Amongst the given set of reactants, the most appropriate for 2º amine is-

1º R-NH2  +  RCHO followed by H₂/ Pt

The correct answer is option 3. \(1^o R-NH_2  +  RCHO\) followed by \(H_2/ Pt\).

Let us break down the options for synthesizing a secondary amine (2º amine) and explain why the third option is the best choice:

1. \(2^o R-Br + NH_3\)

This involves a secondary alkyl halide \((2^o R-Br)\) reacting with ammonia \((NH_3)\). The primary product of this reaction would be a mixture of different amines. Ammonia can react to give primary, secondary, and tertiary amines, and even quaternary ammonium salts. This process is not selective and does not favor the formation of secondary amines.

2. \(2^o R-Br + NaCN\) followed by \(H_2/ Pt\)

A secondary alkyl halide \((2^o R-Br)\) reacts with sodium cyanide \((NaCN)\) to form a nitrile \((R-CN)\), which is then reduced by hydrogen \((H_2)\) in the presence of platinum \((Pt)\) to form an amine. The product of this sequence would be a primary amine \((R-CH_2NH_2)\). The nitrile group \((R-CN)\) gets converted to a primary amine during the reduction step. This method does not provide a secondary amine.

3. \(1^o R-NH_2 + RCHO\) followed by \(H_2/ Pt\)

A primary amine \((1^o R-NH_2)\) reacts with an aldehyde \((RCHO)\) to form an imine \((R-CH=NH-R')\), which is then reduced by hydrogen \((H_2)\) in the presence of a platinum catalyst \((Pt)\) to form a secondary amine \((2^o R-NH-R')\). This method specifically forms a secondary amine. The initial imine formation is a key step, and the subsequent hydrogenation step selectively reduces the imine to a secondary amine without over-reduction.

4. \(1^o R-Br \)(2 mol) + potassium phthalimide followed by \(H_3O^+ / heat\)

This is known as the Gabriel synthesis, where a primary alkyl halide \((1^o R-Br)\) reacts with potassium phthalimide to form an N-alkylphthalimide intermediate. Upon hydrolysis \((H_3O^+/heat)\), this intermediate gives a primary amine \((1^o R-NH_2)\). This method is used for synthesizing primary amines, not secondary amines.

Conclusion: \(1^o R-NH_2  +  RCHO\) followed by \(H_2/ Pt\) is the most appropriate method for synthesizing a secondary amine. The reaction sequence involves forming an imine intermediate, which is then selectively reduced to a secondary amine. This method is well-suited for producing secondary amines with high specificity and yield