Carbonyl Reductions
Carbonyl reductions convert carbonyl compounds into alcohols or related reduced products.
The main exam decision is not just “reduction happens”; it is which carbonyl class reacts with which reducing agent.
Core pattern
Section titled “Core pattern”Hydride reducing agents deliver H- to the electrophilic carbonyl carbon.
After hydride addition, protonation during solvent or work-up gives the alcohol product.
Common reducing agents
Section titled “Common reducing agents”| Reagent | Strength | Typical scope |
|---|---|---|
| NaBH4 | Mild | Aldehydes and ketones |
| LiAlH4 | Strong | Aldehydes, ketones, esters, carboxylic acids, amides |
| DIBAL-H | Controlled strong reagent | Esters and nitriles to aldehydes under selected conditions |
| H2 / catalyst | Catalytic hydrogenation | Alkenes, alkynes, some carbonyls depending on catalyst and conditions |
Aldehydes and ketones
Section titled “Aldehydes and ketones”Both NaBH4 and LiAlH4 reduce aldehydes and ketones.
- Aldehyde -> primary alcohol
- Ketone -> secondary alcohol
NaBH4 is often preferred when only aldehydes and ketones need to be reduced.
Esters and carboxylic acids
Section titled “Esters and carboxylic acids”Esters and carboxylic acids are usually not reduced by NaBH4 under standard undergraduate conditions.
LiAlH4 is normally used for full reduction:
- Ester -> primary alcohols
- Carboxylic acid -> primary alcohol
Acid chlorides
Section titled “Acid chlorides”Acid chlorides are very reactive carbonyl derivatives.
They can be reduced all the way to alcohols, but selective reduction to aldehydes requires controlled reagents and conditions.
For exam answers, do not assume NaBH4 is always selective enough unless the reagent and conditions are stated.
Selectivity checklist
Section titled “Selectivity checklist”Before choosing a product, ask:
- What carbonyl derivative is present?
- Is the reagent mild or strong?
- Is the work-up acidic or protic?
- Are there other reducible groups in the molecule?
- Does the question ask for chemoselectivity?
Common exam traps
Section titled “Common exam traps”- Do not use NaBH4 to reduce esters by default.
- Do not stop LiAlH4 ester reduction at the aldehyde unless a selective reagent is specified.
- Do not forget the protonation step after hydride addition.
- Do not draw hydride attack on the oxygen; hydride attacks the carbonyl carbon.
- Check whether an aldehyde product can be further reduced under the conditions.
Related pages
Section titled “Related pages”Quick summary
Section titled “Quick summary”NaBH4 is the standard mild choice for aldehydes and ketones.
LiAlH4 is the stronger reagent for carboxylic acid derivatives, but it is less tolerant and requires careful work-up.