Q2(d). Bouveault–Blanc reduction and acyloin condensation

Introduction. Both reactions start with single-electron transfer from sodium to the ester C=O, giving a ketyl radical anion. What happens next depends on the solvent. In ethanol (a proton donor) the radical anion is protonated and reduced further (Bouveault–Blanc reduction). In aprotic xylene two radical centres couple (acyloin condensation); with a diester the coupling is intramolecular and closes a ring.

(i) Excess Na, EtOH: Bouveault–Blanc reduction

1. RCO₂Et + e⁻ → RC(O⁻)OEt (ketyl); EtOH protonates it; a second electron and loss of EtO⁻ give the aldehyde RCHO.
2. The aldehyde is reduced in the same way (e⁻, H⁺, e⁻, H⁺) to RCH₂OH. Each ester uses 4 Na and 4 EtOH.
3. Both ester groups are reduced: product heptane-1,7-diol, HO(CH₂)₇OH (C₇H₁₆O₂).

Scheme: Each CO₂Et becomes CH₂OH.

(ii) Na, xylene, then water: acyloin condensation

Scheme: Ring closure by radical coupling of the two ester ends.

1. Sodium surface gives one electron to each ester C=O; the two ketyl radical anions on the same chain couple to form a C–C bond (ring closure; the reaction occurs on the metal surface, so high dilution is not needed).
2. Both tetrahedral centres expel EtO⁻, giving cycloheptane-1,2-dione.
3. Two more electrons reduce the dione to the enediolate (sodium salt), which is stable in xylene.
4. Water protonates the enediolate to the enediol, which tautomerises to the α-hydroxy ketone.
5. Product: 2-hydroxycycloheptan-1-one (an acyloin, C₇H₁₂O₂): the five CH₂ plus the two former ester carbons give a seven-membered ring.

Conclusion. Excess Na/EtOH: heptane-1,7-diol (Bouveault–Blanc). Na/xylene, then water: 2-hydroxycycloheptanone (intramolecular acyloin condensation).

Application. The acyloin condensation is the best route to medium and large rings (10–20 members) and was used to make catenanes; Bouveault–Blanc reduction made fatty alcohols for detergents before LiAlH₄ and catalytic hydrogenolysis.