An aromatic compound ' A ' (molecular formula $\mathrm{C}_8 \mathrm{H}_8 \mathrm{O}$ ) gives positive $2,4-$ DNP test. It gives a yellow precipitate of compound 'B' on treatment with iodine and sodium hydroxide solution. Compound 'A' does not give Tollen's or Fehling's test. On drastic oxidation with potassium permanganate it forms a carboxylic acid 'C' (molecular formula $\mathrm{C}_7 \mathrm{H}_6 \mathrm{O}_2$ ), which is also formed alongwith the yellow compound in the above reaction. Identify $\mathrm{A}, \mathrm{B}$ and C and write all the reactions involved.
$$\begin{array}{ll} \text { Molecular formula }=\mathrm{C}_8 \mathrm{H}_8 \mathrm{O} \\ \text { Degree of unsaturation } & =\left(\mathrm{C}_n+1\right)-\frac{\mathrm{H}_n}{2} \\ & =(8+1)-\frac{8}{2}=9-4=5 \end{array}$$
Degree of unsaturation $>5$ i.e., it may contain benzene ring having degree of unsaturation equal to 4 and one degree of unsaturation must be carbonyl group.
Thus, possible structures are
According to question, compound ' $A$ ' does not respond to Tollen's or Fehling's test, So, it is a ketone not aldehyde. Therefore, structure I is correct. Complete reaction sequence is as follows
Write down functional isomers of a carbonyl compound with molecular formula $\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}$. Which isomer will react faster with HCN and why? Explain the mechanism of the reaction also. Will the reaction lead to the completion with the conversion of whole reactant into product at reaction conditions? If a strong acid is added to the reaction mixture what will be the effect on concentration of the product and why?
$$\begin{aligned} &\text { Functional isomers of } \mathrm{C}_3 \mathrm{H}_6 \mathrm{O} \text { containing carbonyl group are }\\ &\underset{\text { Propanal }}{\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHO}} \text { and } \underset{\text { Propanone }}{\mathrm{CH}_3 \mathrm{COCH}_3} \end{aligned}$$
(a) Propanal, $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHO}$ will react faster with HCN because there is less steric hindrance and electronic factors, which increases its electrophilicity.
(b) The reaction mechanism is as follow
The reaction does not lead to completion because it is a reversible reaction. Equilibrium is established.
(c) If a strong acid is added to the reaction mixture, the reaction is inhibited because production of $\overline{\mathrm{C}} \mathrm{N~ions}$ prevented.
When liquid ' $A$ ' is treated with a freshly prepared ammoniacal silver nitrate solution it gives bright silver mirror. The liquid forms a white crystalline solid on treatment with sodium hydrogen sulphite. Liquid ${ }^{\mathrm{B}}$ ' also forms a white crystalline solid with sodium hydrogen sulphite but it does not give test with ammoniacal silver nitrate. Which of the two liquids is aldehyde? Write the chemical equations of these reactions also.
Since the liquid A reduces ammoniacal silver nitrate, (Tollen's reagent), it 'A' is aldehyde.
