LiAlH4 (lithium aluminum hydride) is one of the strongest reducing agents used in organic chemistry. It converts carbonyl-containing functional groups into alcohols or amines by delivering hydrogen to electron-poor carbon atoms. If you’re studying organic chemistry, it’s one of the first reagents you’ll encounter for reduction reactions, and understanding what it can and can’t reduce is essential for predicting reaction products.
Functional Groups LiAlH4 Reduces
LiAlH4 reduces a wide range of functional groups, and the pattern is straightforward once you see it. Anything with a carbon-oxygen double bond (or carbon-nitrogen multiple bond) is fair game. Here’s what it does to each:
- Aldehydes are reduced to primary alcohols.
- Ketones are reduced to secondary alcohols.
- Carboxylic acids are reduced to primary alcohols.
- Esters are reduced to primary alcohols (two alcohols form, one from each side of the ester linkage).
- Acid chlorides are reduced to primary alcohols.
- Amides are reduced to amines.
- Nitriles are reduced to primary amines.
- Oximes (C=N-OH groups) are reduced to amines.
The key theme: LiAlH4 breaks carbon-oxygen and carbon-nitrogen multiple bonds and replaces them with carbon-hydrogen and oxygen-hydrogen (or nitrogen-hydrogen) bonds. It’s essentially adding hydrogen across those polar bonds.
Why It’s Stronger Than NaBH4
The most common comparison in organic chemistry courses is between LiAlH4 and sodium borohydride (NaBH4). Both deliver a hydride (H⁻) to electrophilic carbons, but they differ dramatically in strength. Aluminum is less electronegative than boron, which makes the Al-H bond more polar and more reactive. That extra reactivity is what lets LiAlH4 tackle functional groups that NaBH4 simply can’t touch.
NaBH4 only reduces aldehydes and ketones. It is not strong enough to convert carboxylic acids, esters, or amides. So if an exam question asks you to reduce an ester to an alcohol, LiAlH4 is the reagent you need. If you only need to reduce a ketone and want to leave an ester group elsewhere in the molecule untouched, NaBH4 is the better choice because of its selectivity.
A useful way to remember this: NaBH4 is the gentle option, LiAlH4 is the sledgehammer. LiAlH4 will reduce almost every reducible functional group in the molecule, so you lose selectivity in exchange for power.
How the Reduction Works
The mechanism centers on hydride transfer. The aluminum hydride ion (AlH4⁻) carries four hydrogens, each capable of acting as a nucleophile. In a typical reduction, the hydride attacks the electrophilic carbon of a carbonyl group, breaking the carbon-oxygen double bond and forming a new carbon-hydrogen bond. The lithium ion (Li⁺) coordinates with the oxygen, helping to activate the carbonyl and position the reagent for attack. For the reaction to proceed, the LiAlH4 needs to line up in the same plane as the carbonyl group.
For carboxylic acids, the process involves an extra step. The acidic hydrogen on the carboxylic acid is removed first (deprotonation), and then the hydride attacks the carbonyl carbon. An aldehyde forms as an intermediate, but you can’t isolate it because it’s more reactive than the starting carboxylic acid and gets reduced immediately to the primary alcohol.
For nitriles, the hydride attacks the electrophilic carbon of the C≡N triple bond, forming an imine intermediate. A second hydride delivery and then addition of water during workup converts this intermediate into a primary amine. This is one of the most reliable ways to make a primary amine from a nitrile in the lab.
Each molecule of LiAlH4 has four hydrides available, so in principle one equivalent can reduce up to four equivalents of substrate, though in practice excess reagent is typically used to ensure complete reaction.
Solvent Requirements
LiAlH4 must be used in dry, aprotic solvents. The two standard choices are anhydrous diethyl ether and tetrahydrofuran (THF). Water, alcohols, and any solvent with an O-H or N-H bond will react violently with LiAlH4 instead of letting it do its job on your substrate. Even trace moisture in your solvent can cause problems. In practice, adding a small amount of LiAlH4 to the solvent first consumes any residual water before the actual substrate is introduced.
This solvent restriction is one of the practical tradeoffs of using LiAlH4 over NaBH4. Sodium borohydride can be used in methanol or even aqueous ethanol, making it far more convenient for simple aldehyde and ketone reductions.
Safety Concerns
LiAlH4 is one of the more hazardous reagents in a typical organic chemistry lab. It reacts violently with water, producing hydrogen gas that can ignite or explode. Even moist air can trigger a dangerous reaction. In powdered form, it can ignite from friction alone. The NFPA hazard diamond gives it the special “W” designation, meaning it reacts violently or explosively with water.
Pouring a solvent that contains traces of water or alcohol onto a significant quantity of LiAlH4 (roughly 5 grams or more) can start a fire. Because of this, LiAlH4 must be stored under dry, inert conditions and handled with care. Spills are covered with plastic sheeting to keep the powder dry, and cleanup requires specialist supervision.
Quenching the Reaction Safely
After the reduction is complete, you still have excess LiAlH4 and aluminum salts in your flask that need to be destroyed before you can isolate your product. This step is called the workup, and it has to be done carefully because you’re now intentionally adding water to a mixture that contains a water-reactive reagent.
The most common approach is the Fieser workup. You first dilute the reaction with ether and cool it to 0°C. Then, for every gram of LiAlH4 used, you slowly add that same number of milliliters of water, followed by the same volume of 15% sodium hydroxide solution, then three times that volume of water again. After warming to room temperature and stirring, you add a drying agent, filter off the aluminum salts, and collect your product. The slow, sequential addition prevents the violent hydrogen evolution that would occur if you dumped water in all at once.
An alternative method uses Glauber’s salt (hydrated sodium sulfate), which releases water slowly and in a controlled fashion. You add it at 0°C until hydrogen gas stops bubbling, then continue stirring and filter. Both methods achieve the same goal: safely destroying leftover reagent while keeping your product intact.