Reduction of carbonyls Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols. Aldehydes will be reduced to primary alcohols Ketones will be reduced to secondary alcohols. propanone C C C O H H H H H H C H H C H C H H H O H C + 2[H] H O C H H C H H H H + 2[H] C O H H H C H H C H H H propanal Propan-1-ol Propan-2-ol Reagents: NaBH4 In aqueous ethanol Conditions: Room temperature and pressure NaBH4 contain a source of nucleophilic hydride ions (H-) which are attracted to the positive carbon in the C=O bond. Nucleophilic Addition Mechanism H+ from water or weak acid Catalytic Hydrogenation Carbonyls can also be reduced using catalytic hydrogenation Reagent: hydrogen and nickel catalyst Conditions: high pressure CH3CHO + H2 CH3CH2OH Example Equations CH3COCH3 + H2 CH3CH(OH)CH3
3.3.8 Aldehydes and ketones (A-level only)
Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH4 in aqueous solution.
These reduction reactions are examples of nucleophilic addition.
Students should be able to:
• write overall equations for reduction reactions using [H] as the reductant
• outline the nucleophilic addition mechanism for reduction reactions with NaBH4 (the nucleophile should be shown as H– )