Question

Propose a mechanism to account for the formation of 3,5-dimethyl pyrazole from hydrazine and 2,4-pentanedione. Look carefully to see what has happened to each carbonyl carbon in going from starting material to product.

Short answer

It is a heterocyclic compound containing two nitrogen atoms in its ring structure.

Pyrazole is an aromatic compound as it follows the Huckel’s rule of aromaticity. 3, 5-dimethyl pyrazole is a member of the pyrazole family.

Step-by-step solution

Pyrazole

It is a heterocyclic compound containing two nitrogen atoms in its ring structure.

Pyrazole is an aromatic compound as it follows the Huckel’s rule of aromaticity. 3, 5-dimethyl pyrazole is a member of the pyrazole family.

Mechanism of the reaction

It is the nucleophilic addition reaction of the carbonyl compound.

Formation of 3,5-dimethylpyrazole

In this reaction, first of all, hydrazine gets protonated first then lone pair of carbonyl oxygen abstracts the hydrogen of protonated hydrazine. In the next step, the lone pair of nitrogen attacks the carbon of carbonyl groups again the hydroxyl group abstracts the hydrogen and in the next step, the water molecule is lost. Again the charge on nitrogen is neutralized.

Again, in the next step same second carbonyl group is protonated and further intramolecular rearrangements take place as shown in the mechanism.

Mechanism