Introduction to Sodium Borohydride
Sodium Borohydride is a very commonly known reducing agent used in Chemistry. It has the formula NaBH4, which is made up of Sodium(Na), Boron(B), and Hydrogen(H). It is widely used in Chemistry labs as it is safer than other reagents for reduction. We know that reduction is the process of adding electrons or removal of hydrogen ions. For this purpose, different kinds of chemical reagents are available, which differ in properties, reactivity, and yield of the product given. We will discuss the structures, properties, and reactivity of NaBH4.
Methods of Preparation
There are several methods available for the synthesis of Sodium Borohydride, out of which Bayer Process and the Brown-Schlesinger process are famous ones. In the Brown-Schlesinger process, Sodium hydride (NaH) and trimethyl borate are made to react together at temperatures of 523-533 K.
This is the best method for synthesis of Sodium Borohydride at industrial scale, tons of compound is synthesized annually via this method. Instead of Na, Mg hydride can be used for synthesis.
Structure of Sodium Borohydride
Sodium and Boron hydride are attached through positive and negative interactions known as Ionic Bond. Boron hydride has 4 H atoms attached to B atoms. These impart negative charges overall. So, the structure is tetrahedral. Sodium is present as Na+. They belong to the cubic lattice structure the same as NaCl.
Physical Properties of Sodium Borohydride
The compound has the formula NaBH4 and a molar mass of 37.83 gm per mole.
It has a clear white color powder type texture and it is relatively more stable than hydrides of Lithium. The melting point of Sodium Borohydride is 673K; heating above this temperature leads to the decomposition of the compound.
Chemical Properties of Sodium Borohydride
The most important property of this compound is its reducing nature. Now the most important question is why is it preferable to other reducing agents? The answer is because of its mild nature and safety issues. Lithium hydride reacts violently in air and leads to disasters and also reduces from carbonyl compounds i.e. From carboxylic acid to alcohol in a single run. It is generally very difficult to stop the reaction at intermediate stages i.e. at aldehyde and ketone stages. For this purpose Sodium Borohydride is a mild reagent and it reacts slowly with polar protic solvents, hence can be stopped at aldehyde and ketone stages. Even if we want to slow it down further, we can replace H atoms of the boron hydride portion with methyl groups.
NaBH4 can liberate Hydrogen when treated with organometallic catalysts.
For the production of various medicinal compounds in industries, it is used as a reducing agent because it's cheaper.
It is highly selective, which means when we treat other compounds with this, it removes certain atoms from particular positions without affecting the others in the molecular structure and hence giving us the desired compound. It should be noted that reagents that react at a slower rate have selective properties while the others which react faster, have higher reactivity but less selectivity.
Reactivity of this compound depends upon solvents too. It works effectively in polar protic solvents but with water and ethanol, its reactivity decreases significantly.
Another feature is that we can use this to reduce aldehyde and ketones in presence of other functional groups like lactones, conjugated systems also.
FAQs on Sodium Borohydride
1. What is sodium borohydride and what is its main purpose in chemistry?
Sodium borohydride, with the chemical formula NaBH₄, is a versatile and selective reducing agent. Its primary use in organic chemistry is to convert aldehydes and ketones into their corresponding primary and secondary alcohols. It is widely used because it is milder and safer to handle than more powerful reducing agents.
2. What is the chemical formula and structure of sodium borohydride?
The chemical formula is NaBH₄. It is an ionic compound made of a sodium ion (Na⁺) and a borohydride ion (BH₄⁻). The borohydride ion has a tetrahedral shape, with a central boron atom bonded to four hydrogen atoms.
3. Is sodium borohydride (NaBH₄) a reducing or an oxidising agent?
Sodium borohydride is a reducing agent. It works by donating a hydride ion (H⁻) to another molecule. When it gives away the hydride, it reduces the other compound while the boron in NaBH₄ gets oxidised.
4. How exactly does sodium borohydride reduce an aldehyde or a ketone?
The reduction occurs via a nucleophilic attack. The borohydride ion (BH₄⁻) releases a hydride ion (H⁻), which is a nucleophile. This hydride attacks the slightly positive carbon atom in the carbonyl group (C=O) of the aldehyde or ketone. In a second step, a solvent like water or alcohol is added to provide a proton (H⁺), which attaches to the oxygen atom to form the final alcohol product.
5. Why is sodium borohydride considered a milder reducing agent than LiAlH₄?
Sodium borohydride is milder than lithium aluminium hydride (LiAlH₄) because the boron-hydrogen (B-H) bond is less polar than the aluminium-hydrogen (Al-H) bond. This makes the hydride ion in NaBH₄ less reactive. As a result, NaBH₄ is selective, reducing only aldehydes and ketones, while the more powerful LiAlH₄ can also reduce less reactive groups like esters and carboxylic acids.
6. Can sodium borohydride be used to reduce every type of carbonyl compound?
No, it cannot. Sodium borohydride is a selective reagent and is generally not strong enough to reduce very stable carbonyl compounds. Its main targets are aldehydes and ketones. It typically does not react with:
- Carboxylic acids
- Esters
- Amides
For these functional groups, a stronger reducing agent like LiAlH₄ is required.
7. What happens when sodium borohydride is mixed with water or alcohol?
Sodium borohydride reacts slowly with protic solvents like water and alcohols to produce hydrogen gas (H₂) and a borate salt. This reaction is generally slow enough to allow these solvents to be used for the reduction process. The reaction is much faster in acidic conditions and slower in basic (alkaline) conditions.
8. What are the key safety precautions for handling sodium borohydride?
When handling sodium borohydride, it's important to be cautious. It is a flammable solid that can release flammable hydrogen gas if it comes into contact with acid or large amounts of water. It can also cause irritation to the skin and eyes. Therefore, it should always be handled in a well-ventilated area, and you should wear protective gear like gloves and safety glasses.
