Lithium aluminum hydride (LiAlH4) is a powerful reducing agent that delivers hydrogen to electron-poor atoms in organic molecules. It converts carbonyl groups like aldehydes, ketones, esters, and carboxylic acids into alcohols, and it reduces amides and nitriles into amines. Each molecule of LiAlH4 carries four hydrogen atoms bonded to aluminum, and because aluminum holds onto those hydrogens loosely, the reagent is reactive enough to break into functional groups that milder reducing agents leave untouched.
How LiAlH4 Delivers Hydrogen
The key to understanding LiAlH4 is the aluminum-hydrogen bond. Aluminum is not very electronegative, so it doesn’t pull electron density away from hydrogen the way a more electronegative atom would. That makes each hydrogen carry extra electron density, effectively turning it into a hydride ion (H⁻), a hydrogen with a pair of electrons ready to form a new bond.
When LiAlH4 encounters a carbonyl group (a carbon double-bonded to oxygen), the electron-rich hydride attacks the carbon of that carbonyl. Oxygen is pulling electron density away from carbon through the double bond, leaving carbon partially positive and vulnerable to attack. Computational studies suggest the process starts with a single electron transferring to the carbonyl carbon, followed by formation of a bridging bond between aluminum, hydrogen, and carbon, and finally full transfer of the hydrogen as the bond to aluminum breaks. The result is that the carbon-oxygen double bond becomes a carbon-oxygen single bond, with a new carbon-hydrogen bond in its place.
What It Reduces and What You Get
LiAlH4 is sometimes called a “universal” carbonyl reducing agent because it handles nearly every common carbonyl-containing functional group. Here’s what it converts and the products you get:
- Aldehydes become primary alcohols (one carbon bonded to the OH group).
- Ketones become secondary alcohols (two carbons bonded to the carbon bearing OH).
- Carboxylic acids become primary alcohols. This is notable because milder reagents like NaBH4 can’t do this.
- Esters become primary alcohols (plus a second alcohol from the leaving group portion).
- Acid chlorides become primary alcohols.
- Amides become amines. A primary amide gives a primary amine, a secondary amide gives a secondary amine, and a tertiary amide gives a tertiary amine.
- Nitriles become primary amines under standard addition. With inverse addition (adding the nitrile to the reagent slowly), you can stop the reduction partway and isolate an imine instead.
- Oximes (C=NOH groups) become primary amines.
One important limitation: LiAlH4 does not reduce isolated carbon-carbon double bonds or aromatic rings under normal conditions. Its reactivity is targeted at polar bonds where one atom is significantly more electronegative than the other.
Why It’s Stronger Than NaBH4
Students often encounter LiAlH4 alongside sodium borohydride (NaBH4), and the distinction matters for predicting reaction outcomes. NaBH4 reduces aldehydes and ketones but generally leaves esters, carboxylic acids, and amides alone. LiAlH4 reduces all of them.
The reason comes down to bond polarity. Boron is more electronegative than aluminum, so the boron-hydrogen bond in NaBH4 is less polar than the aluminum-hydrogen bond in LiAlH4. Less polar means the hydride is held more tightly and delivered less aggressively. LiAlH4’s hydrides are more reactive, giving the reagent enough energy to break into the less electrophilic carbonyls found in esters and carboxylic acids, where the oxygen lone pairs partially stabilize the carbon and make it harder to attack.
In practice, this means you choose NaBH4 when you want to selectively reduce an aldehyde or ketone while leaving an ester elsewhere in the molecule intact. You reach for LiAlH4 when you need to reduce that ester, carboxylic acid, or amide all the way down.
Why It Requires Dry Solvents
LiAlH4 reacts violently with water. One mole of LiAlH4 reacting with four moles of water produces four moles of hydrogen gas, along with lithium hydroxide and aluminum hydroxide. The heat generated can be enough to ignite the hydrogen on contact. Even trace moisture in a solvent or condensation on glassware can trigger a dangerous reaction, particularly with larger quantities of the reagent.
For this reason, LiAlH4 reactions are always run in anhydrous (water-free) solvents. The two standard choices are dry diethyl ether and dry tetrahydrofuran (THF). These solvents dissolve LiAlH4 well enough for the reaction to proceed, and they don’t contain the kind of acidic hydrogens that would destroy the reagent. Alcohols, water, and any other protic solvent are off the table. In fact, when a small amount of LiAlH4 is added to an ethereal solvent, it reacts with and removes any trace water present, effectively drying the solvent further.
How the Reaction Is Set Up
A typical LiAlH4 reduction follows a straightforward sequence. The reagent is dissolved or suspended in dry ether or THF under an inert atmosphere (usually nitrogen or argon to exclude moisture and air). The substrate, meaning the molecule you want to reduce, is then added slowly, often dissolved in the same solvent. The reaction usually proceeds at room temperature or with gentle warming, depending on the functional group.
After the reaction is complete, the product is trapped as an aluminum alkoxide salt, a complex where the newly formed oxygen or nitrogen is still coordinated to aluminum. To free the product, you need to carefully add water in a controlled fashion to break apart these aluminum complexes. This quenching step is the most hazardous part of the process because you are deliberately introducing water to a mixture that may still contain unreacted LiAlH4. The water is added very slowly, typically dropwise, to a cold and dilute mixture so that hydrogen gas evolves at a manageable rate. The aluminum salts that form during quenching often create a thick, gel-like slurry that can trap your product, so the workup usually involves adding aqueous acid or base to dissolve those salts and make filtration or extraction easier.
Safety Considerations
LiAlH4 is classified as both flammable and corrosive. The dry powder can ignite from friction, grinding, or even static sparks. It reacts violently not only with water but also with acids, alcohols, and moist air. On contact with skin or eyes, moisture triggers formation of lithium hydroxide, a corrosive alkali.
Storage requires a cool, dry location under inert gas, kept far from any source of moisture. It should also be stored away from ketones, aldehydes, and other compounds it could reduce spontaneously. Spills of unreacted LiAlH4 should not be cleaned up without expert supervision, and disposal involves controlled reaction with anhydrous agents like calcium hydroxide to safely consume the remaining hydride before the aluminum byproducts can be sent to landfill.