Question

compare the rreducing capability of Lithium,Almunium hydride to those of sodium borohydride??

Answer 1

Lithium aluminum hydride

LiAlH₄ is a strong, unselective reducing agent for polar double bonds, most easily thought of as a source of H^-. It will reduce aldehydes, ketones, esters, carboxylic acid chlorides, carboxylic acids and even carboxylate salts to alcohols. Amides and nitriles are reduced to amines. In each case the partially negative hydrogen reacts with the partially positive carbon of the substrate. It can also be used to reduce nitro groups and even as a nucleophile to displace halide from an sp³ carbon or open an epoxide.

Why does it work? Remember that aluminum is a metal with a low electronegativity; thus, the Al-H bond is strongly polarized with Al positive and H negative. The abnormal polarization (and oxidation state of -1) for hydrogen, which is normally positive, results in a high reactivity, especially with atoms that can accept electrons (be reduced), allowing the hydrogen to become positive again (normal oxidation state of +1).

Precautions: It reacts with other positive centers as well, especially any slightly acidic hydrogens, like those of alcohols, water, carboxylic acids or alkynes to produce hydrogen gas, which is highly flammable and can readily explode (e.g. the Hindenberg). LiAlH₄ requires anhydrous conditions for the reaction and usually an excess of the reagent (to soak up any water that was missed in the glassware or reagents); the most common solvent for the reactions is diethyl ether. "Workup" at the end of the reaction is usually done by careful addition of aqueous acid (remember the flammable hydrogen gas and ether), followed by extraction of the organic products from the water-soluble salts.

To make LiAlH₄ less reactive and more selective, the hydride is made more hindered, e.g., in the compound LiAl(OtBu)₃H. It can reduce acid chlorides (and some esters) quickly, but is slow to react with aldehydes; therefore LiAl(OtBu)₃H provides a convenient way to synthesize aldehydes by reduction of acid chlorides, something that can't be done with LiAlH₄ or NaBH₄.

Sodium borohydride

NaBH₄ is less reactive than LiAlH₄ but is otherwise similar. It is only powerful enough to reduce aldehydes, ketones and acid chlorides to alcohols: esters, amides, acids and nitriles are largely untouched. It can also behave as a nucleophile toward halides and epoxides. It is also convenient that, although LiAlH₄ is strong enough to reduce the C=C of a conjugated carbonyl compound, NaBH₄ is not; thus the carbonyl group can be reduced without the alkene.

Precautions: NaBH₄ is unreactive enough that the reductions can be done in alcohol solution, or even water (as long as they don't take too long); this can be advantageous for polar compounds which can be pretty insoluble in ether. Hydrolysis with acid and water followed by extraction is used to isolate the product (hydrogen gas is produced).

Lithium aluminum hydride

LiAlH₄ is a strong, unselective reducing agent for polar double bonds, most easily thought of as a source of H^-. It will reduce aldehydes, ketones, esters, carboxylic acid chlorides, carboxylic acids and even carboxylate salts to alcohols. Amides and nitriles are reduced to amines. In each case the partially negative hydrogen reacts with the partially positive carbon of the substrate. It can also be used to reduce nitro groups and even as a nucleophile to displace halide from an sp³ carbon or open an epoxide.

Why does it work? Remember that aluminum is a metal with a low electronegativity; thus, the Al-H bond is strongly polarized with Al positive and H negative. The abnormal polarization (and oxidation state of -1) for hydrogen, which is normally positive, results in a high reactivity, especially with atoms that can accept electrons (be reduced), allowing the hydrogen to become positive again (normal oxidation state of +1).

Precautions: It reacts with other positive centers as well, especially any slightly acidic hydrogens, like those of alcohols, water, carboxylic acids or alkynes to produce hydrogen gas, which is highly flammable and can readily explode (e.g. the Hindenberg). LiAlH₄ requires anhydrous conditions for the reaction and usually an excess of the reagent (to soak up any water that was missed in the glassware or reagents); the most common solvent for the reactions is diethyl ether. "Workup" at the end of the reaction is usually done by careful addition of aqueous acid (remember the flammable hydrogen gas and ether), followed by extraction of the organic products from the water-soluble salts.

To make LiAlH₄ less reactive and more selective, the hydride is made more hindered, e.g., in the compound LiAl(OtBu)₃H. It can reduce acid chlorides (and some esters) quickly, but is slow to react with aldehydes; therefore LiAl(OtBu)₃H provides a convenient way to synthesize aldehydes by reduction of acid chlorides, something that can't be done with LiAlH₄ or NaBH₄.

Sodium borohydride

NaBH₄ is less reactive than LiAlH₄ but is otherwise similar. It is only powerful enough to reduce aldehydes, ketones and acid chlorides to alcohols: esters, amides, acids and nitriles are largely untouched. It can also behave as a nucleophile toward halides and epoxides. It is also convenient that, although LiAlH₄ is strong enough to reduce the C=C of a conjugated carbonyl compound, NaBH₄ is not; thus the carbonyl group can be reduced without the alkene.

Precautions: NaBH₄ is unreactive enough that the reductions can be done in alcohol solution, or even water (as long as they don't take too long); this can be advantageous for polar compounds which can be pretty insoluble in ether. Hydrolysis with acid and water followed by extraction is used to isolate the product (hydrogen gas is produced).