Question

Sodium triacetoxyborohydride, $\mathbf{NaBH}{\left(\mathbf{OAc}\right)}_{\mathbf{3}}$, is a mild reducing agent that reduces aldehydes more quickly than ketones. It can be used to reduce aldehydes in the presence of ketones, such as in the following reaction:

(a) Draw a complete Lewis structure for sodium triacetoxyborohydride.

(b) Propose a mechanism for the reduction of an aldehyde by sodium triacetoxyborohydride.

Short answer

(a) The complete Lewis structure of sodium triacetoxyborohydride is shown below.

(b) The mechanism for the reduction of an aldehyde by sodium triacetoxyborohydride is shown below.

Mechanism of the Reaction

Step-by-step solution

Introduction

Lewis structures, also known as Lewis dot diagrams, are defined as structural representations of molecules. Dots are used to indicate electron positions around an atom, and lines (or pairs of dots) indicate covalent bonds between atoms. Each dot in a Lewis diagram represents a valence electron and one bond is represented as two electrons.

Lewis structure of sodium triacetoxyborohydride, NaBH(OAc)3

(a) The Lewis structure of sodium triacetoxyborohydride, NaBH(OAc)₃is shown below:

Lewis structure of $\text{NaBH}{\left(\text{OAC}\right)}_{\text{3}}$

Mechanism for the reduction of aldehyde with NaBH(OAc)3

Sodium triacetoxyborohydride is a mild reducing agent. Sodium triacetoxyborohydride reduces aldehydes to alcohols but ketones remain unreactive in the presence of sodium triacetoxyborohydride.

Hydride reduction of an aldehyde is an example of nucleophilic addition where hydride ion$\left({\text{H}}^{-}\right)$serves as a nucleophile.

The given reaction is shown below.

In the above reaction, the attack of hydride to $3\text{- oxo - butanal}$takes place to form an alkoxide followed by protonation with a solvent ${\text{(CH}}_{\text{3}}\text{COOH)}$to give the desired product $(3-\text{oxo}-\text{butanol)}$. The ketone remains unreactive while the aldehyde is reduced to alcohol in the presence of sodium triacetoxyborohydride. The mechanism part of the reaction is written in line-angle structure for better visualisation.