Views: 6 Author: Site Editor Publish Time: 2022-11-23 Origin: Site
Sodium borohydride is a relatively mild reducing agent. It has a good effect on reducing aldehydes and ketones.Its common solvents include alcohol, tetrahydrofuran,DMF, and water.It generally does not reduce ester groups, carboxyl groups and amides.However, it can be used in the reduction of weak carbonyl groups such as esters in combination with appropriate solvents or under the high temperature catalysis of Lewis acids.Gentle and efficient reduction of aldehydes and ketones.Basic operation:Use methanol or ethanol as solvent, mix aldehyde, ketone carbonyl compound and sodium borohydride at a ratio of 1:1.Heating can be carried out step by step,for example,starting from 50 degrees, after a sufficient time such as 1 hour, the reflux reaction is carried out; at the same time,TLC is used to monitor the progress.The response is usually very thorough In general, the amount of solvent used in the reaction is sufficient as long as it can avoid white sticky paste after the reaction is completed.Dryness need not be strictly maintained during the reaction; there are even cases where water is used as solvent. For example, in the reduction of formyl benzoic acid, the formyl group (formaldehyde) is reduced, and the carboxyl group is first neutralized with sodium hydroxide, and then the formyl group is successfully reduced by reacting in water.Sodium borohydride can quickly decompose and release hydrogen gas under acidic conditions, so it cannot react under acidic conditions and can be used under alkaline conditions.Sodium borohydride decomposes rapidly when it encounters acid to release hydrogen, and it cannot reduce acid alone, so it should be used in conjunction with iodine. React with carboxylic acid first, add iodine after the bubbles stop, and continue to deflate.Then add borate decomposed by hydrochloride to obtain alcohol.Note: The reaction should be kept in anhydrous tetrahydrofuran, and tetrahydrofuran must be refluxed with sodium until the benzophenone turns blue before use! Otherwise an emulsion would appear instead of a clear liquid when the carboxylic acid reacts with sodium borohydride.Sodium borohydride reacts with anhydrous zinc chloride (dried above 200 degrees) in anhydrous tetrahydrofuran for 3 hours to generate zinc borohydride.This solution mixture does not require isolation and purification before becoming zinc borohydride. When used to reduce carboxylic acids or esters in THF at reflux temperature, yields are good, but some double bonds may be affected. For example,reduction of cinnamic acid produces a small fraction of double bond reduction products.
Contact with sodium borohydride can cause sore throat,cough, shortness of breath,headache,abdominal pain,diarrhea,dizziness,conjunctival hyperemia and pain.Dustproof,enhanced ventilation or wear protective masks should be used during construction.Pay attention to eye protection when working,wear protective glasses,and do not eat,drink,or smoke. After poisoning, leave the scene quickly,rest in a semi-recumbent position,breathe fresh air,rinse eyes with plenty of water,remove contaminated clothing,and rinse the whole body; if it enters the digestive tract,rinse mouth immediately,drink plenty of water to induce vomiting, and go to the hospital immediately.In the event of a spill, wear a protective mask filter to clean up the spill.
Air & Water Reactions
Hydrolysis generates enough heat to ignite adjacent combustible materials.Dissolving in water gives off heat,which can produce steam and splash.The solution is alkaline (basic). Water reacts with borohydride to release flammable hydrogen gas. Sodium borohydride burns in air.
It is mildly corrosive to skin.Oral ingestion or intravenous administration of the solid or its solution is highly toxic to animals.Lethal at doses of 160 mg/kg (NIOSH 1986).
After adding NaBH4 (10 g) to freshly distilled diglyme (120 mL) in a dry three-necked flask equipped with a stirrer, nitrogen inlet and outlet, the mixture was stirred at 50o for 30 min until almost all the solids had dissolved .Stop stirring, and after the solids have settled, force the supernatant under N2 pressure through a sintered glass filter into a dry flask. [Centrifuge the residue to obtain more solution, which is added to the main body.The solution was cooled slowly to 0°, then decanted from the separated white needles.Dry the crystals by evacuation for 4 h to obtain anhydrous NaBH4.Alternatively, after filtration at 50°, the solution was heated at 80° for 2 h to give an essentially anhydrous white precipitate of NaBH4,which was collected on a sintered glass filter under N2 and then evacuated at 60° for 2 h .NaBH4 has also been crystallized from isopropylamine by dissolving it in solvent at reflux, cooling, filtering and allowing the solution to settle in a filter flask attached to a dry ice/acetone trap. After most of the solvent entered the cold trap, the crystals were removed with tweezers, washed with anhydrous ether and dried under vacuum.Crystals of slightly lower purity can be obtained more quickly by performing Soxhlet extraction using only a small amount of solvent and extracting for about 8 hours.The crystals formed in the flask were filtered off, washed and dried as before.Other solvents used for crystallization include water and liquid ammonia.
It can be destroyed in a number of ways. Dissolve or dilute the solid or its solution in a large amount of water.Then slowly add diluted acetic acid or acetone to this solution in a well-ventilated area. Hydrogen gas produced by borohydride decomposition should be vented carefully. Adjust the pH to 1.Then let the solution sit for several hours. It was then neutralized to 7, and the solution was evaporated to dryness.The residue is then buried in a landfill approved for hazardous waste disposal.Sodium borohydride can be destroyed in the laboratory by the alternative methods for the other hydrides.