Bouveault-Blanc反应

将羧酸酯用金属钠和无水醇直接还原生成相应地伯醇的反应。主要用于高级脂肪羧酯的还原。此反应最早由Bouveault和Blanc在1903年报道，反应中钠是单电子还原基，乙醇是质子供体。其他醇也可以作为质子供体，但是如果反应体系中没有质子供体，则会发生酮醇缩合反应（acyloin condensation）。在一些情况下芳环也会被还原发生Birch还原反应。后来由于氢化铝锂的广泛应用，此法在实验室中已经很少采用，但是因为其简便易行，在工业上仍然在广泛应用。

反应机理

Bouveault-Blanc反应的历程可能如下：首先，酯从钠获得一个电子还原为自由基负离子，然后从醇中夺取一个质子转变为自由基，自从钠得到一个电子变成碳负离子，消除烷氧基成为醛，醛再经过相同的步骤还原成醇钠，醇钠再得到一个质子变成相应的醇。

反应实例

The central neck of a 5-l. three-necked round-bottomed flask is fitted with astopper carrying a mercury-sealed mechanical stirrer. One of the side necks isconnected by means of a short piece of heavy rubber tubing to a large refluxcondenser about 2 m. long, with an inner tube 2.5 cm. in diameter. The thirdneck is fitted with a separatory funnel.

In the flask are placed 70 g. (3 moles) of sodium and200 cc. of dry toluene. The flask is heated in an oil bath until the sodium ismelted. The stirrer is then started; when the sodium is finely divided, the oilbath is removed and the mixture allowed to cool. Stirring must be continuedduring the cooling in order to keep the sodium finely divided.

Whenthe mixture has cooled to about 60°, there are added from the separatoryfunnel, first, a solution of 114 g. (0.5 mole) of ethyl laurate in 150 cc. ofabsolute alcohol, then 500 cc. more of alcohol, as rapidly as is possiblewithout loss of material through the condenser. The time required for theaddition of the ester solution and the alcohol is less than five minutes,usually two or three minutes. When the reaction has subsided, the flask isheated on a steam bath until the sodium is completely dissolved. The mixture isthen steam-distilled to remove the toluene and ethyl alcohol. The contents ofthe flask are transferred to a separatory funnel while still hot and washedthree times with 200-cc. portions of hot water to remove the sodium laurate.The lauryl alcohol is extracted with ether from the cooled mixture and thewashings. The combined ether extracts are washed with water, sodium carbonatesolution, and again with water, and dried over anhydrous magnesium sulfate. Theether is evaporated and the lauryl alcohol distilled under diminished pressure.The yield is 60–70 g. (65–75 per cent of the theoretical amount) of a productboiling at 143–146°/18 mm. or 198–200°/135 mm.

【Organic Syntheses, Coll. Vol. 2,p.372; Vol. 10, p.62.】

【J. Am. Chem. Soc., 1946, 68, 1932】

【J. Org. Chem., 1998, 63,3141】

参考资料

Comprehensive Organic Name Reactions and Reagents, by Zerong Wang，P493-495.