Q.(a) Explain the reducing action of formic acid with example. OR

(a) Formic acid behaves as a reducing agent (reducing Tollens', Fehling's and acidified $KMnO_4$) because it structurally contains an aldehyde-type $H-CO-$ group; (b) the carbylamine reaction is a specific test for primary amines, and the Gabriel phthalimide synthesis is a method to prepare pure primary amines.

(a) Reducing action of formic acid:

Formic acid, $HCOOH$, has the structure $H-COOH$, in which a hydrogen atom is directly attached to the carbonyl carbon - structurally resembling an aldehyde group ($-CHO$) fused with a carboxylic acid group. Because of this hidden aldehydic character, formic acid (unlike other carboxylic acids such as acetic acid) shows reducing properties, being itself oxidised to carbon dioxide and water:

1. Reduces Tollens' reagent (ammoniacal silver nitrate) to metallic silver, depositing a bright silver mirror on the test-tube walls:

   $HCOOH + 2[Ag(NH_3)_2]^+ + 2OH^- \rightarrow 2Ag\downarrow + CO_2 + 2H_2O + 4NH_3$

2. Reduces Fehling's solution (alkaline cupric tartrate complex) to a red/brick-red precipitate of cuprous oxide:

   $HCOOH + 2CuO \rightarrow Cu_2O\downarrow(\text{red}) + H_2O + CO_2$

3. Decolorises acidified potassium permanganate solution, being oxidised to $CO_2$ while $Mn^{7+}$ is reduced to $Mn^{2+}$:

   $5HCOOH + 2KMnO_4 + 3H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 8H_2O + 5CO_2$

4. It also reduces mercuric chloride solution to mercurous chloride (and, with excess formic acid, further to metallic mercury).

These reactions are used as chemical tests to distinguish formic acid from other carboxylic acids (e.g. acetic acid), which lack this aldehydic hydrogen and do not show such reducing behaviour.

(b)(i) Carbylamine reaction:

When a primary amine (aliphatic or aromatic, $R-NH_2$ or $Ar-NH_2$) is heated with chloroform and alcoholic potassium hydroxide, an offensive-smelling isocyanide (carbylamine) is formed:

$R-NH_2 + CHCl_3 + 3KOH(alc.) \xrightarrow{\Delta} R-NC + 3KCl + 3H_2O$

The mechanism involves alcoholic $KOH$ generating dichlorocarbene ($:CCl_2$) from chloroform, which is attacked by the amine's nitrogen lone pair, followed by loss of two more $HCl$ equivalents to give the isocyanide. Since secondary and tertiary amines do not have an $N-H$ available to react in this way, they give no such reaction - making this a specific, distinguishing test for primary amines.

(b)(ii) Gabriel phthalimide synthesis:

This is a method used to prepare pure primary amines (avoiding the mixture of primary, secondary and tertiary amines that direct ammonolysis of alkyl halides gives). The steps are:

1. Phthalimide (which has an acidic $N-H$, due to the two flanking carbonyl groups) is treated with alcoholic $KOH$ to form potassium phthalimide (its potassium salt). …