Reversing the Polarity of Carbon
The radical chemistry you just met in this rung broke bonds evenly, sending one electron each way. Now we shift to a metal-mediated world where electron PAIRS move again, but with a twist you have not seen before. All through the earlier rungs, a carbon attached to oxygen, nitrogen, or a halogen was the electron-POOR partner — think of a carbonyl carbon, left δ+ because the greedy oxygen pulls the shared electrons away. That carbon waits to be attacked by a nucleophile. The organometallic trick turns this on its head: it makes carbon itself the attacker.
The whole secret is electronegativity. Carbon, sitting in the middle of the periodic table, is more electronegative than any ordinary metal. So when you bond carbon to magnesium or lithium, the shared pair slumps back TOWARD carbon — exactly the opposite of what happens with oxygen. The carbon becomes δ−, electron-rich, practically a carbanion. A carbon that was born to be δ+ is now δ−. Chemists have a name for this deliberate flip of an atom's usual polarity: umpolung, a German word meaning, almost literally, 'polarity reversal'. It is one of the most powerful ideas in synthesis, because it lets you connect two atoms that, in their natural states, would simply repel.
Grignard and Organolithium: Making the Reagent
The workhorse organometallic is the Grignard reagent, written R-MgBr. You make it by stirring an alkyl, vinyl, or aryl halide R-X with magnesium turnings in scrupulously dry ether. The magnesium atom physically inserts itself INTO the carbon-halogen bond — the metal slides between R and X — leaving the carbon bonded to magnesium and now nucleophilic. Its fiercer relative is the organolithium reagent, R-Li, made the same way but with lithium metal. Because lithium is even less electronegative than magnesium, the carbon in R-Li is more strongly δ−: more reactive, and a stronger base.
R-X + Mg --dry ether--> R-MgBr (the carbon, once delta+, is now delta-) R-X + 2 Li --dry ether--> R-Li + LiX (fiercer cousin; carbon even more delta-)
There is one limit you must respect or you will fail in the flask, not on paper. Because these reagents behave like R− and R− is the conjugate base of an alkane (an impossibly weak acid), the reagent is a ferociously strong base. It will rip a proton off ANY -OH, -NH, or even -C(triple)CH in the flask — including water itself — and instantly die as a harmless alkane R-H, never reaching its intended target. This is exactly why everything must be bone dry, and why a substrate carrying its own acidic O-H or N-H must be fitted with a protecting group first. The reagent is too eager to tell apart 'the proton I should ignore' from 'the carbon I should attack'.
Addition to Carbonyls, Revisited
You already met the carbonyl addition of these reagents in the earlier carbonyl rung; here we look at it again, now that you see WHY it works. The δ− carbon and the δ+ carbonyl carbon are made for each other — opposite charges snapping shut like a magnet finding its pole. The result is a nucleophilic addition that builds a brand-new carbon-carbon bond, the single fact that makes these reagents central to synthesis. Decorating a molecule is useful; growing its carbon skeleton is real power.
- The electron-rich metal-bound carbon attacks the δ+ carbonyl carbon. A curved arrow runs FROM the C-metal bonding pair TO the carbonyl carbon — it moves an electron pair, not the atom; the carbon does not fly across, its electrons reach out.
- As the new C-C bond forms, the C=O pi bond breaks and its electrons fold up onto oxygen. The carbonyl carbon, once flat and sp2, rehybridizes to sp3 and goes tetrahedral; oxygen now carries the negative charge as a metal alkoxide.
- In a SEPARATE workup step, add dilute acid or water. The alkoxide grabs a proton and you isolate an alcohol that is now one R group bigger than the carbonyl you started with. Never add the water with the reagent still alive — it would kill the Grignard before step one.
The class of alcohol reads straight off the carbonyl partner, like a dial. Grignard plus formaldehyde (H2C=O) gives a primary alcohol; plus any other aldehyde gives a secondary alcohol; plus a ketone gives a tertiary alcohol. Push further: with carbon dioxide (O=C=O) the reagent adds once and, after workup, gives a carboxylic acid (R-COOH) — a tidy way to turn R-X into a chain one carbon longer. That ladder is the single most useful pattern here, because it is how you stitch small fragments into a bigger carbon framework, choosing the product by choosing the partner.
Gilman Reagents: The Gentler Cousin
Grignards and organolithiums are blunt instruments — fierce, basic, and indiscriminate. The Gilman reagent, an organocuprate written R2CuLi, is the same idea dialed down to finesse. You make it by treating two equivalents of an organolithium with a copper(I) salt; the copper holds the carbon more loosely, so the carbon is still nucleophilic but far gentler and far less basic. That milder temper unlocks two things its rowdy cousins cannot do cleanly: conjugate addition, and coupling with alkyl halides.
Picture an alpha,beta-unsaturated ketone — a C=C conjugated with a C=O, like C=C-C=O. It offers a nucleophile two doors. The carbonyl carbon (the '1,2' spot) is one; the far end of the double bond (the 'beta' or '1,4' spot) is the other. A Grignard, hard and reactive, tends to barge through the nearer door, adding straight to the carbonyl. A Gilman reagent, soft and patient, prefers the far door: it does conjugate addition, also called 1,4-addition, delivering its R group to the beta carbon and leaving the C=O intact. This is the carbon-nucleophile version of the Michael-type addition you saw with enolates — same regiochemistry, different nucleophile. When you specifically want to install an R group at the beta position of an enone, the Gilman is the reagent of choice.
The second trick is coupling. A Gilman reagent will displace the halide of a second alkyl halide R'-X, joining R and R' into a brand-new R-R' carbon-carbon bond — a coupling that the harsh Grignard does messily but the cuprate does cleanly. It even works on the tough vinyl and aryl halides that ordinary substitution chokes on. Think of it as a hand-built precursor to the metal-catalyzed cross-couplings coming up next in this rung: the copper does the same job of stitching two carbon fragments together, just stoichiometrically rather than catalytically.
Reading the Toolkit Backwards
Lay the three reagents side by side and a strategy appears, each chosen by the bond you want to end up holding. Need a new C-C bond AND an alcohol from a carbonyl? Reach for a Grignard or organolithium; pick the lithium reagent when a hindered or stubborn carbonyl shrugs off the milder magnesium. Need to add an R group to the beta carbon of an enone without touching its C=O? Use a Gilman. Need to weld two carbon fragments together at their halide ends? A Gilman does it stoichiometrically, and a transition-metal catalyst will do it even better in the next guide.
Step back and notice the single idea threading through all of it: a metal less greedy than carbon hands carbon a δ− character it could never have alone, and that umpolung-reversed carbon becomes a builder of carbon-carbon bonds. This is the same logic that, scaled up with a palladium catalyst, earned a Nobel Prize and let chemists assemble molecules once thought impossible. The reagent in your flask and the reaction in the textbook share one root insight — carbon, given the right metal partner, can stop waiting and start attacking.