The oxidation-reduction reactions of carbonyl compounds
Oxidation
Aldehydes can be easily oxidised to carboxylic acids.
This oxidation from primary alcohols is a two stage process.
This is more simply represented as:
Ketones cannot be oxidised without the molecule being broken up.
Aldehydes are powerful reducing agents possible to:
- Reduce copper 2+ to copper (I)
- Silver (I) to metallic silver
There are used as a basis to distinguish aldehydes from ketones.
Reduction
It is possible to make primary and secondary alcohols by reduction of aldehydes and ketones.
Aldehydes give 1° alcohols and ketones 2° alcohols.
Other reducing agents include:
- Lithium tetrahydridoaluminate (III) LiAlH4 needs to be in ethoxyethane – water eliminated.
- Sodium in ethanol
- Hydrogen gas under pressure with platinum catalyst
Remember: Oxidising agents maybe written as [O] and reducing agents [H]. Remember to balance these equations even when using these symbols
Questions
1. Write balanced equations (using [H]) for the reaction of sodium tetrahydridoaluminate (III) in water with
a) CH3CHO
b) CH3CH2COCH3
c) CH3COCH2CHO
2. How would you expect each of the following compounds A, B and C, to react:
a) when refluxed with acidified potassium dichromate (VI) solution
b) with sodium tetrahydridoaluminate (III) in water
A – CH3CH2CHO
B – (CH3)2CHOH
C – CH3COCH3
3 2-Oxopropanal, CH3COCHO, is thought to be one of the chemicals responsible for the smell in burnt sugar
a) This compound contains two carbonyl groups. Explain the difference between the two groups.
b) What reagent(s) would you use to reduce this compound?
c) Draw the full displayed formula of the compound formed in the reduction reaction.
d) If the oxopropanal is refluxed with acidified potassium dichromate, a new compound is formed. Write and equation for the reaction using [H] as appropriate.