Lithium Aluminium Hydride- LiAlH4

What is LiAlH₄?

Lithium aluminium hydride, also known as LAH, is a reducing agent that is commonly used in modern organic synthesis.

• It is a nucleophilic reducing agent that is best suited for reducing multiple polar bonds such as C=O.
• The LiAlH₄ reagent is capable of converting aldehydes to primary alcohols, ketones to secondary alcohols, carboxylic acids and esters to primary alcohols, amides and nitriles to amines, epoxides to alcohols, and lactones to diols.
• The LAH reagent, lithium aluminium hydride, cannot reduce isolated non-polar multiple bonds such as C=C. The double or triple bonds in conjugation with the multiple polar bonds, on the other hand, can be reduced.

Table of Contents

• Structure of LiAlH4
• Preparation of LiAlH4
• Properties of LiAlH4
• What makes LiAlH4 so effective?
• Applications of LiAlH4
• Frequently Asked Questions – FAQs

Structure of Lithium Aluminium Hydride – LiAlH₄

In aluminium hydride, AlH₄^– ion, hydrogens are arranged in a tetrahedral arrangement around Al³⁺. The coordination of hydride, H^– ions to Al³⁺ ions results in its formation.
Li⁺ centres are surrounded by five AlH₄^– tetrahedra in the structure. A bipyramid arrangement is formed by the Li⁺ centres being bonded to one hydrogen atom from each of the surrounding tetrahedra.

Preparation of LiAlH₄

1. The reaction of lithium hydride and aluminium chloride produces lithium aluminium hydride.

4LiH + AlCl₃ → LiAlH₄ + 3LiCl

2. In addition to this method, the industrial synthesis requires the initial preparation of sodium aluminium hydride under high pressure and temperature:

Na + Al + H₂ → NaAlH₄

Following that, LiAlH4 is synthesised via a salt metathesis reaction as follows:

NaAlH₄ + LiCl → LiAlH₄ + NaCl

It proceeds in a high yield. Filtration is used to remove LiCl from an ethereal solution of LAH.

Properties of LiAlH₄

• Although lithium aluminium hydride, LAH, is a white solid, commercial samples are usually grey due to impurities.
• It produces hydrogen gas when it reacts violently with water. As a result, it should not be exposed to moisture, and the reactions should take place in inert and dry surroundings. This reaction can be used to generate hydrogen in the laboratory.

LiAlH₄ + 4H₂O → LiOH + Al(OH)₃ + 4H₂

• The reduction reaction with LiAlH₄ as the reducing agent must be carried out in anhydrous nonprotic solvents such as diethyl ether, THF, and so on.
• It has a high solubility in diethyl ether. However, due to the presence of catalytic impurities, it may spontaneously decompose in it. Despite its low solubility, THF is the preferred solvent for LAH.
• Usually, the reactions are carried out with an excess of LiAlH₄. To remove moisture if any, a small amount of the reagent is added to the solvent.

Workup:

• During the workup, the reaction mixture is first chilled in an ice bath, and then the Lithium aluminium hydride is quenched by a careful and very slow addition of ethyl acetate, followed by methanol, and finally cold water.
• Sometimes the reagent is decomposed by gradually adding undried solvent, and then dilute sulphuric acid to the reaction mixture.

What makes LiAlH₄ so effective?

Aluminium is a metal with low electronegativity. Therefore, the Al-H bond is strongly polarised, with Al being positive and H negative. The abnormal polarisation (and oxidation state of -1) of hydrogen, which is normally positive, results in a high reactivity, The abnormal polarisation (and oxidation state of -1) of hydrogen, which is normally positive, results in a high reactivity, particularly with atoms that can accept electrons (be reduced), allowing the hydrogen to become positive again (normal oxidation state of +1). Precautions: It also reacts with other positive centres, particularly slightly acidic hydrogens, such as those found in alcohols, water, carboxylic acids, or alkynes, to produce hydrogen gas, which is highly flammable and easily explodes.

Applications of LiAlH₄

1. LiAlH₄ reduces aldehydes or ketones to the corresponding primary or secondary alcohols.

Acetaldehyde, for example, is reduced to ethyl alcohol, while acetone is reduced to isopropyl alcohol.

2. Lithium aluminium hydride is used to reduce carboxylic acids, esters, and acid halides to their corresponding primary alcohols.

For example, LiAlH₄ reduction of acetic acid, methyl acetate, and acetyl chloride yield the same ethyl alcohol.

3. Lithium aluminium hydride, LiAlH₄, reduces the amides to amines. This method is particularly useful for obtaining secondary amines.

Diethylamine, for example, can be synthesised from acetyl chloride.

4. LiAlH₄ reduces the nitriles to primary amines.

For example, LiAlH₄ reduces acetonitrile to ethyl amine.

5. The oxiranes (epoxides) are converted to alcohol by lithium aluminium hydride. The mechanism involves a hydride attack on the epoxide’s less hindered side.

For example, 2-methyloxirane primarily yields 2-propanol.

6. Lithium aluminium hydride is used to convert haloalkanes and haloarenes to their corresponding hydrocarbons.

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