Oxidation of Aldehydes and Ketones
Oxidation of Aldehydes
Aldehydes are easily oxidised to carboxylic acids containing the same number of carbon atoms with oxidising agents like acid dichromate (K2Cr2O7/H2SO4) and KMnO4. The oxygen of an oxidising agent is usually represented by [O].
Oxidation of Ketones
Ketones do not undergo oxidation easily and on vigorous oxidation yield acids with lesser number of carbon atoms.
Unsymmetrical ketones
In case of unsymmetrical ketones, the C=O bond remains with the smaller chain. This rule is known as Popoff's rule. Study the following example for better understanding :
Oxidation of butan-2-one
The oxidation involves the cleavage either between carbon atoms numbered 1 and 2; or between atoms numbered 2 and 3.
Cleavage involving carbons 1 and 2 :
In this case, the C=O group is with longer chain, and hence, is not preferred.
Cleavage involving carbons 2 and 3 :
In this case, the C=O group is attached to the smaller alkyl group. This results the major products.
Products formed
Hence the reaction in one step can be written as :
Symmetrical Ketones
In case of symmetrical ketones, only one type of cleavage occurs. A mixture of two acids is formed in this case. Take a look at the following example :
Oxidation of Pentan-3-one
Clearly, in case of pentan-3-one, it does not matter whether the cleavage occurs between 1,2 or 2,3 as the chain containing the carbonyl group will always contain 3 carbon atoms and the other one will contain 2 carbon atoms. Hence, the products formed will be CH3CH2COOH and CH3COOH.
The reaction in one step may be written as :
Distinction between Aldehydes and Ketones
Weak acids like Cu2+, Ag+, bromine water have potential to oxidise aldehydes. Ketones, however, can not be oxidised by weak acids. This difference in property between aldehydes and ketones is used to distinguish them from each other.
Tollen's test
When an aldehyde (aliphatic or aromatic) is warmed with ammoniated silver nitrate (Tollen's reagent), a bright silver mirror is formed on the inner sides of the test tube due to reduction of Tollen's reagents.
Unsaturated aldehydes are oxidised by Tollen's reagent to unsaturated acids.
Ketones do not react with Tollen's reagents.
Tollen's test is also known as silver mirror test.
Fehling's test
Fehling's solution contains copper (II) ions complexed with sodium potassium tartrate (Rochelle salt). In this reaction, Cu (II) ions are reduced to Cu(I) oxide which is a red brick ppt.
Note
- Ketones do not react with Fehling's solution.
- Aromatic aldehydes do not react with Fehling's solution either.
Why do ketones not give Tollen's test and Fehling's test
All aldehydes have a hydrogen atom attached to the carbonyl group.
Due to the presence of the H-atom, aldehydes are easily oxidised by even weak oxidising agents like Ag+, Cu2+ ions.
On the other hand, ketones do not have any hydrogen atom attached to the carbonyl group.
In ketones, oxidation involves the cleavage of C−C bond and, consequently,can only be oxidised by strong oxidising agents.
Haloform reaction
Aldehydes and ketones having methyl group attached to >C=O are oxidised by sodium hypohalite (NaOX or X2+NaOH) to haloforms.
Iodoform test
When sodium hypoiodide (NaOI or I2/NaOH) is used in the above haloform reaction, yellow ppt of iodoform is formed. Due to this reason, this reaction is used for detection of CH3CO group — or CH3CH(OH) which is discussed in distinction between alcohols.
Questions
Question : Give simple chemical tests to distinguish between the following pairs of compounds:
- Propanal and propanone
- Acetophenone and benzophenone
- Pentan-2-one and pentan-3-one
- Benzaldehyde and acetophenone
- Ethanal and propanal
Answer :
1. Propanal (CH3CH2CHO) and Propanone (CH3COCH3)
- Tollen's test: Propanal being an aldehyde gives Tollen's test but propanone, which is a ketone,
does not.
Reaction of propanal with Tollen's reagent:
Propanone being a ketone does not give Tollen's test:
- Iodoform test: Since propanone is a methyl ketone, it gives yellow ppt. with iodoform. Propanal
does not give this test.
Propanone gives iodoform test.
Propanal does not give iodoform test
2. Acetophenone (C6H5COCH3) and Benzophenone (C6H5COC6H5)
- Iodoform test: Acetophenone is a methyl ketone; therefore, it gives positive iodoform test. Benzophenone is not a methyl ketone; hence, does not give this test.
3. Pentan-2-one (CH3COCH2CH2CH3) and Pentan-3-one (CH3CH2COCH2CH3)
- Iodoform test: Pentan-2-one being a methyl ketone gives iodoform test. Pentan-3-one does not give this test.
4. Benzaldehyde and acetophenone
- Tollen's test: Benzaldehyde is an aldehyde; hence, it reduces Tollen's reagents. Acetophenone being a ketone does not.
- Iodoform test: Acetophenone gives iodoform test because it is a methyl ketone. Benzaldehyde does not give this test.
Ethanal (CH3CHO) and propanal (CH3CH2CHO)
- Iodoform test: Ethanal gives iodoform test wheareas propanal does not (why?).