10.3.4 REDUCTION REACTIONS

Reduction of aldehydes, ketones and carboxylic acids

• Common reducing agents are :*lithium aluminium hydride (LiAlH4) *Sodium borohydride (NaBH4)

• LiAlH4 are the stronger reducing agent • Can be used to reduce carboxylic acids,

aldehydes and ketones• NaBH4 only strong enough to reduce

aldehydes and ketones • NaBH4 is much easier to use than LiAlH4 and

usually preferred for the reduction of aldehydes and ketones

ALDEHYDES BASIC REACTION :

NaBH4 Aldehydes primary alcohol

methanol

RH

OR

H

OH+ [H]

Examples

CH3 O [H]+CH3

OHEthanal Ethanol

O

[H]+

OH

benzaldehyde benzylalcohol

• A balanced equation for the reaction can be written using [H] to represent hydrogen from reducing agent:

CH3CHO + 2[H] CH3CH2OH

Ketones BASIC REACTION :

NaBH4 ketone secondary alcohol

methanol

NaBH4

methanol

Carboxylic acidsBASIC REACTION :

(i) LiAlH4 in ethoxyethaneCarboxylic acids primary alcohol

(ii) H+/H2O

(i) LiAlH4 in ethoxyethane

(ii) H+/H2O

• This can also be written as a balanced equation using [H] to represent hydrogen from the reducing agent:

• CH3COOH +4[H] CH3CH2OH + H2O

If you need to make an aldehyde from a carboxylic acids, the carboxylic acids must be reduced to a primary alcohol using LiAlH4 and then the primary alcohol must be oxidised back to an aldehyde . (partial oxidation with distillation)

Reduction of nitrobenzene

• First step could be written as :C6H5NO2 + 6[H] + H+ C6H5NH3

+ + 2H2O• And the second as :C6H5NH3

+ + OH- C6H5NH2 + H2O

BASIC REACTION :

(i) Sn/ conc. HCl HEATNitrobenzene + 6[H] phenylamine aniline

(ii) NaOH