硼氢化钠

Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are used for bleaching wood pulp. The compound is insoluble inether, and soluble in glyme solvents, methanol and water, but reacts with the latter two in the absence of base.

The compound was discovered in the 1940s by H. I. Schlesinger, who led a team that developed metal borohydrides for wartime applications. Their work was classified and published only in 1953.

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Physical properties

Sodium borohydride is an odorless white to gray-white microcrystalline powder which often forms lumps. It is soluble in water, without decomposition, however it reacts vigorously with acid solutions. The salt can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.

[Structure

NaBH4 is a salt, consisting of the tetrahedral BH4 anion. The solid is known to exist as three polymorphs: α, β and γ. The stable phase at room temperature and pressure is α-NaBH4, which is cubic and adopts an NaCl-type structure, in the Fm3mspace group. At a pressure of 6.3 GPa, the structure changes to the tetragonal β-NaBH4 (space group P421c) and at 8.9 GPa, the orthorhombic γ-NaBH4 (space group Pnma) becomes the most stable

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Synthesis and handling

Sodium borohydride is prepared industrially following the original method of Schlesinger: sodium hydride is treated withtrimethyl borate at 250-270 °C:

B(OCH3)3 + 4 NaH → NaBH4 + 3 NaOCH3

Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent.Sodium borohydride can also be produced by the action of NaH on powdered borosilicate glass.

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Reactivity

NaBH4 will reduce many organic carbonyls, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It will reduce acyl chlorides, thiol esters and imines. Under typical conditions, it will not

reduce esters, amides, or carboxylic acids. At room temperature, the only acid derivatives it reduces are acyl chlorides, which are exceptionally electrophilic.

Many other hydride reagents are more strongly reducing. These usually involve replacing hydride with alkyl groups, such aslithium triethylborohydride and L-Selectride (lithium tri-sec-butylborohydride), or replacing B with Al. Variations in the counterion also affect the reactivity of the borohydride.

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The reactivity of NaBH4 can be enhanced or augmented by a variety of compounds.

[11][12] Oxidation with iodine intetrahydrofuran gives the BH3-THF complex, which

[13]can reduce carboxylic acids.

[14] Likewise, the NaBH4-MeOH system, formed by the addition

of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols.

Mixing water or an alcohol with the borohydride converts some of it into unstable

hydride ester, which is more efficient at reduction, but the reductant will eventually decompose spontaneously to give hydrogen gas and borates. The same reaction can run also

intramolecularly: an α-ketoester converts into a diol, since the alcohol produced will attack the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester.

[15] The combination of NaBH4 withcarboxylic acids results in the formation of

acyloxyborohydride species. These can perform a variety of reductions not normally associated with borohydride chemistry, such as alcohols to hydrocarbons and nitriles to primary amines

Coordination chemistry

BH4 is a ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH4 (or the LiBH4) on the corresponding metal halide. One example is the titanocene derivative:

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2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2 [edit]Hydrogen source

In the presence of metal catalysts, sodium borohydride releases hydrogen. Exploiting this reactivity, sodium borohydride is used in prototypes of the direct borohydride fuel cell. The hydrogen is generated for a fuel cell by catalyticdecomposition of the aqueous borohydride solution:

NaBH4 + 2 H2O → NaBO2 + 4 H2 (ΔH < 0)

Applications

The principal application of sodium borohydride is the production of sodium dithionite from sulfur dioxide: Sodium dithionite is used as a bleaching agent for wood pulp and in the dyeing industry.

Sodium borohydride reduces aldehydes and ketones to give the related alcohols. This reaction is used in the production of various antibiotics

includingchloramphenicol, dihydrostreptomycin, and thiophenicol. Various steroids and vitamin A are prepared using sodium borohydride in at least one step.

Sodium borohydride has been considered as a solid state hydrogen storage candidate. Although practical temperatures and pressures for hydrogen storage have not been achieved, in 2012 a core-shell nanostructure of sodium borohydride was used successfully to store, release and reabsorb hydrogen under moderate conditions.[edit]Safety

Sodium borohydride is a source of basic borate salts which can be corrosive, and hydrogen or diborane, which are both flammable. Spontaneous ignition can result from solution of sodium borohydride in dimethylformamide.

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硼氢化钠是一种无机化合物，分子式NaBH4。硼氢化钠为白色粉末，容易吸水潮解，可溶于水和低级醇，在室温下与甲醇迅速反应生成氢气。在无机合成和有机合成中硼氢化钠常用做还原剂。

硼氢化钠给有机化学家们提供了一种非常便利温和的还原醛酮类物质的手段。在此之前，通常要用金属/醇的办法来还原羰基化合物，而硼氢化钠可以在非常温和的条件下实现醛酮羰基的还原，生成一级醇、二级醇。还原步骤是先把底物溶于溶剂，一般是甲醇或者乙醇，然后用冰浴冷却，将硼氢化钠粉末加入混合物搅拌至反应完全即可。反应过程可以用薄层层析监测。如果溶剂不是醇，那么需要另加甲醇或者乙醇一同反应。硼氢化钠是一种中等强度的还原剂，所以在反应中表现出良好的化学选择性，只还原活泼的醛酮羰基，而不与酯、酰胺作用。

Cyclohexanone is the organic compound with the formula (CH2)5CO. The molecule consists

of six-carbon cyclic molecule with a ketone functional group. This colorless oil has an odor reminiscent of peardrop sweets as well as acetone. Over time, samples assume a yellow color due to oxidation. Cyclohexanone is slightly soluble in water, but miscible with common organic solvents. Billions of kilograms are produced annually, mainly as a precursor to nylon.

Production

Cyclohexanone is produced by the oxidation of cyclohexane in air, typically using cobalt catalysts:

C6H12 + O2 → (CH2)5CO + H2O

This process co-forms cyclohexanol, and this mixture, called \KA oil\is the main feedstock for the production of adipic acid. The oxidation involves radicals and the intermediacy of the hydroperoxide C6H11O2H. In some cases, purified cyclohexanol, obtained by hydration of cyclohexene, is the precursor. Alternatively, cyclohexanone can be produced by the partial hydrogenation of phenol:

C6H5OH + 2 H2 → (CH2)5CO

This process can also be adjusted to favor the formation of cyclohexanol. [edit]Laboratory methods

Cyclohexanone can be prepared from cyclohexanol by oxidation with chromic oxide. An alternative method utilizes the safer and more readily available oxidant sodium hypochlorite

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Uses

The great majority of cyclohexanone is consumed in the production of precursors to Nylon 6,6 and Nylon 6. About half of the world's supply is converted to adipic acid, one of two precursors for nylon 6,6. For this application, the KA oil (see above) is oxidized with nitric acid. The other half of the cyclohexanone supply is converted to the oxime. In the presence of sulfuric acid catalyst, the oxime rearranges to caprolactam, a precursor to nylon 6:

Safety

Like cyclohexanol, cyclohexanone is not carcinogenic and is only moderately toxic, with a TLV of 25 ppm for the vapor. It is an irritant.

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A recent study of plastic tubing used in medical procedures that circulate blood outside the body suggests a link between this compound and decreased heart function, swelling, loss of taste and short term memory loss.[7]