Mechanism of synthesis of barbituric acid from diethyl malonate and urea

Initial reaction
Diethyl malonate ($C_7H_ {12} O_4 $) and urea ($CH_4N_2O $) Start the reaction under appropriate conditions. Diethyl malonate has an active methylene, and its $α-H $is affected by two adjacent ester groups and has a certain acidity. Under the action of basic catalyst, the methylene $H $of diethyl malonate is taken away, forming carbonanion.

\ [C_7H_ {12} O_4 + B ^ -\ longrightarrow C_7H_ {11} O_4 ^ - + HB\]

The diethyl malonate carbon anion generated by nucleophilic addition
acts as a nucleophilic reagent to attack the carbonyl carbon attached to the nitrogen atom in urea. In urea molecules, carbonyl carbon has a certain positive electricity and is vulnerable to attack by nucleophilic reagents. After the nucleophilic addition reaction occurs, a new intermediate is formed.

\ [C_7H_ {11} O_4 ^ - + CH_4N_2O\ longrightarrow\ text {intermediate}\]

Molecular cyclization
The intermediate further undergoes molecular cyclization. The atoms in the molecule form a cyclic structure through the rearrangement and cyclization of chemical bonds. During this process, the basic skeleton of barbituric acid is gradually constructed after a series of electron transfer and chemical bond cleavage and formation.

Follow-up reaction and product generation
The intermediate after cyclization may also need to go through some reaction steps such as elimination and proton transfer, and finally generate barbituric acid ($C_4H_4N_2O_3 $). Throughout the reaction process, through ingenious chemical changes, diethyl malonate and urea atoms are recombined to construct barbituric acid molecules with specific structures and properties.

This is the general mechanism of the synthesis of barbituric acid from diethyl malonate and urea. The specific conditions and rates of each step of the reaction are affected by many factors such as reaction temperature, catalyst type and concentration.