Chapter 18, Problem 18.28P

Interpretation Introduction

Interpretation:

The product of the reaction is to be predicted in which $\text{Phenylethanal}$ is added slowly to a basic solution of $\text{Dimethylpropanal}$.

Concept introduction:

The reaction is a crossed aldol reaction. The base will deprotonate one aldehyde, and the resulting enolate anion will attack another aldehyde. As we have two aldehydes, base deprotonates the aldehyde with $\text{α-H}$. $\text{Dimethylpropanal}$ has no $\text{α-}$ hydrogens, so it cannot form enolate anion. $\text{Phenylethanal}$ has $\text{α-}$ hydrogens, so it forms the enolate nucleophile. Because $\text{Phenylethanal}$ is added slowly, there is never a substantial concentration of it in its uncharged form, so the major reaction occurs between $\text{Phenylethanal}$ enolate anion and $\text{Dimethylpropanal}$. Enolate anion undergoes nucleophilic addition with $\text{Dimethylpropanal}$ and forms an aldol product. If an aldehyde with $\text{α-}$ hydrogens is added slowly to a basic solution of an aldehyde without $\text{α-}$ hydrogens, primarily one aldol product is formed. Because the reaction is heated, dehydration leads to the condensation product.