Organic Chemistry 2 · Claisen condensation and conjugate addition
Claisen of ethyl isobutyrate with trityl sodium
Conditions: Ph₃C⁻ Na⁺ (1 equiv), Et₂O; then H3O⁺
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The mechanism, step by step
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Deprotonate irreversibly with the trityl anion
The trityl anion (pKa of triphenylmethane ≈ 31) removes the single α-hydrogen of ethyl isobutyrate (pKa ≈ 25). Because the base is about six pKa units stronger than the enolate, this deprotonation runs to completion and does not reverse.
The curved arrows
- The lone pair on the trityl carbon bonds to the tertiary α-hydrogen.
- The C–H pair stays on the α-carbon, giving the fully formed ester enolate.
Enolate adds to a second ester
The tertiary enolate carbon attacks the carbonyl carbon of a second ethyl isobutyrate molecule. Both partners are crowded, so this addition is slow, but nothing competes with it once all the ester enolate has formed.
The curved arrows
- The enolate carbon lone pair forms the new C–C bond to the second ester carbonyl carbon.
- The C=O π pair moves onto oxygen, giving a tetrahedral alkoxide.
Collapse and expel ethoxide
The alkoxide re-forms the C=O bond and pushes out ethoxide, giving the β-keto ester. Its carbon between the carbonyls carries two methyls and no hydrogen, so no stabilized anion can form; the full conversion was secured by the first, irreversible deprotonation instead.
The curved arrows
- An oxygen lone pair re-forms the C=O π bond.
- The C–O bond to the ethoxy group breaks and ethoxide leaves with the pair.
Acidic workup
Hydronium, added after the reaction, protonates the ethoxide released in the condensation. The neutral β-keto ester is isolated before any base can cleave it.
The curved arrows
- The ethoxide oxygen lone pair bonds to a proton of hydronium.
- The O–H pair returns to oxygen, releasing water.
Ethyl 2,2,4-trimethyl-3-oxopentanoate
The product is ethyl 2,2,4-trimethyl-3-oxopentanoate, with triphenylmethane, ethanol and water as by-products. Sodium ethoxide fails here because the product cannot be trapped as an anion, so retro-Claisen cleavage would undo it.
What you'll practice
Draw the Claisen condensation of ethyl isobutyrate driven by trityl sodium: irreversible enolate formation, addition to a second ester, loss of ethoxide and the workup to ethyl 2,2,4-trimethyl-3-oxopentanoate.
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