MCQ Detailed View

Ozonolysis is a classical reaction of alkenes in which ozone (O₃) adds across the carbon–carbon double bond to give a primary ozonide (molozonide), which rapidly rearranges to a more stable ozonide. On hydrolysis or in the usual reductive workup, this ozonide cleaves to carbonyl fragments. Ethene (ethylene), CH₂=CH₂, is the simplest symmetrical alkene. Cleavage of its C=C bond yields two identical one-carbon carbonyl units. The only one-carbon carbonyl compound obtainable under these conditions is methanal, commonly called formaldehyde, with formula HCHO. Thus, hydrolysis of the ozonide derived from ethene furnishes formaldehyde.

The outcome follows the Criegee mechanism: ozone cycloadds to the double bond forming a molozonide; the molozonide fragments to a carbonyl oxide and an aldehyde intermediate; recombination produces the ozonide. Subsequent hydrolysis (often in the presence of a reducing agent such as Zn/H₂O or dimethyl sulfide, though the question states simply “on hydrolysis”) breaks the ozonide to the corresponding aldehydes or ketones. Because each alkene carbon in ethene carries one hydrogen, the fragments are both aldehydic and identical, giving two molecules of HCHO.

The distractors map to different reactions of ethene and highlight why ozonolysis is diagnostic for carbonyl formation rather than epoxidation or dihydroxylation. Ethylene oxide is formed by epoxidation of ethene, typically with a peracid such as mCPBA or by catalytic oxidation, not by ozonolysis. Ethylene glycol results from syn dihydroxylation of ethene using reagents like cold, dilute KMnO₄ or OsO₄ followed by hydrolysis. Ethyl alcohol (ethanol) arises from hydration of ethene (acid-catalyzed addition of water) or from catalytic hydrogenation of acetaldehyde, but not from cleavage of a double bond by ozone.