Chapter 18, Problem 18.8P

Interpretation Introduction

Interpretation:

A complete and detailed mechanism for the reactions in Equation $18-15$ is to be drawn.

Concept introduction:

If an aldehyde or ketone is treated with a large excess of an alcohol under acidic conditions, then the hemiacetal produced from nucleophilic addition reacts further to form an acetal, in which two alkoxy ($\text{RO}$) groups are bonded to the same carbon. The complete mechanism for this acetal formation reaction is shown in Equation $18-13$:

The equation $18-15$ is:

The reversibility of acetal formation enables us to use acetals as protecting groups for ketones and aldehydes. Ketones and aldehydes are susceptible to an attack by nucleophiles (at the carbonyl carbon) and bases (at the α carbon), but acetals are inert to both strong nucleophiles and strong bases. One popular alcohol used to protect ketones and aldehydes is $\text{1},\text{2}-\text{ethanediol}$ (ethylene glycol), as shown in Equation $18-15$. $\text{1},\text{2}-\text{ethanediol}$ has two $\text{OH}$ functional groups per molecule, so only one equivalent of the diol is needed. It also forms a five membered ring, which helps favor acetal formation.