the most reliable combination.

Quick rules

• Primary alcohol, RCH₂OH: RMgX + formaldehyde (one carbon added) or RMgX + ethylene oxide for RCH₂CH₂OH (two carbons added).
• Secondary alcohol: RMgX + aldehyde, two possible disconnections.
• Tertiary alcohol with two identical groups: an ester + 2 RMgX.
• Carboxylic acid with one more carbon than an alkyl halide: R–Br → RMgBr → CO₂ → H₃O⁺.
• Ketone: RMgX + a nitrile or a Weinreb amide, not an ester.

The Grignard reaction in the lab: practical tips

Many students run their first Grignard reaction in the teaching lab, often the synthesis of triphenylmethanol from phenylmagnesium bromide and methyl benzoate (two equivalents) or benzophenone (one equivalent). The same rules apply in research labs and in industry.

1. Dry everything. Oven- or flame-dry the glassware, use anhydrous ether or THF, protect the flask with a drying tube or an inert gas (nitrogen or argon), and keep the halide and the carbonyl compound dry.
2. Start the reagent with a small portion of halide, activating the magnesium if needed (an iodine crystal or a drop of 1,2-dibromoethane). Wait for signs of reaction (cloudiness, gentle reflux, the iodine colour fading) before adding the rest.
3. Add the halide slowly to control the exotherm. Adding a large amount before the reaction has started can lead to a sudden, violent reflux once it does.
4. Add the carbonyl compound dropwise, often with cooling, after the reagent has formed.
5. Work up separately: pour the mixture into cold aqueous ammonium chloride or dilute acid. Saturated NH₄Cl is mildly acidic and is preferred for tertiary alcohols that would dehydrate in stronger acid.
6. Remove biphenyl (from Wurtz coupling of phenyl radicals) by washing the crude solid with a non-polar solvent such as hexane or petroleum ether, in which the biphenyl dissolves and the polar alcohol does not.

Safety: diethyl ether is extremely flammable and its vapour is heavier than air, so no open flames; Grignard reagents react violently with water. In industry and research, the concentration of a Grignard solution is measured by titration before use, and "turbo Grignard" reagents (iPrMgCl·LiCl, introduced by Paul Knochel's group in 2004) make functionalized aryl Grignards by halogen–magnesium exchange under mild conditions.

Grignard reagents vs organolithium and organocuprate reagents

In curved-arrow terms all three behave the same way: the electrons of the carbon–metal bond attack an electrophilic carbon. The metal changes how fast that happens, which electrophile wins, and how basic the reagent is.

Sources

1. Grignard, V. "Sur quelques nouvelles combinaisons organométalliques du magnésium et leur application à des synthèses d'alcools et d'hydrocarbures." Comptes Rendus Acad. Sci. 1900, 130, 1322–1324.
2. The Nobel Prize in Chemistry 1912: Victor Grignard and Paul Sabatier. NobelPrize.org. nobelprize.org
3. Seyferth, D. "The Grignard Reagents." Organometallics 2009, 28, 1598–1605. doi.org
4. Garst, J. F.; Soriaga, M. P. "Grignard reagent formation." Coordination Chemistry Reviews 2004, 248, 623–652. doi.org
5. Krasovskiy, A.; Knochel, P. "A LiCl-Mediated Br/Mg Exchange Reaction for the Preparation of Functionalized Aryl- and Heteroarylmagnesium Compounds from Organic Bromides." Angew. Chem. Int. Ed. 2004, 43, 3333–3336. pubmed.ncbi.nlm.nih.gov
6. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press, 2012; chapters 6 and 9.