定义Definition
硼氢化钠 NaBH₄ 是白色固体，由 Na⁺ 和四面体形的 BH₄⁻ 组成，是常用的温和还原剂。它能把醛还原为伯醇、把酮还原为仲醇，一般不还原羧酸、酯和碳碳双键。
Sodium borohydride, NaBH₄, is a white solid made of Na⁺ and tetrahedral BH₄⁻ ions, and a common mild reducing agent. It reduces aldehydes to primary alcohols and ketones to secondary alcohols, but generally leaves carboxylic acids, esters and C=C double bonds untouched.
还原机理Mechanism
BH₄⁻ 相当于提供氢负离子 H⁻ 的来源。H⁻ 作为亲核试剂进攻羰基碳（亲核加成），C=O 的 π 键断开，生成烷氧负离子；随后烷氧负离子从水或醇溶剂中得到质子，生成醇。书写方程式时常用 [H] 表示还原剂提供的氢：CH₃CHO + 2[H] → CH₃CH₂OH。
BH₄⁻ acts as a source of hydride, H⁻. Hydride attacks the carbonyl carbon as a nucleophile (nucleophilic addition), breaking the C=O π bond to give an alkoxide ion, which then takes a proton from the water or alcohol solvent to give the alcohol. Equations often use [H] for the hydrogen supplied: CH₃CHO + 2[H] → CH₃CH₂OH.
与氢化铝锂的比较Comparison with LiAlH₄
NaBH₄ 较温和，可在水、甲醇或乙醇中使用，操作安全，选择性好。LiAlH₄ 还原性强得多，能还原羧酸、酯、酰胺和腈，但与水剧烈反应，必须在无水乙醚或 THF 中使用。需要只还原醛酮而保留酯基时选用 NaBH₄。
NaBH₄ is milder; it can be used in water, methanol or ethanol, is safer to handle and more selective. LiAlH₄ is much stronger and also reduces carboxylic acids, esters, amides and nitriles, but reacts violently with water and must be used in dry ether or THF. NaBH₄ is chosen to reduce an aldehyde or ketone while leaving an ester intact.
安全Safety
NaBH₄ 与酸或水（尤其在酸性条件下）反应放出易燃的氢气，应密封干燥保存，远离酸和火源。它有毒，对皮肤和眼睛有刺激和腐蚀作用。
NaBH₄ reacts with acids and with water, especially acidic water, releasing flammable hydrogen; store it sealed and dry, away from acids and flames. It is toxic and irritates or burns skin and eyes.
化学性质Chemical properties
把醛还原为伯醇：RCHO + 2[H] → RCH₂OH
Reduces aldehydes to primary alcohols: RCHO + 2[H] → RCH₂OH
把酮还原为仲醇：R₂CO + 2[H] → R₂CHOH
Reduces ketones to secondary alcohols: R₂CO + 2[H] → R₂CHOH
与酸反应放出氢气：NaBH₄ + HCl + 3H₂O → NaCl + B(OH)₃ + 4H₂
Reacts with acid to release hydrogen: NaBH₄ + HCl + 3H₂O → NaCl + B(OH)₃ + 4H₂
年表Timeline
- 1942
施莱辛格与布朗在研究挥发性铀化合物的过程中发现硼氢化钠
Schlesinger and Brown discover sodium borohydride while studying volatile uranium compounds
- 1953
相关研究解密发表，NaBH₄ 开始用作有机还原剂
The work is declassified and published, and NaBH₄ comes into use as an organic reducing agent
- 1979
布朗因有机硼化学获诺贝尔化学奖
Brown receives the Nobel Prize in Chemistry for organoboron chemistry