Question

In Ozonolysis of benzene, how many molecules of ozone are added?

Answer 1

Hint: Ozonolysis Reaction: It is an organic reaction where the unsaturated bonds of alkenes, alkynes, azo compounds etc. cleaned with Ozone. Alkenes and Alkynes form organic compounds with multiple carbon – carbon double bonds get replaced by carbonyl groups. The ozone molecule firstly attaches to the double bond and then further reaction takes place. Hence, the number of ozone molecules used will depend upon the number of double bonds the compound has.

Complete step by step answer:
During ozonolysis reaction the benzene adds three molecules of ozone and forms benzene triozonide. This reaction takes place in 2 steps:
Benzene on treatment with 3 molecules of ozone gives benzene triozonide. Three \[{{O}_{3}}\] molecules are added over \[{{C}_{6}}{{H}_{6}}\] in this reaction.
\[{{C}_{6}}{{H}_{6}}+3{{O}_{3}}\to {{C}_{6}}{{H}_{6}}{{O}_{9}}\]

Benzene triozonide is formed in step 1 which on decomposition with water and zinc metal gives three molecules of glyoxal. The presence of zinc metal aids to the reductive ozonolysis and thus the final product of reaction is an aldehyde not a carboxylic acid as it is the case with oxidative ozonolysis.
\[{{C}_{6}}{{H}_{6}}{{O}_{9}}+3{{H}_{2}}O\to 3CHO-CHO+3{{H}_{2}}O\]

Benzene on ozonolysis gives glyoxal. This process thus needs 3 molecules of ozone.

Note: Ozonolysis reaction is an oxidation reaction. Here, oxygen atoms are added to the substrate and hydrogen atoms are removed. It is a useful reaction to convert unsaturated hydrocarbons to aldehydes and ketones. When there is no metal added with ozonolysis reagent it gives oxidative ozonolysis which further oxidizes the aldehydes to carboxylic acids. When metals are used reaction stops at formation of aldehyde and this is known as reductive ozonolysis.