What Happens When a Ketone Is Reduced?

October 21, 2025 •

3 min read

The reduction of a ketone is a fundamental transformation in organic chemistry, reliably converting a carbonyl group ($>C=O$) into a hydroxyl group ($-OH$). This reaction results in the formation of a secondary alcohol, a crucial building block in synthetic chemistry. The process involves the addition of two hydrogen atoms across the carbon-oxygen double bond, mediated by specific reducing agents.

Quick Summary

Reduction of a ketone yields a secondary alcohol by adding two hydrogen atoms to the carbonyl group via a nucleophilic addition mechanism. Common reducing agents include sodium borohydride and lithium aluminum hydride, each with distinct reactivity. The reaction's stereochemistry can be controlled in certain cases.

Key Points

Formation of Secondary Alcohols: The reduction of a ketone results in the formation of a secondary alcohol, where the carbonyl group ($>C=O$) is converted to a hydroxyl group ($-OH$) bonded to a carbon with two other carbon atoms.
Nucleophilic Addition Mechanism: The reaction proceeds via a two-step nucleophilic addition, where a hydride ion attacks the electrophilic carbonyl carbon, followed by a protonation step.
Use of Metal Hydrides: The most common reducing agents are sodium borohydride ($NaBH_4$) and lithium aluminum hydride ($LiAlH_4$).
Reactant Selectivity: $NaBH_4$ is a milder reagent used in protic solvents and selectively reduces only ketones and aldehydes, whereas $LiAlH_4$ is stronger, requires anhydrous solvents, and can reduce a broader range of functional groups.
Enantioselectivity Concerns: The reduction of asymmetrical ketones can create a new chiral center, potentially leading to a racemic mixture unless specific chiral reagents are used to direct the stereochemical outcome.
Complete Deoxygenation: In specialized reactions like the Wolff-Kishner or Clemmensen reduction, a ketone's carbonyl group can be completely removed to form an alkane.

From Ketone to Secondary Alcohol: The Reduction Process

The reduction of a ketone is a core reaction in organic chemistry, defined by the conversion of its carbonyl group ($>C=O$) into a secondary alcohol ($>CHOH$). This process is essentially the opposite of alcohol oxidation, where a secondary alcohol is converted back to a ketone. The chemical reaction involves the gain of electrons for the carbonyl carbon, lowering its oxidation state and converting it into a new functional group. A variety of reagents can be used to accomplish this, leading to different practical considerations for chemists.

The Mechanism of Ketone Reduction

The most common method for reducing ketones involves using metal hydride reagents, such as sodium borohydride ($NaBH_4$) or lithium aluminum hydride ($LiAlH_4$). The mechanism is a two-step process involving nucleophilic addition to the electrophilic carbonyl carbon. Due to the high electronegativity of oxygen, the carbon atom in the carbonyl group carries a partial positive charge, making it susceptible to attack by a hydride ion ($H^−$).

Step 1: Nucleophilic Attack

The reducing agent delivers a hydride ion ($H^−$).
This hydride ion attacks the partially positive carbonyl carbon.
An intermediate alkoxide ion ($R_2CHO^−$) is formed.

Step 2: Protonation (Workup)

An acidic workup is performed.
The alkoxide ion is protonated to form the final secondary alcohol product ($R_2CHOH$).

Common Reducing Agents for Ketones

Several reagents are used for ketone reduction, each with specific advantages and limitations.

Sodium Borohydride ($NaBH_4$): Milder and safer, used in protic solvents. It selectively reduces aldehydes and ketones.
Lithium Aluminum Hydride ($LiAlH_4$): More powerful, requires anhydrous ethereal solvents. It can reduce a broader range of functional groups.
Catalytic Hydrogenation: Uses hydrogen gas with a metal catalyst. It can also reduce other double bonds.

Comparison of Common Reducing Agents


Feature	Sodium Borohydride ($NaBH_4$)	Lithium Aluminum Hydride ($LiAlH_4$)	Catalytic Hydrogenation ($H_2$/Metal)
Reducing Strength	Milder	Stronger and more reactive	Highly versatile
Solvent Compatibility	Protic solvents (e.g., ethanol, water)	Anhydrous, aprotic solvents (e.g., ether, THF)	No specific solvent requirement, reaction is heterogeneous
Reaction Conditions	Milder, room temperature	Vigorous reaction, often requires cooling	Elevated temperature and pressure may be needed
Selectivity	High. Only reduces aldehydes and ketones.	Lower. Can reduce many functional groups.	May reduce other double bonds along with the carbonyl.
Handling	Safer and easier to handle	Reactive and potentially hazardous	Requires specialized equipment for high-pressure hydrogen
Product	Secondary alcohol	Secondary alcohol	Secondary alcohol

Stereoselectivity in Ketone Reduction

Reducing asymmetrical ketones can create a chiral center, potentially yielding a mixture of stereoisomers. Stereochemical outcomes can be influenced by factors like chiral reducing agents or substrate structure. For example, the CBS reduction and enzymatic methods can achieve high enantioselectivity. You can find more information on chiral reducing agents here.

Complete Deoxygenation: Reducing to Alkanes

Ketones can also be completely reduced to alkanes. Common methods include:

Wolff-Kishner Reduction: Converts the ketone to an alkane using a hydrazone intermediate and a strong base.
Clemmensen Reduction: Uses zinc amalgam and concentrated hydrochloric acid to reduce the ketone to an alkane under acidic conditions.

Conclusion

When a ketone is reduced, its carbonyl group is converted to a hydroxyl group, forming a secondary alcohol. The reaction typically involves a nucleophilic addition mechanism with hydride-donating agents like sodium borohydride or lithium aluminum hydride. The choice of reducing agent depends on desired conditions and selectivity. For asymmetrical ketones, stereochemistry might need control using specific reagents, and in certain cases, a ketone can be fully deoxygenated to an alkane. Ketone reduction is a versatile and essential tool in organic synthesis.

Frequently Asked Questions

When a ketone is reduced, the primary product is a secondary alcohol. This occurs because the carbonyl carbon, which is bonded to two other carbon groups in a ketone, becomes a chiral center upon reduction.

The two most common reducing agents for ketones are sodium borohydride ($NaBH_4$) and lithium aluminum hydride ($LiAlH_4$). Other methods include catalytic hydrogenation and specialized chiral reagents.

The mechanism is similar, but the product is different. Ketone reduction yields a secondary alcohol, while aldehyde reduction yields a primary alcohol. This is because an aldehyde has at least one hydrogen bonded to its carbonyl carbon, while a ketone has two carbon groups.

The hydride ion ($H^−$) acts as a nucleophile, meaning it has a negative charge and an available electron pair. It attacks the positively polarized carbonyl carbon, initiating the addition reaction.

$LiAlH_4$ is more reactive because the aluminum-hydrogen bond is more polar and has a more ionic character than the boron-hydrogen bond in $NaBH_4$. This makes the hydride ion delivered by $LiAlH_4$ more nucleophilic and powerful.

Yes, a ketone can be fully reduced to an alkane (a hydrocarbon) through specific reactions that achieve complete deoxygenation. Examples include the Wolff-Kishner reduction and the Clemmensen reduction.

When an asymmetrical ketone is reduced with a non-chiral reagent, a new chiral center is created, and a racemic mixture of two enantiomeric secondary alcohols is typically produced. Specialized chiral reagents are needed for enantioselective reduction.