Lithium Aluminum Hydride (LiAlH₄) CAS 16853-85-3

Lithium Aluminum Hydride (LiAlH₄): Structure, Properties, and Applications

1. Introduction

Lithium aluminum hydride (LiAlH₄), commonly abbreviated as LAH, is one of the most powerful and versatile reducing agents in organic chemistry. First synthesized in the mid‑20th century, it quickly became indispensable for converting a wide range of functional groups – including aldehydes, ketones, esters, carboxylic acids, and amides – into their corresponding alcohols or amines. Its strong reducing power, high hydride content, and clean work‑up compared to other hydride reagents make it a reagent of choice in pharmaceutical, perfume, and fine chemical synthesis. Beyond organic synthesis, LiAlH₄ is also investigated as a potential solid‑state hydrogen storage material due to its high theoretical hydrogen capacity.

2. Chemical Structure and Nomenclature

Molecular formula: AlH₄Li (LiAlH₄)

Molecular weight: 37.95 g/mol

IUPAC name: Lithium tetrahydroaluminate(—)

Common names: Lithium aluminum hydride, lithium aluminium hydride, LAH, Lithal

CAS number: 16853-85-3

Structure: LiAlH₄ consists of a lithium cation (Li⁺) and a tetrahedral alanate anion [AlH₄]⁻, where aluminum is in the +3 oxidation state and hydrogen is in the –1 oxidation state (hydride). The Al–H bond is highly polar, making LAH a much stronger reducing agent than sodium borohydride (NaBH₄).

3. Physicochemical Properties

Appearance: White crystalline powder; commercial samples often appear gray due to impurities.

Molar mass: 37.95 g/mol

Density: 0.917 g/cm³ (solid); 0.97 g/mL at 20°C

Melting point: 150°C (decomposes); some sources report 125°C (decomposes)

Boiling point: Decomposes before boiling; ~0°C (sublimation under vacuum)

Solubility: Soluble in ether and tetrahydrofuran (THF); reacts violently with water, releasing hydrogen gas

Reactivity: LiAlH₄ reacts vigorously with water, alcohols, acids, and other protic solvents. It must be handled under an inert atmosphere (argon or nitrogen) and stored in dry, airtight containers. It is also sensitive to oxygen and carbon dioxide.

4. Synthesis

LiAlH₄ is produced industrially by the reaction of lithium hydride (LiH) with aluminum chloride (AlCl₃) in an ether solvent:

4 LiH + AlCl₃ → LiAlH₄ + 3 LiCl

The product is separated by filtration and purified by recrystallization. It is typically supplied as a solid powder, pellets, or as a solution in THF.

5. Mechanism of Action

As a source of hydride ions (H⁻), LiAlH₄ delivers a nucleophilic hydride to the electrophilic carbonyl carbon of aldehydes, ketones, esters, carboxylic acids, and amides. The reduction proceeds via a tetrahedral intermediate; for esters, an aldehyde intermediate is formed, which is further reduced to the primary alcohol. Carboxylic acids and esters, which are less reactive toward nucleophiles, can only be reduced by the stronger LAH and not by NaBH₄.

6. Applications

Organic synthesis: LiAlH₄ is used to reduce a broad spectrum of functional groups:

• Aldehydes and ketones → primary and secondary alcohols

• Esters and carboxylic acids → primary alcohols

• Acyl chlorides → primary alcohols

• Amides → amines

• Nitriles → primary amines

• Azides → amines

• Epoxides → alcohols (ring opening)

• Phosphine oxides → phosphines

Pharmaceuticals and fine chemicals: LAH is a key reagent in the synthesis of active pharmaceutical ingredients (APIs), perfumes, and other fine organic compounds.

Hydrogen storage: LiAlH₄ has a high theoretical gravimetric hydrogen capacity of 10.5–10.6 wt%, making it a promising candidate for solid‑state hydrogen storage in fuel cell applications. Research is ongoing to improve its dehydrogenation kinetics and reduce its high decomposition temperature through catalysis and composite formation.

Preparation of other hydrides: LiAlH₄ is used as a precursor for the synthesis of other alanates, such as sodium aluminum hydride (NaAlH₄), potassium aluminum hydride (KAlH₄), and magnesium alanate (Mg(AlH₄)₂).

Catalysis and polymerization: LAH serves as a source of hydrogen, a propellant, and a catalyst in certain polymerization reactions.

7. Safety and Handling

Hazards: LiAlH₄ is highly reactive and poses significant fire and explosion risks. It reacts violently with water, releasing flammable hydrogen gas and heat. Contact with moisture, protic solvents, or oxidizers can lead to ignition. Dust is irritant to the respiratory tract, skin, and eyes.

Storage: Store under inert gas (argon or nitrogen) in tightly sealed containers in a cool, dry place, away from combustible materials. Protect from moisture and light. Store below +30°C.

Personal protective equipment (PPE): Wear safety glasses, impervious gloves, a fire‑retardant laboratory coat, and respiratory protection when handling powders. Use only in a well‑ventilated area or fume hood.

Spill and fire: Do not use water, sand, or CO₂ to extinguish LiAlH₄ fires. Use graphite powder, copper powder, or anhydrous solids such as NaCl, CaO, or LiCl. For small spills, cover with dry sand or other non‑combustible material and avoid contact with water.

Disposal: Quench residues carefully with a protic solvent (e.g., alcohol) in a controlled environment, followed by aqueous work‑up. Dispose of as hazardous waste according to local regulations.

8. Conclusion

Lithium aluminum hydride (LiAlH₄) is a cornerstone reagent in modern organic chemistry, valued for its exceptional reducing power, versatility, and clean reaction profiles. Its ability to reduce a wide array of functional groups makes it indispensable in the synthesis of pharmaceuticals, perfumes, and fine chemicals. In addition, its high hydrogen content positions it as a promising material for future energy storage applications, though challenges in reversibility and kinetics remain active areas of research. Due to its extreme reactivity with moisture and air, strict safety protocols are essential for its handling, storage, and disposal.