Animated Solution for Chemistry - Organic Chemistry: The most appropriate reagent for conversion of ${\text{C}}_2{\text{H}}_5\text{CN}$ into ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{NH}}_2$ is

Let's embark on a journey to understand how we can transform a molecule. The problem asks us to convert ethyl cyanide, chemically known as propanenitrile (${\text{C}}_2{\text{H}}_5\text{CN}$), into propan-1-amine (${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{NH}}_2$).

If we look closely at the structures, the core of the transformation lies at the functional group. We are converting a carbon-nitrogen triple bond ($-\text{C}\equiv \text{N}$) into a carbon-nitrogen single bond ($-{\text{CH}}_2-{\text{NH}}_2$).

This is a classic reduction reaction. Specifically, we are adding four hydrogen atoms across the triple bond—two hydrogens attach to the carbon atom, and two hydrogens attach to the nitrogen atom.

To break a strong triple bond and force four hydrogen atoms onto the molecule, we need a chemical sledgehammer—a strong reducing agent. Let's evaluate the options provided in our chemical arsenal:

1. ${\text{NaBH}}_4$ (Sodium Borohydride): This is a relatively mild reducing agent. It is fantastic for reducing aldehydes and ketones to alcohols, but it simply lacks the thermodynamic punch to reduce a stubborn nitrile group.

2. ${\text{Na(CN)BH}}_3$ (Sodium Cyanoborohydride): This is even milder than ${\text{NaBH}}_4$. The electron-withdrawing cyano group stabilizes the boron-hydrogen bonds, making it less reactive. It is primarily used in reductive aminations, not for reducing nitriles.

3. ${\text{CaH}}_2$ (Calcium Hydride): While it contains hydride ions, it is primarily used as a desiccant (drying agent) to remove trace water from organic solvents. It is not a standard reagent for reducing organic functional groups.

This leaves us with ${\text{LiAlH}}_4$ (Lithium Aluminium Hydride). This reagent is the heavyweight champion of hydride donors.

Why is it so strong? The aluminium atom is larger and less electronegative than boron. This makes the $\text{Al}-\text{H}$ bond weaker and more polarized than the $\text{B}-\text{H}$ bond, allowing it to deliver hydride ions (${\text{H}}^{-}$) with immense nucleophilic force.

The hydride ion aggressively attacks the electrophilic carbon of the nitrile group, pushing the pi electrons onto the nitrogen. A sequence of these hydride attacks, followed by an aqueous workup to provide protons, completely reduces the nitrile to a primary amine.

Therefore, ${\text{LiAlH}}_4$ is the perfect and most appropriate reagent for this conversion.