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The Claisen Condensation

Take the aldol you already know, swap the aldehyde for an ester, and one new thing happens at the end: a leaving group walks off. That single twist turns two ordinary esters into a beta-keto ester — and it is the very same move cells use to build fatty acids two carbons at a time.

The Aldol, With One New Ending

Earlier in this rung you watched the aldol reaction: an enolate — the nucleophilic carbon born by pulling an alpha proton off one carbonyl — reaches over and adds to the carbonyl of a second molecule. The product was a beta-hydroxy carbonyl, with a fresh C-C bond stitched together. The Claisen condensation is that same opening move, played with esters instead of aldehydes or ketones. An ester has its own slightly acidic alpha hydrogens, so a base can deprotonate it to make an ester enolate, and that enolate carbon attacks the carbonyl of a second ester. So far, beat for beat, it is the aldol.

Then comes the difference, and it is the whole story. When an enolate adds to an aldehyde, the new oxygen-bearing carbon has nowhere to send its negative charge except back up to oxygen — so it just grabs a proton and you keep an alcohol. But an ester carbonyl carries something an aldehyde does not: an OR group sitting right on the carbonyl carbon, a built-in leaving group. The alkoxide that briefly forms collapses back down, kicks that OR group out, and rebuilds the C=O. The result is not a beta-hydroxy compound at all but a beta-keto ester — two carbonyls separated by one CH2, with one ester oxygen lost as an alkoxide ion. The leaving group is the single feature that tells Claisen and aldol apart.

Walking the Mechanism

Let us run the textbook case: two molecules of ethyl acetate, CH3CO2CH2CH3, treated with sodium ethoxide (CH3CH2O minus, Na plus) — and notice the base is chosen to match the ester's own OR group, a detail we will explain in a moment. Keep your eye on where the negative charge lives at each beat; the mechanism is just a charge being passed along like a hot coal.

  1. Make the nucleophile. Ethoxide plucks an alpha hydrogen off one ester to give an ester enolate — the carbon next to the C=O now carries the negative charge, cushioned by resonance onto the carbonyl oxygen. This deprotonation is slow and incomplete: an ester alpha C-H has a pKa near 25, while ethanol is about 16, so at any instant only a trace of enolate exists. That trace is enough.
  2. Attack. The enolate carbon, lone pair forward, adds to the carbonyl carbon of a second, intact ester. Arrow from the enolate carbon to the C of the C=O; the pi electrons of that C=O fold up onto oxygen. The carbonyl carbon is now sp3 and bears four groups, and the charge has moved onto oxygen — a tetrahedral intermediate.
  3. Collapse and kick. That alkoxide is unstable as a four-bonded carbon; the oxygen's lone pair drops back down to re-form the C=O, and to make room the C-OR bond breaks — ethoxide leaves with its electron pair. This is the step the aldol can never take, because an aldehyde has no leaving group here. The new bond between the two halves survives; out pops a beta-keto ester (ethyl acetoacetate) plus a free ethoxide.
  4. The deprotonation that pays for everything. So far the equilibria have been unfavourable — each step is roughly uphill or balanced. But the product sits between two carbonyls, so its central CH2 is unusually acidic (pKa near 11). The ethoxide floating around removes that proton, dropping the product into a deep, stable enolate well. That final, strongly favourable deprotonation is what drags the whole reaction forward. On workup you add acid to reprotonate and isolate the neutral beta-keto ester.
  2  CH3-C(=O)-OEt   --NaOEt-->   CH3-C(=O)-CH2-C(=O)-OEt  +  EtO(-)
  (ethyl acetate)                 (ethyl acetoacetate = a beta-keto ester)

   enolate carbon ---> attacks 2nd C=O ---> tetrahedral O(-) ---> kicks out OEt
   then: EtO(-) removes the central H (pKa ~11)  <-- this step pulls it forward

   leaving group present  ==> Claisen (acyl substitution, restores C=O)
   no leaving group       ==> aldol   (addition, keeps the C-OH)
Two esters become one beta-keto ester. The OR leaving group is what lets the carbonyl reform; the acidic central CH2 (pKa ~11) is what makes the final deprotonation drive the equilibrium.

Getting the Conditions Right

Two practical rules fall straight out of that mechanism. First: match the base to the ester's alkoxy group. If you used hydroxide on ethyl acetate, the OH would attack the ester carbonyl and simply hydrolyse it (saponification) — the very reaction from the acyl-substitution rung. Worse, if you used methoxide on an ethyl ester, methoxide could displace ethoxide and scramble your starting material. Using ethoxide with an ethyl ester means any such attack just swaps like for like and changes nothing. So the classic recipe is sodium ethoxide for ethyl esters, sodium methoxide for methyl esters.

Second: the ester must have at least two alpha hydrogens. Look again at why. The reaction only goes because the final product can be deprotonated at its central CH2 to fall into the stable enolate well — that is the thermodynamic engine. A product carbon flanked by two carbonyls needs at least one hydrogen of its own to lose; trace it back and the donor ester must have started with two alpha hydrogens (one is spent forming the enolate that attacks, one survives into the product to be removed at the end). An ester like ethyl isobutyrate, (CH3)2CH-CO2Et, has only a single alpha hydrogen, so its Claisen product has no acidic central proton, the driving deprotonation is impossible, and under ordinary ethoxide conditions the reaction simply fails to accumulate product.

Dieckmann: A Claisen That Closes a Ring

Now tie both ends of the rope to the same molecule. The Dieckmann condensation is just an intramolecular Claisen: take one molecule that carries two ester groups — a diester — far enough apart along a chain, and let the alpha carbon of one ester reach around and attack the carbonyl of the other. Same enolate, same tetrahedral intermediate, same expelled alkoxide. The only new thing is that the new C-C bond now closes a ring, because both reacting groups were tethered to one backbone from the start.

The product is a cyclic beta-keto ester. Geometry decides which ring you get: five- and six-membered rings form smoothly and selectively, because a chain reaching around to make a 5- or 6-ring meets little strain and the two ends find each other easily, whereas three- or four-membered rings are too strained and very large rings demand the two ends meet against long odds. A diester with six carbons in its chain, for instance, neatly delivers a five-membered ring bearing a ketone and an ester. Dieckmann is a favourite for the same reason the intramolecular aldol is: tethering the partners turns a sluggish bimolecular meeting into a quick local one.

Mixed Claisens, and the Biosynthetic Echo

What if you mix two different esters that both have alpha hydrogens? You inherit the same headache as the crossed aldol from earlier in this rung: each ester can be the enolate or the target, so you get a messy stew of up to four products. The clean trick is the same too — pair one ester that has alpha hydrogens (the donor) with a partner that has NONE but is a good electrophile and cannot make its own enolate. Diethyl carbonate, ethyl formate, and diethyl oxalate are the usual stooges. Only the donor can become the nucleophile, only the partner can be attacked, and you get one product. This is the crossed Claisen, and it is how chemists install a carbonyl exactly where they want it.

Here is the part that should make you sit up. Your own cells run a Claisen condensation millions of times a second to build fatty acids. The reaction that lengthens a fatty-acid chain is a Claisen: an acetyl unit, carried as a thioester on a carrier protein, has its alpha carbon condense onto a growing acyl chain, forging a new C-C bond and expelling the carrier as the leaving group — exactly the move we just traced, only with a sulfur-based leaving group instead of OR. That is why fatty acids almost universally have an even number of carbons: nature builds them two carbons at a time, one Claisen step per round.

Be honest about how far the echo carries, though. The flask version and the biological version share the same heart — an ester (or thioester) enolate attacking an acyl carbon and expelling a leaving group — but biology cheats in clever ways the flask cannot. The building block is not simple acetyl but malonyl, a unit carrying an extra carboxyl that is lost as CO2 by decarboxylation at the very moment of the C-C bond formation; that release of a stable gas yanks the equilibrium forward, the same trick of 'losing something stable to drive an uphill reaction' that the final deprotonation pulled in the flask. And each step is shepherded by an enzyme holding the partners in place. Same chemical logic, different and far more controlled stagecraft.