NEET 2020 Chemistry Question 133 Solution

Ozonolysis of alkenes involves the addition of an ozone molecule to the carbon-carbon double bond to form an ozonide intermediate. Subsequent cleavage of the ozonide with zinc dust and water ($$\text{Zn} + \text{H}_2\text{O}$$) cleaves the double bond completely, converting each doubly bonded carbon into a carbonyl ($$\text{C}=\text{O}$$) group: $$\text{R}_1\text{R}_2\text{C}=\text{CR}_3\text{R}_4 \xrightarrow{\text{1. O}_3, \text{ 2. Zn/H}_2\text{O}} \text{R}_1\text{R}_2\text{C}=\text{O} + \text{O}=\text{CR}_3\text{R}_4$$

Methanal (formaldehyde, $$\text{HCHO}$$) has the structure $$\text{H}_2\text{C}=\text{O}$$. Therefore, for methanal to be produced, one end of the alkene double bond must have been an unsubstituted methylene carbon ($$=\text{CH}_2$$), which only occurs in terminal alkenes.

- Structure (1): 1-methylcyclohexene. The double bond is inside the ring (endocyclic). Ozonolysis breaks the ring to yield 6-oxoheptanal (a single dicarbonyl compound), with no formaldehyde. - Structure (2): Cyclohexene with internal methyl substituent, gives a dicarbonyl upon ring opening. - Structure (3): Methylenecyclohexane (cyclohexane ring with an exocyclic $$=\text{CH}_2$$ group). Ozonolysis cleaves the exocyclic double bond, yielding Cyclohexanone and Methanal ($$\text{HCHO}$$). - Structure (4): Internal endocyclic double bond.

Option (3) is uniquely capable of generating methanal upon ozonolysis.