Reduction Reactions
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
- Lithium aluminium hydride, LiAlH4, in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid
- Both of these reagents produce the nucleophilic hydride ion, H-
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 acid to a primary alcohol (using LiAlH4 refluxed in dry ether, followed by dilute acid)
- Remember that NaBH4 cannot reduce carboxylic acids
- Aldehyde to a primary alcohol (using LiAlH4 or NaBlH4)
- Ketone to a secondary alcohol (using LiAlH4 or NaBlH4)
Exam Tip
Take care if you are asked about the formation of an aldehyde from a carboxylic acidYou have to use LiAlH4 refluxed in dry ether, followed by dilute acid but this reaction cannot be stopped at the aldehyde because the LiAlH4 is too powerfulTo 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
Reduction of nitrobenzene
- Nitrobenzene, C6H5NO2, can be reduced to phenylamine, C6H5NH2, according to the following two-stage reaction:
The two-stage reduction reaction of nitrobenzene to phenylamine
Stage 1 - Reduction of nitrobenzene
- C6H5NO2 (l) + 3Sn (s) + 7H+ (aq) → C6H5NH3+ (aq) + 3Sn2+ (aq) + 2H2O (l)
- Nitrobenzene, C6H5NO2, is reacted with tin, Sn, and concentrated hydrochloric acid, HCl
- The reaction mixture is heated under reflux in a boiling water bath
- The phenylammonium ions, C6H5NH3+, are protonated due to the acidic conditions
Stage 2 - Formation of phenylamine
- C6H5NH3+ (aq) + OH- (aq) → C6H5NH2 (l) + H2O (l)
- The phenylammonium ions, C6H5NH3+, are deprotonated by the addition of sodium hydroxide solution, NaOH (aq)
