Solved Examples on Aldehydes, Ketons & Carboxylic Acids:

Question 1: An aromatic compound (A) C₈H_9­Br reacts with CH₂(COOC₂H₅)₂ in the presence of C₂H₅ONa to give B. Compound B on refluxing with dilute H₂SO₄ gives (C) which on vigorous oxidation gives (D). The compound (D) is a dibasic acid but on heating does not give an anhydride. It forms a mononitro derivative (E) in which all the substitutents are equidistant from one another. Give structures of (A) to (E) with proper reasoning.

Solution:

Compound A is  

The reactions involved are: 

The compound (E), a mononitro derivative in which each group is at same distance in benzene nucleus means that it is symmetrical tri substituted (1,3,5) product and thus, two groups in (A), (B), (C) and (D) are at meta positions.

Question: 2. A compound A reduces Fehling’s solution and gives positive silver mirror test. On warming with dilute alkali, followed by dehydration it gives a product B which also responds to both the above tests. In addition, it decolourises the colour of bromine water. On treatment with hydrogen in the presence of nickel under pressure the compound B is converted to C which does not give any one of the above three test. Molar mass of the compound C is 74g mol^–1. Deduce the structures of A and B giving the chemical equations involved.

Solution: The compound A must be an aldehyde as it reduces Fehling’s solution and gives positive silver mirror test. The compound A also contains a-hydrogen atom as it undergoes aldol condensation, the dehydration of which gives B. The latter contains unsaturation as bromine water is decolourised. Let the structure of A be RCH₂CHO. The reactions involved are

Since the molar mass of C is 74 g mol^–1, R in the compound C must be hydrogen atom.

Hence,The structures of A, B and C are.

A = CH₃CHO,

B = CH₃CH = CHCHO

C = CH₃CH₂CH₂CH₂OH

Question:3. An organic compound A on treatment with ethyl alcohol gives a carboxylic acid B and compound C. Hydrolysis of C under acidic conditions gives B and D. Oxidation of D with KMnO₄ also gives B. The compound B on heating with Ca(OH)₂ gives E (molecular formula C₃H₆O). E does not give Tollen’s test and does not reduce Fehling’s solution but forms a 2, 4-dinitrophenylhydrazone. Identify A, B, C, D and E.

Solution:The given reactions are as follows.

The compound E must be ketonic compound as it does not give Tollens test and does not reduce Fehling’s solution but forms a 2, 4-dinitrophenyl-hydrazone. Therefore, its structure would be CH₃COCH₃ (acetone).

Since E is obtained by heating B with Ca(OH)₂, the compound B must be CH₃COOH (acetic acid).

Since B is obtained by oxidation of D with KMnO₄, the compound D must be an alcohol with molecular formula CH₃CH₂OH (ethanol).

Since B and D are obtained by acid hydrolysis of C, the compound C must be an ester CH₃COOC₂H­₅ (ethyl acetate).

Since the compounds B (acetic acid) and C (ethyl acetate) are obtained by treating A with ethanol, the compound A must be an anhydride (CH₃CO)_­2O (acetic anhydride). The given reactions are