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  The Claisen Condensation Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Claisen Condensation

The aldol reaction is a significant and helpful reaction of aldehydes and ketones where the carbonyl group serves both as an electrophilic reactant and the source of a nucleophilic enol species. Esters go through the same transformation called the Claisen Condensation. Four instances of this base-induced reaction, which generally forms beta-ketoester products, are displayed in the following diagram. Greek letter assignments for the ester results are given in blue color in the diagram. Equation no.1 exists the synthesis of the main reagent ethyl acetoacetate, and no.2 demonstrates the normal form of the Claisen condensation. Intramolecular reactions, like no.3, lead to rings (generally five or six-membered) and are considered as Dieckmann Condensations. The last equation displays a mixed condensation among two esters, one of that has no alpha-hydrogens. The result in this example is a phenyl substituted malonic ester rather than a ketoester.

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In a identical mode to the aldol reaction, the basic event in the Claisen condensation is a dimerization of two esters by an alpha C-H addition of one reactant to the carbonyl group of a second reactant. This bonding is followed by the alcohol elimination from the resultant hemiacetal. The eventual formation of a resonance stabilized beta-ketoester enolate anion as displayed on the third row of the technique, presents a thermodynamic driving force for the condensation. Note: this stabilization is only achievable if the donor has two reactive alpha-hydrogens.

The Claisen condensation different from the aldol reaction in various important ways.

(i) The aldol reaction might be catalyzed by acid or base but most Claisen condensations require base. 
(ii) In difference to the catalytic base employed for aldol reactions, a full equivalent of base (or more) must be used for the Claisen condensation. The extra base is needed since beta-ketoesters having acidic hydrogens at the alpha-carbon are stronger acids (by about 5 powers of ten) than the alcohol co-product. Subsequently, the alkoxide base released after carbon-carbon bond formation (upper right structure in the mechanism picture) instantaneously removes an alpha proton from the beta-ketoester result. As noticed above, formation of this doubly-stabilized enolate anion presents a thermodynamic driving force for the condensation.
(iii) The aldol reaction might be catalyzed by hydroxide ion, but the Claisen condensation needs that alkoxide bases be employed, in order to avoid ester hydrolysis. The particular alkoxide base employed should match the alcohol component of the ester to avoid ester exchange reactions. Very strong bases like LDA may also be employed in this reaction.
(iv) The stabilized enolate result might be neutralized by aqueous acid in order to get the beta-ketoester product.

Transformations identical to the Claisen condensation may be influenced with mixed carbonyl reactants, that may include ketones and nitriles with esters. Esters generally serve as the electrophilic acceptor component of the condensation. Anhydrides and Acyl chlorides would also be good electrophilic acceptors, but they are more costly than esters and do not tolerate the alcohol solvents frequently employed for Claisen condensations. 
In the example of mixed condensations, complex result mixtures are generally avoided by using an acceptor ester that has no alpha-hydrogens. Instances of such type of reactants are: ethyl formate (HCO2C2H5), diethyl carbonate (C2H5OCO2C2H5), ethyl benzoate (C6H5CO2C2H5) and diethyl oxalate (C2H5O2C-CO2C2H5). Equations no. 2, 3 & 4 below demonstrate the make use of such type of acceptors with ester, nitrile and ketone are the donor compounds. Nucleophilic enol species from the nitrile in no.4 may be written as: C6H5CH=C=N(-). The 2-formylcyclohexanone result from reaction #3 presents predominantly as its hydrogen-bonded enol. Most beta-ketoesters have important enol concentrations, but the formyl group has an outstanding bias for this tautomer.

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Equation no.1 displays a condensation in which both reactants might serve either as donors or acceptors. Selective formation of one of the four probable condensation products is because of the reversibility of these reactions and the driving force presented by resonance stabilization of the enolate anion of 2,4-pentanedione (pKa=9). The Protonation of this anion gives the result. The last equation (no.5) provides an interesting example of selectivity. There are 3 ester functions, each of them has at least one alpha-hydrogen. Only one of which, that on the left side, has two alpha-hydrogens and will yield an enolizable beta-ketoester by functioning as the donor in a Dieckmann cyclization. The Strained four-membered rings are not favored by reversible condensation reactions, thus ring closure to the ester drawn below the horizontal chain does not take place. The only reasonable result is the five-membered cyclic ketoester.
Even though many Claisen condensations are carried out with a full equivalent of the alkoxide base, an effective alternative technique, employed in reaction no.5, uses sodium hydride (NaH) together with a catalytic amount of alcohol. The catalytic alcohol responds with NaH to produce alkoxide, this starts a condensation reaction and the result alcohol then reacts with more NaH to give alkoxide.

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