Grignard Reaction

What a Grignard does

An ordinary alkyl halide R-X (X = Cl, Br, I) has a δ⁺ carbon — it is an electrophile. Stir it with magnesium turnings in dry diethyl ether and something dramatic happens: the polarity inverts. The carbon becomes δ⁻ and the magnesium picks up the δ⁺ end:

    R-X      +    Mg    ──Et₂O, dry──→    R-Mg-X
    δ⁺ δ⁻         (0)                       δ⁻  δ⁺

This is "umpolung" — polarity inversion — in its simplest form. The carbon that used to be the electrophile is now a nucleophile. The species in solution is not literally a free carbanion; it is a polar covalent C-Mg bond with significant ionic character (about 50% ionic by Pauling electronegativity). For the purposes of arrow-pushing, treat R⁻ as if it were free.

Once made, the Grignard attacks essentially any electrophile in sight: carbonyls, epoxides, nitriles, CO₂, alkyl halides (sometimes), even Mo and Ti alkyls in catalytic cycles. By far the most useful targets are carbonyls — aldehydes, ketones, esters — which give alcohols after aqueous workup.

Step 1: Forming the Grignard

1. Activate the magnesium. The metal surface is coated in MgO. Add a crystal of I₂ or scratch the metal with a glass rod to expose fresh Mg. Some labs add a few drops of 1,2-dibromoethane as an "initiator" — it reacts with Mg to give MgBr₂ + ethylene and roughens the surface.
2. Add the alkyl halide slowly. Dissolve RX in dry Et₂O or THF, drip into a flask containing Mg turnings under N₂ at room temperature. The reaction is exothermic; uncontrolled addition can boil the solvent.
3. Watch for the cloudy gray haze. The solution turns cloudy as RMgX forms and Mg metal disappears. With reactive halides (alkyl iodides, allyl bromides) this happens in minutes; with chlorides, it may need gentle reflux.
4. Titrate (optional). Real concentration is rarely the nominal concentration. Titrate against a standard such as sec-butanol with 1,10-phenanthroline indicator before any quantitative work.

The reaction is a single-electron transfer (SET) process at the magnesium surface, going through R• and Mg-X•⁻ radical-pair intermediates that recombine to RMgX. This is why some halides give partial racemization of an existing stereocenter on R (a free radical loses chirality), and why secondary halides occasionally undergo cyclization (radical clock reactions) before recombining.

Worked example: CH₃MgBr + acetone → t-butanol

    1) form reagent

       CH₃-Br  +  Mg    ──Et₂O, 25 °C, 30 min──→    CH₃-Mg-Br

    2) attack the ketone

                                        O⁻ MgBr⁺
                  O                     |
                  ‖                     |
       CH₃-Mg-Br + CH₃-C-CH₃   →   CH₃-C-CH₃     (alkoxide intermediate)
                                        |
                                        CH₃

    3) aqueous workup (NH₄Cl, H₂O)

       alkoxide  +  H⁺   →   (CH₃)₃C-OH    t-butanol

• Reagents. Acetone 1.0 equiv, methylmagnesium bromide 1.1 equiv (3.0 M in Et₂O), 25 °C.
• Time. Drip the ketone into a stirred Grignard solution at 0 °C, then warm to 25 °C for 1 h.
• Workup. Quench with saturated NH₄Cl (gentle acid) at 0 °C, extract with Et₂O, dry over MgSO₄, distill.
• Yield. 80-90% t-butanol.

The same logic generalizes to the carbonyl-class lookup table that every organic student memorizes:

Electrophile	Grignard adds once or twice?	Product after H⁺ workup
Formaldehyde HCHO	Once	Primary alcohol R-CH₂-OH
Aldehyde RCHO	Once	Secondary alcohol R-CH(OH)-R'
Ketone R₂C=O	Once	Tertiary alcohol R₃C-OH
Ester RCOOR'	Twice (if R'MgX excess)	Tertiary alcohol with two of R'
CO₂ (dry ice)	Once	Carboxylic acid R-COOH
Nitrile RC≡N	Once, then hydrolyze	Ketone R-C(=O)-R'
Epoxide	Once (opens at less hindered C)	Alcohol with new C-C bond

Grignard vs Organolithium vs Organozinc

	Grignard (RMgX)	Organolithium (RLi)	Organozinc (R₂Zn or RZnX)
Polarity / reactivity	Strongly nucleophilic, basic	Even stronger; aggressive	Mild — much less basic
Typical solvent	Et₂O, THF	Hexanes, Et₂O, THF	Et₂O, toluene, hexanes
Functional-group tolerance	Poor (no OH, NH, ester)	Worst (attacks even amides)	Good — tolerates esters, ketones, even some OH
Stability of reagent	Hours in dry Et₂O	Decomposes; use cold, fresh	Months sealed, no glove box for many
Adds to esters?	Twice (gives 3° alcohol)	Twice	Usually not — stops at ketone
Catalytic asymmetric C-C?	Limited	Limited	Yes — Negishi coupling, Reformatsky, Et₂Zn additions
Pyrophoric?	No (handles under N₂)	Yes (n-BuLi, t-BuLi)	Generally no
Most common use	Make 2°/3° alcohols	Deprotonate, lithiate aromatics	Cross-coupling, mild conjugate additions

The pattern is "more ionic = more reactive but less selective." Lithium is the most electropositive of the three; magnesium sits in the middle; zinc is the gentlest. For a route that requires reacting in the presence of an ester or ketone elsewhere on the molecule, an organozinc (or its Reformatsky cousin) is often the only choice.

Real-world syntheses

• Tamoxifen. The classical Bristol-Myers route uses a Grignard step: 4-methoxyphenyl magnesium bromide adds to a ketone bearing the rest of the carbon framework, generating the tertiary benzylic alcohol that dehydrates to the trisubstituted alkene of this breast-cancer drug.
• Antihistamine diphenhydramine (Benadryl). Phenylmagnesium bromide adds twice to ethyl benzoate, giving triphenylmethanol — adapted to install both phenyl rings of the active.
• Fragrance industry. Iso E Super and ambroxide intermediates use Grignard additions to set the quaternary stereocenter that defines the olfactory profile.
• Industrial scale. Multi-tonne ibuprofen and agrochemical intermediates run alkyl Grignards in continuous-flow reactors to manage the exotherm.

Variants and modern developments

• Turbo Grignards (Knochel). RMgCl·LiCl. The lithium chloride breaks up Mg aggregates, dramatically accelerating both formation and addition steps; functional-group tolerance closer to organozincs.
• Magnesium-halogen exchange. Treat ArI with i-PrMgCl·LiCl at -40 °C; iodine and Mg trade places. Bypasses Mg insertion for sensitive substrates.
• Reformatsky reagent. Zn instead of Mg, with α-halo esters. Milder, tolerates esters elsewhere on the molecule.
• Catalytic Grignard cross-coupling. Kumada coupling: ArMgX + ArX → biaryl with Ni or Pd catalysis.
• Continuous-flow Grignards. Industrial chemistry now generates Grignards on demand in microreactor channels — better heat management and no large stockpiles of reactive reagent.

Common pitfalls

• Trace water destroys the reagent. Even 50 ppm water in your THF is enough to consume a few mol% of Grignard. Distill solvents over Na/benzophenone or use molecular sieves; flame-dry glassware; work under N₂.
• Acidic protons elsewhere on the molecule. Free OH, NH₂, COOH, or terminal alkyne C-H proton-quench your Grignard. Either protect them (silyl ether, Boc-amine, methyl ester) or use 2+ equivalents of Grignard so one is sacrificed to the proton.
• Wurtz coupling. If your alkyl halide is highly reactive (allyl, benzyl) and your Grignard concentration is high, R-X + RMgX → R-R + MgX₂ becomes a parasitic side reaction. Use lower temperatures and slow addition.
• Two additions to esters. A Grignard adds once to an ester to give a ketone, but the ketone is more electrophilic and the second equivalent adds before workup. To stop at the ketone, use a Weinreb amide instead — the chelated tetrahedral intermediate is stable until workup.
• Storage assumptions. Commercial Grignard solutions decompose slowly even at -20 °C. The bottle of MeMgBr that has been on the shelf for two years is probably not 3.0 M anymore; titrate before use.