Reduction of Carboxylic Acids, Aldehydes & Ketones
Carbonyl compounds
- Alcohols can be oxidised to carbonyl compounds in the presence of a suitable oxidising agent
- Primary alcohol → aldehyde → carboxylic acid
- Secondary alcohol → ketone
- Tertiary alcohol - no reaction
- These reactions can be reversed in the presence of a suitable reducing agent
- Carboxylic acid → aldehyde → primary alcohol
- Ketone → secondary alcohol
- The two most common reducing agents for carbonyl compounds are:
- Lithium aluminium hydride, LiAlH4, in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid
- This is the stronger of these reducing agents and can reduce carboxylic acids
- Sodium borohydride, NaBH4, in aqueous or alcoholic solutions
- This is the less hazardous of these reducing agents but it cannot reduce carboxylic acids
- Both of these reagents produce the nucleophilic hydride ion, H–
- Lithium aluminium hydride, LiAlH4, in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid
Exam Tip
- You can be expected to know typical conditions and reagents of all reactions, e.g. catalysts, reducing agents, reflux, etc
- However, you do not need to know more precise details such as specific temperatures
Reduction Reactions
- Equations for reduction reactions can be written using [H] to represent the reducing agent
Carboxylic acids
- Carboxylic acids are reduced to primary alcohols
- This requires heating with LiAlH4 under reflux in dry ether followed by dilute acid
- Remember that NaBH4 cannot reduce carboxylic acids
Reduction of a carboxylic acid
Carboxylic acids are reduced to primary alcohols with LiAlH4
Aldehydes
- Aldehydes are reduced to primary alcohols
- This can be with LiAlH4 or NaBH4
Reduction of an aldehyde
Aldehydes are reduced to primary alcohols with LiAlH4 or NaBH4
Ketones
- Ketones are reduced to secondary alcohols
- This can be with LiAlH4 or NaBH4
Reduction of a ketone
Ketones are reduced to secondary alcohols with LiAlH4 or NaBH4
Exam Tip
- Take care if you are asked about the formation of an aldehyde from a carboxylic acid
- You have to use LiAlH4 refluxed in dry ether, followed by dilute acid
- This reaction cannot be stopped at the aldehyde because the LiAlH4 is too powerful
- To form an aldehyde from a carboxylic acid, you have to reduce the carboxylic acid down to a primary alcohol and then oxidise it back up to the aldehyde