Difficulty: Intermediate | Prerequisites: Hydride reagents (LT8 Part 1), SN2 mechanism, electronegativity, curved-arrow notation, carbonyl reactivity hierarchy.
Hydride reagents let you reduce carbonyls, but they cannot form new carbon-carbon bonds. This section introduces organometallic reagents (Grignards, alkyl lithiums, Gilman reagents) that act as carbon nucleophiles, enabling you to build molecular complexity by attaching new carbon fragments to carbonyl electrophiles. This is one of the most important tools in synthetic organic chemistry. You should already be comfortable with hydride reductions and the idea that irreversible nucleophiles form stable products through nucleophilic addition.
Grignard reagents (RMgBr) and alkyl lithiums (RLi) are powerful carbon nucleophiles that add to carbonyls to form alcohols while creating new C-C bonds. Gilman reagents (R2CuLi) are milder, "softer" versions that react selectively with acid chlorides to give ketones without over-adding.
Grignard reagent (RMgBr)
An organometallic compound formed by reacting an alkyl or aryl halide with magnesium metal. The carbon bonded to magnesium behaves as a carbanion (R:⁻) and attacks electrophilic carbons. Think of it as a carbon with a negative charge, ready to form a new C-C bond.
Organometallic
Any compound with a direct carbon-metal bond. In this context, Grignards (C-Mg), alkyl lithiums (C-Li), and Gilman reagents (C-Cu) are the three types you need.
Carbanion
A carbon atom bearing a formal negative charge and a lone pair. In simple terms, a carbon that acts as a nucleophile. The conjugate acid of a methyl carbanion (CH4) has pKa ≈ 60, making carbanions extremely strong bases and irreversible nucleophiles.
Alkyl lithium (RLi)
Formed by treating an alkyl halide with two equivalents of lithium metal. Reacts very similarly to a Grignard for carbonyl additions. The C-Li bond is the most polar of the three organometallics, making RLi the most reactive.
Gilman reagent (R2CuLi / lithium dialkyl cuprate)
A milder organometallic made by reacting an alkyl lithium with 0.5 equivalents of CuCl. It is a "soft" nucleophile that reacts selectively with acid chlorides to yield ketones, stopping after a single addition.
Umpolung (polarity reversal)
The concept of flipping the natural polarity of a bond. When you convert an alkyl halide (electrophilic carbon) into a Grignard reagent (nucleophilic carbon), you have performed an umpolung. In simple terms, you turned the carbon from electron-poor to electron-rich.
Before organometallics, you already know two carbon nucleophiles from SN2 chemistry: sodium acetylide (NaC≡CH) and sodium cyanide (NaCN). Both attack alkyl halides via SN2. Organometallics extend this idea to carbonyl electrophiles.
A Grignard reagent is prepared by treating an alkyl or aryl halide with magnesium metal in an aprotic solvent (THF or diethyl ether). This is a reduction: the electrophilic carbon in the alkyl halide becomes nucleophilic.
R-Br + Mg → R-MgBr
Carbon (EN = 2.55) is more electronegative than magnesium (EN = 1.31), so the C-Mg bond is polarised with partial negative charge on carbon. The carbon end behaves as a carbanion ("R:⁻") with pKaH ≈ 60.
Alkyl lithiums are made similarly: R-Br + 2 Li → R-Li + LiBr. The C-Li bond is even more polar (Li EN = 0.98), making alkyl lithiums the most reactive of the three organometallics.
The mechanism mirrors hydride reduction: the carbanion (HOMO) attacks the electrophilic carbonyl carbon (LUMO, pi-star), forming a tetrahedral alkoxide intermediate. An acid workup (H3O⁺) then protonates to give the alcohol.
The key difference from hydride reduction: a Grignard adds a carbon group instead of hydrogen, so a new C-C bond forms.
With aldehydes: Grignard + aldehyde → secondary alcohol (after workup). One equivalent of Grignard adds once.
With ketones: Grignard + ketone → tertiary alcohol (after workup). One equivalent of Grignard adds once.
With esters: The same over-addition problem seen with LAH. The first equivalent of Grignard adds to the ester, forming a tetrahedral intermediate. The leaving group (alkoxide, pKa ≈ 16) departs, giving a ketone. The ketone is more reactive than the ester, so a second equivalent of Grignard immediately adds. Final product after workup: a tertiary alcohol with two identical R groups from the Grignard. You need two equivalents of Grignard for every equivalent of ester.
The functional group that results when any carbonyl compound reacts with a Grignard is an alcohol.
Because Grignards and alkyl lithiums over-add to esters and acid chlorides, chemists developed the Gilman reagent (R2CuLi) for controlled, single carbanion additions.
Synthesis: Alkyl halide → (2 Li) → alkyl lithium → (0.5 eq CuCl) → R2CuLi + LiCl
Reactivity: Gilman reagents react with acid chlorides to yield ketones. No acid workup is required because the product is a neutral ketone, not an alkoxide.
Acid chloride + R2CuLi → Ketone + RCu + LiCl
The reaction stops at the ketone because the Gilman reagent is less reactive than a Grignard or alkyl lithium. It is a "soft" nucleophile (low charge density, larger atom) and is not reactive enough to attack the ketone product.
The reactivity of organometallic reagents follows the polarity of the C-Metal bond. The greater the electronegativity difference, the more polar the bond, and the more reactive the reagent.
RLi: ΔEN = 2.55 - 0.98 = 1.57 (most reactive, most polar)
RMgBr: ΔEN = 2.55 - 1.31 = 1.24
R2CuLi: ΔEN = 2.55 - 1.90 = 0.65 (least reactive, least polar)
RLi and RMgBr behave nearly identically for carbonyl additions. R2CuLi is distinctly milder.
Grignard synthesis: R-Br + Mg (in THF) → RMgBr
Alkyl lithium synthesis: R-Br + 2 Li → RLi + LiBr
Gilman reagent synthesis: 2 RLi + CuCl → R2CuLi + LiCl
Grignard + aldehyde: RCHO + R'MgBr → (1. addition, 2. H3O⁺) → secondary alcohol (R and R' on the carbinol carbon)
Grignard + ketone: R2C=O + R'MgBr → (1. addition, 2. H3O⁺) → tertiary alcohol
Grignard + ester: RCO2R' + 2 R''MgBr → (1. addition ×2, 2. H3O⁺) → tertiary alcohol with two R'' groups
Gilman + acid chloride: RCOCl + R'2CuLi → R-CO-R' (ketone) + R'Cu + LiCl
Electronegativity values to remember: Li = 0.98, Mg = 1.31, Cu = 1.90, C = 2.55.
Grignard reactions are among the most widely used C-C bond-forming reactions in the pharmaceutical and chemical industries. Any time a synthetic route needs to attach a new carbon fragment to a carbonyl, a Grignard or organolithium is the likely tool. Gilman reagents are used when the target is a ketone and over-addition must be avoided.
Students often think Grignards can stop at the ketone stage when reacting with an ester. They cannot: the ketone intermediate is more reactive than the ester, so the second equivalent of Grignard adds immediately.
Students sometimes forget that Grignard reactions require aprotic solvents. Grignards are destroyed by water, alcohols, and any protic solvent because the carbanion is a strong enough base (pKa ≈ 60) to deprotonate them.
Students confuse Grignard and Gilman reactivity. Gilman reagents react with acid chlorides to give ketones. Grignards would over-add past the ketone.
Students forget the acid workup step for Grignard reactions. Like hydride reductions, the immediate product is an alkoxide that must be protonated.
⚠️ Grignard addition mechanisms (with aldehydes, ketones, and esters) are heavily tested. Be prepared to draw full curved-arrow mechanisms.
⚠️ Predicting the product class: aldehyde + Grignard = secondary alcohol, ketone + Grignard = tertiary alcohol, ester + 2 Grignard = tertiary alcohol with two identical R groups.
⚠️ Know how to synthesise a Grignard, an alkyl lithium, and a Gilman reagent from an alkyl halide.
⚠️ The electronegativity ranking (RLi > RMgBr > R2CuLi in reactivity) is a commonly tested reasoning question.
⚠️ Retrosynthetic analysis questions often require you to identify which Grignard and which carbonyl are needed to make a target alcohol.
True or false: A Grignard reagent can be prepared in water as the solvent.
Fill in the blank: The product of a Grignard reaction with a ketone, after workup, is a(n) ______ alcohol.
True or false: A Gilman reagent reacting with an acid chloride gives an alcohol.
Fill in the blank: The most reactive organometallic (among RLi, RMgBr, R2CuLi) is ______.
True or false: Two equivalents of Grignard are needed per equivalent of ester.
Answers: 1. False (aprotic solvent required). 2. Tertiary. 3. False (it gives a ketone). 4. RLi. 5. True.
Q: Draw the product of benzaldehyde reacting with CH3MgBr followed by aqueous workup.
A: A secondary alcohol: 1-phenylethan-1-ol (PhCH(OH)CH3). The Grignard adds a methyl group to the aldehyde carbon. IR data would show the C=O stretch at ~1734 cm⁻¹ disappearing and a broad O-H stretch appearing at ~3300 cm⁻¹.
Q: Why does a Grignard react with an ester to give a tertiary alcohol rather than a ketone?
A: The first equivalent of Grignard adds to the ester, forming a tetrahedral intermediate. The alkoxide leaving group departs (pKa ≈ 16), generating a ketone. Because the ketone is more reactive than the original ester, the second equivalent of Grignard adds immediately. After workup, the product is a tertiary alcohol.
Q: You want to convert an acid chloride to a ketone. Which reagent do you use, and why not a Grignard?
A: Use a Gilman reagent (R2CuLi). A Grignard would add to the acid chloride but then continue to add to the resulting ketone, giving a tertiary alcohol. The Gilman reagent is milder (softer nucleophile, less polar C-Cu bond) and stops after the first addition.
Q: Rank RLi, RMgBr, and R2CuLi from most to least reactive and explain why.
A: Most reactive: RLi (ΔEN = 1.57). Then RMgBr (ΔEN = 1.24). Least reactive: R2CuLi (ΔEN = 0.65). The larger the electronegativity difference between carbon and the metal, the more polar the bond and the more carbanion-like the carbon, making it a stronger nucleophile.
This connects directly to the hydride reagents section (same LT8), because both hydrides and organometallics are irreversible nucleophiles that undergo nucleophilic addition to carbonyls. The difference is product: hydrides give alcohols with no new C-C bond, while organometallics give alcohols with a new C-C bond (or ketones, with Gilman reagents). This also connects forward to 1,2- vs 1,4-addition with alpha,beta-unsaturated carbonyls (next section of LT8) and to synthesis problems in LT6, LT12, and LT18.
Grignard reagent, RMgBr, organometallic, alkyl lithium, RLi, Gilman reagent, R2CuLi, lithium dialkyl cuprate, dialkyl cuprate, carbanion, carbon nucleophile, C-C bond formation, umpolung, polarity reversal, nucleophilic addition, aldehyde Grignard, ketone Grignard, ester Grignard, acid chloride Gilman, secondary alcohol, tertiary alcohol, organometallic synthesis, CHEM 212, LT8, organic chemistry II, Penn State