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Oxidation of Aldehydes and Ketones

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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 formaldehyde
Oxidation of acetaldehyde
Oxidation of benzaldehyde

Oxidation of Ketones

Ketones do not undergo oxidation easily and on vigorous oxidation yield acids with lesser number of carbon atoms.

Oxidation of ketones

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

Oxidation of unsymmetrical ketones examples

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 :

Oxidation of butan-2-one explaination

In this case, the C=O group is with longer chain, and hence, is not preferred.

Cleavage involving carbons 2 and 3 :

Oxidation of butan-2-one explaination

In this case, the C=O group is attached to the smaller alkyl group. This results the major products.

Products formed

Oxidation of unsymmetrical ketones examples : Popoff's rule

Hence the reaction in one step can be written as :

Oxidation of unsymmetrical ketones examples : Popoff's rule

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

Oxidation of Symmetrical ketones, example 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.

Oxidation of pentan-3-one explaination

The reaction in one step may be written as :

Oxidation of pentan-3-one, products ethanoic acid and propanoic acid

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.

Tollen's test to distinguish aldehydes and ketones

Unsaturated aldehydes are oxidised by Tollen's reagent to unsaturated acids.

Tollen's test to distinguish aldehydes and ketones

Ketones do not react with Tollen's reagents.

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.

Fehling's solution

Note

Why do ketones not give Tollen's test and Fehling's test

All aldehydes have a hydrogen atom attached to the carbonyl group.

Aldehyde structure

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.

Ketone structure

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.

Haloform reaction
Haloform reaction
Haloform reaction
Haloform reaction

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.

Iodoform test

Questions

Question : Give simple chemical tests to distinguish between the following pairs of compounds:

  1. Propanal and propanone
  2. Acetophenone and benzophenone
  3. Pentan-2-one and pentan-3-one
  4. Benzaldehyde and acetophenone
  5. Ethanal and propanal

Answer :

1. Propanal (CH3CH2CHO) and Propanone (CH3COCH3)

2. Acetophenone (C6H5COCH3) and Benzophenone (C6H5COC6H5)

3. Pentan-2-one (CH3COCH2CH2CH3) and Pentan-3-one (CH3CH2COCH2CH3)

4. Benzaldehyde and acetophenone

Ethanal (CH3CHO) and propanal (CH3CH2CHO)

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