Ethyl butanoate is treated with finely dispersed sodium in refluxing dry xylene, and the product is then worked up with water and acid (acyloin condensation). The product is

In an aprotic solvent sodium reduces two ester groups by electron transfer and couples them to an enediolate, which on workup tautomerises to the alpha-hydroxy ketone. Two $\mathrm{C}{}_3\mathrm{H}{}_7\mathrm{CO}$ units are joined, giving $\mathrm{C}{}_3\mathrm{H}{}_7-\mathrm{CO}-\mathrm{CH}(\mathrm{OH})-\mathrm{C}{}_3\mathrm{H}{}_7$, which is 5-hydroxyoctan-4-one. The diol would need a protic medium or Mg/Hg type coupling of a ketone, and the dione needs oxidation.