Q47E - Organic Chemistry (Mcmurry)

Question

In light of your answer to Problem 19-46, propose a mechanism for the formation of 3, 5-dimethylisoxazole from hydroxylamine and 2,4-pentanedione

Step-by-Step Solution

Answer

Protonation of Alcohol

Attack of hydroxylamine

Removal of water

Cyclization reaction

Removal of Water

Formation of-3,5-Oxazole

Step-by-Step Solution Step 1: Reaction of hydrazine with the carbonyl carbon

It is a nucleophilic addition reaction of the carbonyl compound. The reaction is completed in six different steps.

Carbonyl oxygen gets protonated in an acidic medium. Lone pair of hydroxylamine attacks one of the carbonyl carbon as both are equivalent. In the next step, oxygen of the hydroxyl group abstracts the acidic hydrogen of hydroxylamine and ${}^{\text{+}}{\text{OH}}_{\text{2}}$is forms further lone pair of the nitrogen gets delocalized and the water molecule is lost.

Attack of Nucleophile

Protonation

Removal of Water

Step 2: Intramolecular reaction in the mechanism

In step, no 4 lone pair of oxygen attacks on the carbonyl carbon and gives a cyclic ring structure. In the next step, the water molecule is lost and further positive charge on the oxygen molecule is neutralized.

Cyclization reactions

Removal of Water

Formation of Oxazole