Question:

Give simple chemical tests to distinguish between the following pairs of compounds.
(i)Propanal and Propanone 
(ii)Acetophenone and Benzophenone
(iii)Phenol and Benzoic acid
(iv)Benzoic acid and Ethyl benzoate
(v)Pentan-2-one and Pentan-3-one 
(vi)Benzaldehyde and Acetophenone
(vii)Ethanal and Propanal
 

Updated On: Sep 28, 2023
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Solution and Explanation

(i) Propanal and propanone can be distinguished by the following tests.

(a)Tollen's test 

Propanal is an aldehyde.Thus,it reduces Tollen's reagent.But,propanone being a ketone does not reduce Tollen's reagent.


(b) Fehling's test 

Aldehydes respond to Fehling's test,but ketones do not.

Propanal being an aldehyde reduces Fehling's solution to a red-brown precipitate of \(Cu_2O,\)but propanone being a ketone does not. 


(c)Iodoform test:

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to iodoform test.They are oxidized by sodium hypoiodite\((NaOI)\) to give iodoforms.Propanone being a methyl ketone responds to this test,but propanal does not.


(ii)Acetophenone and Benzophenone can be distinguished using the iodoform test.

Iodoform test:

Methyl ketones are oxidized by sodium hypoiodite to give yellow ppt.of iodoform. Acetophenone being a methyl ketone responds to this test,but benzophenone does not.


(iii)Phenol and benzoic acid can be distinguished by ferric chloride test.
Ferric chloride test:

Phenol reacts with neutral \(FeCl_3\) to form an iron-phenol complex giving violet colouration.

But benzoic acid reacts with neutral \(FeCl_3\) to give a buff coloured ppt.of ferric benzoate.


(iv)Benzoic acid and Ethyl benzoate can be distinguished by sodium bicarbonate test.


Sodium bicarbonate test:

Acids react with \(NaHCO_3\) to produce brisk effervescence due to the evolution of \(CO_2\) gas.Benzoic acid being an acid responds to this test,but ethylbenzoate does not.


(v)Pentan-2-one and pentan-3-one can be distinguished by iodoform test.
Iodoform test:

Pentan-2-one is a methyl ketone.Thus,it responds to this test.But pentan-3-one not being a methyl ketone does not respond to this test.


(vi)Benzaldehyde and acetophenone can be distinguished by the following tests.


(a)Tollen's Test:

Aldehydes respond to Tollen's test.Benzaldehyde being an aldehyde reduces Tollen's reagent to give a red-brown precipitate of \(Cu_2O\),but acetophenone being a ketone does not.

(b)Iodoform test

 Acetophenone being a methyl ketone undergoes oxidation by sodium hypoiodite(NaOI)to give a yellow ppt.of iodoform.But benzaldehyde does not respond to this test.


(vii)Ethanal and propanal can be distinguished by iodoform test.

Iodoform test 

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom responds to the iodoform test.Ethanal having one methyl group linked to the carbonyl carbon atom responds to this test.But propanal does not have a methyl group linked to the carbonyl carbon atom and thus,it does not respond to this state.

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Concepts Used:

Aldehydes, Ketones, and Carboxylic Acids

Aldehydes, Ketones, and Carboxylic Acids are carbonyl compounds that contain a carbon-oxygen double bond. These organic compounds are very important in the field of organic chemistry and also have many industrial applications.

Aldehydes:

Aldehydes are organic compounds that have the functional group -CHO.

Preparation of Aldehydes

Acid chlorides are reduced to aldehydes with hydrogen in the presence of palladium catalyst spread on barium sulfate.

Ketones:

Ketones are organic compounds that have the functional group C=O and the structure R-(C=O)-R’.

Preparation of Ketones

Acid chlorides on reaction with dialkyl cadmium produce ketones. Dialkyl cadmium themselves are prepared from Grignard reagents.

Carboxylic Acid:

Carboxylic acids are organic compounds that contain a (C=O)OH group attached to an R group (where R refers to the remaining part of the molecule).

Preparation of Carboxylic Acids

Primary alcohols are readily oxidized to carboxylic acids with common oxidizing agents such as potassium permanganate in neutral acidic or alkaline media or by potassium dichromate and chromium trioxide in acidic media.