Organic Chemistry · Organohalides

Reactions of Alkyl Halides: Grignard Reagents

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A is an organomagnesium compound with the general formula R–Mg–X (R = alkyl, aryl, or vinyl; X = Cl, Br, or I), prepared by inserting magnesium metal into a carbon–halogen bond:

\[ \mathrm{R{-}X + Mg \longrightarrow R{-}Mg{-}X} \]

The reaction was discovered by Victor Grignard in 1900 and earned him the Nobel Prize in Chemistry in 1912. What makes Grignard reagents so valuable is that the carbon–magnesium bond is strongly polarized: the carbon carries a partial negative charge and behaves like a . That carbon is a powerful and base, which means Grignard reagents can build new carbon–carbon bonds — the single most important operation in organic synthesis. An alkyl halide is no longer just a molecule with a leaving group; it becomes a building block that can attack carbonyl groups, carbon dioxide, and epoxides.

Why this matters

Almost every molecule with a carbon skeleton — pharmaceuticals, natural products, polymers, agrochemicals — is assembled by forming carbon–carbon bonds, and Grignard chemistry is one of the classic ways to do it. Converting an alkyl halide to a Grignard reagent and then treating it with a carbonyl compound lets a chemist make primary, secondary, or tertiary alcohols by design. The same reagent reacts with CO₂ to give carboxylic acids, which are everywhere in biology and drug chemistry. Understanding Grignard reagents also teaches two habits that carry through all of organometallic chemistry: keeping water and air out of the reaction, and thinking about polarity (which atom is electron-rich, which is electron-poor) before predicting a product.

The college version

Core Concepts

Polarity of the carbon–magnesium bond

Magnesium is much less electronegative than carbon (1.31 vs 2.55 on the Pauling scale), so the bonding electrons sit closer to carbon. The reagent is often drawn with a formal ionic character, R⁻ MgX⁺, and it reacts as though the carbon carried a full negative charge: a carbanion. This is the opposite polarity of an alkyl halide, where carbon is electron-poor (δ+). Grignard reagents are therefore strong bases — the conjugate acid of a typical alkyl group (an alkane C–H) has a pKa around 50, far higher than water (pKa ≈ 15.7), so a Grignard reagent will deprotonate water, alcohols, amines, and terminal alkynes instantly.

Preparation: the rules of the game

Grignard reagents form when magnesium metal (turnings) reacts with an alkyl, aryl, or vinyl halide in diethyl ether or THF. The ether solvent is not passive: its oxygen lone pairs coordinate to the electron-poor magnesium and stabilize the reagent. Three rules follow directly from the reagent's strong basicity:

No water or alcohol may be present — they would destroy the reagent by protonating the carbanion carbon to give the alkane (R–Mg–X + H₂O → R–H + Mg(OH)X).
No acidic hydrogens in the substrate — a molecule containing –OH, –NH, –SH, or –C≡C–H cannot be converted to its Grignard reagent because the reagent reacts with its own starting material.
Air and moisture excluded — reactions run under inert gas; the reagents also react with O₂.

Fluorides do not form Grignard reagents (the C–F bond is too strong for magnesium to insert), while chlorides, bromides, and iodides work well, with bromides being the usual choice for balance of reactivity and cost.

Reactions with carbonyl compounds

The carbanion carbon attacks the electron-poor carbonyl carbon, and the alkoxide that forms is protonated during an aqueous (typically dilute acid) to give the alcohol:

Formaldehyde (H₂C=O) → primary alcohol, R–CH₂OH
Any other aldehyde → secondary alcohol, R–CH(OH)–R′
Ketone → tertiary alcohol, R–C(OH)(R′)(R″)
Esters → tertiary alcohols, because two equivalents of the Grignard reagent add (the first addition forms a ketone, which reacts with a second equivalent)
CO₂ → carboxylic acid, R–COOH (after acid workup)
Epoxides (oxiranes) → primary alcohols with the chain extended by two carbons (attack occurs at the less hindered carbon)

The workup step

After the Grignard addition, the product is an alkoxide salt, R–O⁻ MgX⁺. Adding aqueous acid protonates the alkoxide to the neutral alcohol. The workup is an essential part of the procedure — the C–C bond-forming step and the protonation step are separate, and skipping the workup leaves the alkoxide salt, not the alcohol.

Limitations and side reactions

Because the carbanion carbon is so reactive, Grignard reagents also react with oxygen, halogens, and any acidic proton they encounter. If the target molecule contains an –OH or –NH group, that group must be protected (converted to a non-acidic derivative) before the Grignard reaction. Allylic and benzylic halides can couple with unreacted starting halide instead of forming clean Grignard reagents, so those substrates are used with care.

How It Works / Step-by-Step Process

Choose the halide: no acidic hydrogens in the molecule, no water in solvent or glassware; use anhydrous ether or THF under inert gas.
Add the halide to magnesium turnings; the reaction initiates (often with a small crystal of iodine or gentle warming) and the metal inserts into the C–X bond.
Add the (aldehyde, ketone, ester, CO₂, or epoxide); the carbanion carbon forms a new C–C bond and gives an alkoxide (or carboxylate) salt.
Quench with aqueous acid to protonate the alkoxide; isolate and purify the alcohol (or carboxylic acid).

Common Confusions

Do Not Confuse	With	Difference
Grignard carbon's polarity	Alkyl halide carbon's polarity	In R–Mg–X the carbon is δ− (nucleophile); in R–X the carbon is δ+ (electrophile).
Grignard + water as a solvent	Grignard + water as a quench	Water is never a solvent; it is deliberately added at the end to destroy the reagent and protonate the alkoxide.
Using an ester	Using a ketone	Esters consume 2 equiv of Grignard reagent (via a ketone intermediate); ketones need only 1 equiv.
Grignard reagent attacking carbonyl	Grignard reagent attacking the alkyl halide's own C–X	The reagent attacks electron-poor carbons of C=O; attacking halides leads to unwanted coupling byproducts.
A protecting group	A catalyst	A protecting group masks a reactive –OH/–NH temporarily; it is not consumed catalytically.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine magnesium metal is a magnet that can pull the halogen off an alkyl halide, leaving the carbon with an extra negative charge — like a carbon carrying a backpack full of electrons. That electron-rich carbon is hungry to share: it attacks electron-poor carbons in carbonyl groups, building longer chains, one carbon–carbon bond at a time. But it is so reactive that even a drop of water would "calm it down" instantly, so everything must be kept perfectly dry.

Worked example

Example 1: Designing a synthesis of 2-methyl-2-butanol

Target: 2-methyl-2-butanol, (CH3)2C(OH)CH2CH3. It is a tertiary alcohol, so a sensible route is a ketone + Grignard reagent: acetone, (CH3)2C=O, plus methylmagnesium bromide, CH3MgBr.

The balanced stoichiometry is 1:1:

\[ \mathrm{(CH_3)_2C{=}O + CH_3MgBr \longrightarrow (CH_3)_2C(OMgBr)CH_3 \xrightarrow{H_3O^+} (CH_3)_2C(OH)CH_3} \]

Suppose you start with 0.10 mol of acetone. You need 0.10 mol of CH3MgBr, which is prepared from 0.10 mol of bromomethane and 0.10 mol of Mg. The mass of magnesium required is:

\[ m(\mathrm{Mg}) = n \times M = 0.10\ \mathrm{mol} \times 24.31\ \frac{\mathrm{g}}{\mathrm{mol}} \approx 2.4\ \mathrm{g} \]

Because the mole ratio of Mg to CH3Br is 1:1, the mol units cancel to give grams directly. In the lab you would use a slight excess of Mg, dry ether, and an inert atmosphere — general safety practice for moisture-sensitive organometallic reagents.

Example 2: From aryl halide to benzoic acid

Bromobenzene reacts with Mg to give phenylmagnesium bromide (C6H5MgBr). Bubbling CO₂ (dry ice) into the solution adds the carbanion to the carbon of CO₂, forming the benzoate salt; acid workup gives benzoic acid:

\[ \mathrm{C_6H_5MgBr + CO_2 \longrightarrow C_6H_5CO_2^-MgBr^+ \xrightarrow{H_3O^+} C_6H_5COOH} \]

This is the classic two-carbon-from-one-carbon story in reverse: the aryl halide carbon becomes the carbonyl carbon of the acid. Stoichiometry is again 1:1, so 0.050 mol of C6H5MgBr would consume 0.050 mol of CO₂ — a good reminder that you must supply the CO₂ in at least equimolar amount (usually a large excess of dry ice is used, and the reaction is worked up with dilute acid once it has warmed).

Key takeaways

Grignard reagents are R–Mg–X, made from alkyl/aryl/vinyl halides + Mg in anhydrous ether or THF.
The carbon is nucleophilic (carbanion-like) — the opposite polarity of the carbon in an alkyl halide.
They must be prepared and used under strictly anhydrous, air-free conditions; water destroys them.
Substrates with acidic H (–OH, –NH, –SH, –C≡CH) cannot be used directly.
Formaldehyde → 1° alcohol; other aldehydes → 2° alcohol; ketones → 3° alcohol; esters → 3° alcohol (2 equiv); CO₂ → carboxylic acid; epoxides → 1° alcohol (chain +2 carbons).
The alkoxide product is converted to the alcohol during the aqueous acid workup.
Grignard chemistry is a premier method for forming carbon–carbon bonds.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

Write the general structure of a Grignard reagent and state which atom is nucleophilic.
Show answer
R–Mg–X; the carbon bonded to magnesium is δ− and acts as the nucleophile (carbanion-like).
Why must Grignard reactions be run in anhydrous ether or THF rather than water or ethanol?
Show answer
Water and alcohols have acidic protons (pKa ≈ 15–16) that immediately protonate the carbanion carbon, converting R–Mg–X to the alkane R–H. Ethers have no acidic hydrogens, and their oxygen coordinates to Mg, stabilizing the reagent.
What alcohol class results from reacting a Grignard reagent with (a) formaldehyde, (b) a ketone, (c) an ester?
Show answer
(a) A primary alcohol, R–CH2OH. (b) A tertiary alcohol. (c) A tertiary alcohol — but two equivalents of the reagent are required because the first addition gives a ketone that reacts with the second equivalent.
Why can't 2-bromoethanol (BrCH2CH2OH) be converted to its Grignard reagent directly?
Show answer
The –OH hydrogen is acidic enough to destroy the reagent as soon as it forms: the reagent would deprotonate the alcohol (pKa ≈ 16) rather than persist. The –OH must be protected first.
How many moles of methylmagnesium bromide are needed to convert 0.25 mol of methyl benzoate (C6H5COOCH3) to the corresponding tertiary alcohol, and why?
Show answer
0.50 mol (2 equiv). Esters react with the first equivalent to give a ketone, which then reacts with the second equivalent to give the tertiary alcohol; each C=O consumes one equivalent.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Grignard reagent: An organomagnesium compound, R–Mg–X, with a nucleophilic carbon.
carbanion: A carbon with a negative charge and an unshared electron pair.
anhydrous: Free of water.
nucleophile: An electron-rich species that attacks electron-poor centers.
electrophile: An electron-poor species that accepts electron pairs.
workup: The step after the reaction that converts salts into isolable products.
protecting group: A temporary modification that masks a reactive group.

Sources & references

openstax.org — Organic Chemistry

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