Lithium Aluminum Hydride (LiAlH₄) and Lithium Borohydride (LiBH₄) are high-performance inorganic lithium hydrides widely used as powerful reducing agents in chemical synthesis, pharmaceuticals, materials science, and advanced energy research.
Both compounds are valued for their high hydrogen content, strong reactivity, and ability to donate hydride ions (H⁻). They are chemical reagents rather than structural materials and require strict moisture-free handling.
Chemical Composition & Physical Properties
| Property | LiAlH₄ | LiBH₄ |
|---|---|---|
| Chemical Formula | LiAlH₄ | LiBH₄ |
| Molar Mass | ~37.95 g/mol | ~21.76 g/mol |
| Appearance | White to gray crystalline powder | White crystalline solid |
| Density | ~0.4 g/cm³ (bulk) | ~0.68 g/cm³ |
| Melting / Decomposition | ~125–190 °C (decomposes) | Melts ~268–278 °C, decomposes ~380 °C |
| Solubility | Soluble in ethers (THF, diethyl ether) | Soluble in ethers (THF, diethyl ether) |
| Reactivity with Water | Violent, releases H₂ | Violent, releases H₂ |
Chemical & Hydride Behavior
Both compounds act as hydride donors, transferring H⁻ ions to electrophilic centers such as carbonyl groups.
Lithium Aluminum Hydride (LiAlH₄):
• One of the strongest reducing agents used in chemistry
• Reduces aldehydes, ketones, esters, carboxylic acids, amides, nitriles, epoxides
• Widely used in pharmaceutical and fine chemical synthesis
Lithium Borohydride (LiBH₄):
• Strong reducing agent with greater selectivity in some reactions
• Can reduce esters, nitriles, and epoxides
• Investigated for hydrogen storage due to very high hydrogen content
Hydrogen Content & Energy Research
| Compound | Hydrogen Content (wt%) |
|---|---|
| LiAlH₄ | ~10.5% |
| LiBH₄ | ~18.5% |
LiBH₄ is of particular interest in hydrogen energy research, although practical applications are limited by high dehydrogenation temperatures and recharge complexity.
Refining & Processing
LiAlH₄:
• Produced via metathesis reactions (e.g., NaAlH₄ + LiCl)
• Purified by recrystallization in dry ether solvents
LiBH₄:
• Produced via boron-hydride chemistry and metathesis routes
• Requires strict moisture and oxygen exclusion
Available Forms
✔ Anhydrous crystalline powders
✔ Ether-based solutions for controlled reactivity
✔ Stabilized formulations (limited cases)
Applications
Chemical & Pharmaceutical Synthesis:
• Reduction of carbonyl compounds, acids, esters, and nitriles
• Production of active pharmaceutical ingredients (APIs)
Materials Science:
• Lithiation agents for metal oxides
• Specialty precursor chemistry
Energy Research:
• Hydrogen storage investigations
• Advanced energy material synthesis
Advantages
✔ Extremely powerful reducing capability
✔ Broad reaction scope (LiAlH₄)
✔ Selective reduction pathways (LiBH₄)
✔ High hydrogen density
Safety & Handling
⚠ React violently with moisture and water
⚠ Release flammable hydrogen gas
⚠ LiAlH₄ may ignite in air
⚠ Require inert atmosphere storage and handling
Why Choose These Hydrides
Choose LiAlH₄ when:
• Maximum reducing power is required
• Broad-spectrum reductions are needed
Choose LiBH₄ when:
• Selectivity is important
• Hydrogen content or specific reduction behavior is desired