Sodium borohydride and its Structure

Sodium borohydride also known as sodium tetrahydroborate and sodium tetrahydroborate is an inorganic compound with the formula NaBH4.This white solid, usually in the form of an aqueous alkaline solution, is a reducing agent used in the paper and dye industries.Also used as a reagent in organic synthesis.The compound was discovered in the 1940s by H. I. Schlesinger, who led a team looking for volatile uranium compounds. The results of this wartime study were declassified and published in 1953.

Characteristics

The compound is soluble in alcohols, some ethers and water, but hydrolyzes slowly. Sodium borohydride is an odorless white to off-white microcrystalline powder that often agglomerates.It can be purified by recrystallization from warm (50 °C) diglyme.Sodium borohydride is soluble in protic solvents such as water and lower alcohols.It also reacts with these protic solvents to form H2; however, these reactions are rather slow. At 20 °C, the complete decomposition of the methanol solution takes almost 90 minutes.Decomposes in neutral or acidic aqueous solutions but is stable at pH 14.

Structure 

NaBH4 is a salt consisting of tetrahedral [BH4]- anions.The solid is known to exist in three polymorphic forms: alpha, beta and gamma.The stable phase at normal temperature and pressure is α-NaBH4, cubic, NaCl type structure, and the space group is Fm3m. At a pressure of 6.3 GPa, the structure changes to tetragonal β-NaBH4 (space group P421c), and at 8.9 GPa, orthorhombic γ-NaBH4 (space group Pnma) becomes most stable.

Synthesis and Processing

For commercial NaBH4 production, the Brown-Schlesinger process and the Bayer process are the most popular methods.Sodium borohydride is prepared industrially from sodium hydride (formed by the reaction of Na and H2) and trimethyl borate in the Brown-Schlesinger process at 250–270 °C:

Organic synthesis

NaBH4 can reduce many organic carbonyl compounds, depending on the specific conditions.Most typically, it is used in laboratories to convert ketones and aldehydes into alcohols.It effectively reduces acid chlorides, anhydrides, alpha-hydroxylactones, thioesters, and imines at room temperature.It reduces esters slowly and inefficiently in excess of reagents and/or at elevated temperatures, whereas carboxylic acids and amides are not reduced at all.NaBH4 reacts with water and alcohols to evolve hydrogen gas and form the corresponding borate, a reaction that is particularly fast at low pH.However, alcohols (usually methanol or ethanol) are usually the solvent of choice for the reduction of ketones and aldehydes with sodium borohydride.The reduction mechanism of ketones and aldehydes has been scrutinized by kinetic studies and, contrary to popular descriptions in textbooks, does not involve a 4-membered transition state like the hydroboration of alkenes,or a 6-membered transition state involving the molecule Alcohol solvent,require hydrogen bond activation because reduction does not occur in aprotic solvents like diglyme.However, the rate order in alcohols is 1.5, whereas both carbonyls and borohydrides are first order, suggesting a more complex mechanism than that involving a six-membered transition state involving only a single alcohol molecule.It has been proposed that carbonyls and borohydrides are simultaneously activated through interactions with alcohol and alkoxide ions, respectively, and that the reactions proceed through open transition states.

α,β-Unsaturated ketones tend to be reduced by NaBH4 at the 1,4 positions, although mixtures are usually formed.The addition of cerium chloride increases the selectivity of the 1,2-reduction (Luche reduction) of unsaturated ketones. α,β-Unsaturated esters also undergo 1,4-reduction in the presence of NaBH4.The NaBH4-MeOH system was formed by adding methanol to sodium borohydride in refluxing THF to reduce the ester to the corresponding alcohol.mixed water or alcohol with borohydride to convert some of them into unstable hydride esters, and the reduction was more efficient, but the reducing agent eventually decomposed spontaneously to produce hydrogen gas and borate.The same reaction can also occur intramolecularly: α-ketoesters are converted to diols as the resulting alcohol attacks the borohydride to produce an ester of the borohydride, which then reduces the adjacent ester.The reactivity of NaBH4 can be enhanced or enhanced by various compounds.

Oxidation

Oxidation with iodine in tetrahydrofuran gives borane-tetrahydrofuran, which reduces carboxylic acids to alcohols.Partial oxidation of borohydride with iodine gives octahydrotriborate:

Coordination Chemistry

BH4− is a ligand for metal ions. Such borohydride complexes are usually prepared by the action of NaBH4 (or LiBH4) on the corresponding metal halides. An example is the titanocene derivative

Protolysis and hydrolysis:

In the presence of a metal catalyst, sodium borohydride hydrolyzes with release of hydrogen gas.Taking advantage of this reactivity, sodium borohydride has been used in prototypes of direct borohydride fuel cells.

Applications

papermaking

The main application of sodium borohydride is the production of sodium dithionite from sulfur dioxide:sodium dithionite is used as a bleaching agent in the wood pulp and dyeing industries.It has been tested as a pretreatment for wood pulping but is too costly to commercialize.

Chemical synthesis

Sodium borohydride reduces aldehydes and ketones to the related alcohols.This reaction is used to produce various antibiotics, including chloramphenicol, dihydrostreptomycin, and thienol.Preparation of various steroids and vitamin A using sodium borohydride in at least one step.Sodium borohydride is considered a way to store hydrogen for hydrogen vehicles because it is safer (stable in dry air) and more efficient on a weight basis than most other alternatives.Hydrogen can be liberated by simple hydrolysis of borohydrides. However, this solution requires an inexpensive and efficient method to recycle the hydrolyzed product sodium metaborate back to borohydride.