Physical and chemical properties of sodium borohydride

Physical properties of sodium borohydride

White crystalline powder, strong hygroscopicity, easy to absorb water and deliquesce. Soluble in water, liquid ammonia and amines. Easily soluble in methanol, slightly soluble in ethanol and tetrahydrofuran. Insoluble in ether, benzene and hydrocarbons. It is stable in dry air, decomposes in wet air and decomposes when heated to 500 ℃.

Chemical properties of sodium borohydride

1. Since the hydrogen in sodium borohydride has a partial negative charge (the electronegativity of B is smaller than that of H), and the hydrogen in - Oh, - NH -, - NH2 in alcohols and amines has more partial positive charge, BH4 - in sodium borohydride can form a double hydrogen bond with the molecules constituting these substances. Therefore, sodium borohydride can be dissolved in water, liquid ammonia, alcohols and amines.

2. Sodium borohydride will react slowly with substances containing hydroxyl groups such as water and alcohol to release hydrogen. At the same time, due to the slow reaction, the loss of sodium borohydride in a short time is very small. Therefore, sodium borohydride can use alkaline solution, methanol and ethanol as solvents.  

NaBH4+2H2O=NaBO2+4H2↑

NaBH4+4HO-R→B(OR)3 +4H2↑+NaOR

3. Sodium borohydride is an inorganic compound with strong selective reducibility. It is often used as reducing agent in Inorganic Synthesis and organic synthesis, and has good chemical selectivity. It can reduce the carbonyl group of aldehydes and ketones under very mild conditions to produce primary and secondary alcohols. A small amount of sodium borohydride can reduce nitrile to aldehyde, and an excessive amount can be reduced to amine. It can selectively reduce carbonyl and aldehyde groups to hydroxyl groups, and can also reduce carboxyl groups to aldehyde groups without interaction with esters and amides, and generally does not react with carbon carbon double bonds and three bonds.

4. Sodium borohydride has a good reduction effect on aldehydes and ketones. Common solvents are alcohol, tetrahydrofuran, dimethylformamide (DMF), water, etc. It generally does not reduce ester groups and amides, but it can reduce weak carbonyl groups such as ester groups at high concentration, high temperature, combined with appropriate solvents or catalyzed by Lewis acid.

5. Sodium borohydride will decompose rapidly under acidic conditions to produce hydrogen, which cannot exist stably, but it can exist stably under neutral or alkaline conditions, and it is the most stable when pH = 14.

NaBH4+H++3H2O=H3BO3+Na++4H2↑

6. Sodium borohydride cannot reduce carboxylic acid alone. It must be used in combination with iodine. First react with carboxylic acid until bubbles stop, then add iodine and continue to release gas. Then add boric acid ester formed by decomposition of hydrochloric acid to obtain alcohol.

7. Zinc borohydride can be prepared by reacting sodium borohydride with anhydrous zinc chloride (dehydrated and dried above 200 ℃) in anhydrous tetrahydrofuran (THF) for 3 hours. The solution mixture can be used as zinc borohydride without separation and purification.