Figure 12. Infra-red Spectra of An Aldehyde
| Contents |
General Introduction |
Structure |
How Carbonyl Compounds React |
Aldehydes And Ketones |
Distinguishing Tests |
Glossary |
It is possible to distingish between an aldehyde and ketone through a few simple experiments.
1. Reacting Ammoniacal silver nitrate solution with the carbonyl compounds. A mild oxidation reaction occurs as the aldehyde is oxidised to a carboxylic acid:
RCHO + [O] = RCOOH
The silver is reduced because it gains electrons:
Ag+ + e- = Ag
This creates a silver mirror solution. The ketone will not react, this is because a ketone cannot be further oxidised.
2. Adding Acidified Potassium Dichromate solution to the aldehyde and ketone. The aldehyde is oxidised to make a carboxylic acid:
RCHO + [O] = RCOOH
The dichromate is reduced. It changes colour from orange to green.
Cr2O72-(aq) + H2O(l) = 2CrO42-(aq) + 2H+(aq)
The ketone does not react as it doesnt oxidise further. Therefore the dichromate stays orange.
3. Adding 2,4-dinitrophenylhydrazine solution to each of the carbonyl compounds. The solution precipitates to give a orange crystal structure. The hydrazone can then be recrystallised then used to give a melting point which will correspond to either the aldehyde or ketone. This test can also be used to establish the existence of a carbonyl group.
| Aldehydes | Ketones | |
|---|---|---|
| Ammoniacal silver nitrate | Silver Mirror | No Reaction |
| Acidified Potassium Dichromate | Orange to Green | Stays Orange |
| 2,4-dinitrophenylhydrazine solution | Orange Precipitate where melting point can be calculated | Orange Precipitate where melting point can be calculated |
Another distinguishing test is the use of spectroscopy. This produces different spectra which are evaluated below.
ALDEHYDE:
An aldehyde will have C-H absorption bands. These are two small absorptions to the right of the spectra around 2900 cm-1 and 2700 cm-1. Look for the carbonyl band (C=O) around 1740-1720 cm-1.
Figure 12.
Infra-red Spectra of An Aldehyde
KETONE:
The aldehyde C-H absorption bands will be absent as a ketone bonds to anoher functional group. A carbonyl C=O band will be around 1725-1705 cm-1.
Figure 13.
Infra-red Spectra of A Ketone