Chapter 17, Problem 17.8P

Interpretation Introduction

Interpretation:

A complete and detailed mechanism for the reactions in Equations 17-12 and 17-13 is to be drawn.

Concept introduction:

Sodium borohydride (${\text{NaBH}}_4$) and lithium aluminum hydride (${\text{LiAlH}}_{\text{4}}$) are reducing agents. These are two common hydride sources in organic chemistry. In both cases, $\text{H}$ has a partial negative charge. The hydride anion ${\text{H}}^{\text{-}}$ is a strong nucleophile and a strong base. Nucleophilic addition of ${\text{NaBH}}_4$ takes place in solvents such as water or alcohol. Nucleophilic addition involving ${\text{LiAlH}}_{\text{4}}$, on the other hand, cannot take place in water or alcohol because LiAlH4 is too reactive.

An imine is reduced with ${\text{NaBH}}_4$ or ${\text{LiAlH}}_{\text{4}}$ to form an amine. $\text{Methanol}$, a weakly acidic solvent, is suitable for the ${\text{NaBH}}_4$ reduction, but the ${\text{LiAlH}}_{\text{4}}$ reduction must take place in a solvent that has no acidic protons such as ether. The hydride ion from ${\text{LiAlH}}_{\text{4}}$ deprotonates the weakly acidic proton from water and alcohols very quickly, producing hydrogen gas. The reaction is so exothermic that the hydrogen can ignite, causing an explosion.