Introduction

Lithium aluminum hydride (LiAlH₄) and sodium borohydride (NaBH₄) are two of the most important reducing agents in organic chemistry. At first glance they look extremely similar, and in many ways they are. Both reagents deliver hydride to electrophilic carbonyl carbons and both are commonly used to convert carbonyl-containing compounds into alcohols.

Despite their similarities, they are not interchangeable. The key difference is strength. Lithium aluminum hydride is a much stronger reducing agent than sodium borohydride, allowing it to reduce a much wider range of carbonyl-containing functional groups. Understanding that difference will help you quickly choose the correct reagent and predict the correct product.

Why These Reagents Are Nucleophiles

Structurally, both reagents contain a central atom bonded to four hydrogens:

• LiAlH₄ contains aluminum bonded to hydrogen.
• NaBH₄ contains boron bonded to hydrogen.

At first glance, you might think the negatively charged aluminum or boron is the reactive center.

It isn't.

Hydrogen is actually more electronegative than aluminum and slightly more electronegative than boron. As a result, the electron density in these bonds is biased toward hydrogen.

This means the reactive site of both reagents is:

The hydrogen atom.

In both cases, the hydride behaves as the nucleophile.

What Do These Reagents Do?

Both reagents are primarily used to reduce carbonyl compounds.

In general:

• Hydride attacks the electrophilic carbonyl carbon.
• Electrons move onto oxygen.
• After acidic workup, an alcohol is formed.

The major difference is not the mechanism.

The major difference is which carbonyls each reagent can reduce.

Why LiAlH₄ Is Stronger

The aluminum-hydrogen bond is weaker than the boron-hydrogen bond.

Because the hydride attached to aluminum is more reactive, lithium aluminum hydride behaves as a stronger nucleophile and a stronger reducing agent.

This increased reactivity allows LiAlH₄ to reduce functional groups that NaBH₄ typically cannot.

Sodium Borohydride: The Gentle Reducing Agent

Sodium borohydride is the weaker reagent.

In practice, it is primarily used to reduce:

• Aldehydes
• Ketones

These reductions are reliable, predictable, and common throughout organic chemistry.

When you see NaBH₄, the first thing you should think is:

Aldehyde or ketone reduction.

Lithium Aluminum Hydride: The Powerful Reducing Agent

Lithium aluminum hydride reduces a much wider range of carbonyl-containing compounds.

It can reduce:

• Aldehydes
• Ketones
• Esters
• Carboxylic acids
• Acid chlorides
• Anhydrides
• Amides

This broad reactivity makes LiAlH₄ one of the most useful reducing agents in synthesis.

If a molecule contains a carbonyl and can be reduced, LiAlH₄ is usually capable of doing the job.

What Products Are Formed?

Most carbonyl-containing functional groups reduced by LiAlH₄ ultimately become alcohols.

Examples include:

Aldehydes

Reduced to:

Primary alcohols

Ketones

Reduced to:

Secondary alcohols

Esters

Reduced to:

Primary alcohols

Carboxylic Acids

Reduced to:

Primary alcohols

Acid Chlorides

Reduced to:

Primary alcohols

Anhydrides

Reduced to:

Alcohol products

The outcome is generally straightforward:

Carbonyl reduction usually gives alcohols.

The Important Exception: Amides

Amides behave differently.

In an amide, nitrogen would have to leave as N⁻ in order to generate the same alcohol-type product seen with other carbonyl derivatives.

Because N⁻ is a very poor leaving group, the reaction follows a different pathway.

As a result:

LiAlH₄ reduces amides to amines, not alcohols.

This is one of the most important exceptions to memorize.

Comparing the Reagents Visually

A simple way to think about these reagents is:

Sodium Borohydride

Reduces:

• Aldehydes
• Ketones

Lithium Aluminum Hydride

Reduces:

• Aldehydes
• Ketones
• Esters
• Carboxylic acids
• Acid chlorides
• Anhydrides
• Amides

When in doubt:

LiAlH₄ does much more than NaBH₄.

Chemoselectivity: The Real Difference

The most important practical distinction appears when a molecule contains multiple carbonyl groups.

Suppose a molecule contains:

• A ketone
• An ester

Using NaBH₄

The ketone is reduced.

The ester remains unchanged.

Using LiAlH₄

Both the ketone and the ester are reduced.

This difference allows chemists to selectively reduce one functional group while leaving another untouched.

A Useful Shortcut

If you are solving predict-the-product problems, a quick shortcut is:

NaBH₄

Think:

Aldehydes and ketones only.

LiAlH₄

Think:

Almost every carbonyl.

That shortcut gets you to the correct answer surprisingly often.

Common Student Mistakes

Treating the Reagents as Interchangeable

They are not.

LiAlH₄ is significantly stronger and reacts with many more functional groups.

Forgetting About Esters

Esters are reduced by LiAlH₄ but generally not by NaBH₄.

Missing the Amide Exception

Most carbonyl derivatives become alcohols after reduction.

Amides become amines.

Thinking the Metal Is the Nucleophile

The reactive site is the hydride, not the aluminum or boron.

Key Takeaways

• Both LiAlH₄ and NaBH₄ are hydride reducing agents.
• In both reagents, the nucleophilic atom is hydrogen.
• Lithium aluminum hydride is the stronger reducing agent.
• Sodium borohydride is primarily used for aldehydes and ketones.
• Lithium aluminum hydride reduces nearly all carbonyl-containing functional groups.
• Most reductions produce alcohols.
• Amides are reduced to amines.
• NaBH₄ can selectively reduce aldehydes and ketones while leaving many acid derivatives untouched.
• LiAlH₄ is the reagent of choice for more demanding reductions.

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