Chapter 20, Problem 20.41P

Interpretation Introduction

Interpretation:

Barbituric acid can be prepared from malonic ester and urea as shown. A complete, detailed mechanism for this reaction is to be provided.

Concept introduction:

In the basic solution, no strong acid should appear in the mechanism, hence, the proton transfer step occurs before the nucleophilic addition-elimination reaction. The nucleophilic addition-elimination mechanism has two steps. Step 1 is nucleophilic addition, in which a nucleophile attacks the electron poor carbonyl carbon. This forces the pair of electrons from the initial $\text{C=O}$ bond onto the O atom which generates a negative charge on O. The product of that step is a tetrahedral intermediate, which, in Step 2, undergoes nucleophile elimination. A lone pair of electrons on the negatively charged O atom is used to regenerate the $\text{C=O}$ double bond and the leaving group departs.