Question

An alkene on ozonolysis gives formaldehyde and. The alkene is
A. Ethene
B. Propene
C. 1-butene
D. 2-butene

Answer 1

Hint: Ozonolysis of alkene is the oxidation of alkenes with the help of ozone which gives aldehydes, ketones, or carboxylic acid as the products. This is a redox reaction in organic chemistry one or more than one product can be formed due to this reaction.

Complete Step by Step Answer:
Ozonolysis of alkene goes through the process of oxidative cleavage. Ozonide is formed when ozone attacks the alkene and ozone breaks the π-bond as well as the σ-bond between the carbon and to break the sigma(σ) bond between the carbons high amount of energy and a very reactive species is required and Ozone is a very strong oxidising agent. Ozonolysis of an alkene with no alkyl substituent will produce aldehyde. When one carbon of alkene has both alkyl substituents it will produce ketone and aldehyde and ketones, and further oxidation will produce carboxylic acid.

The give compounds on ozonolysis will produce the following products:
$CH_2=CH_2\ +\ O_3 \longrightarrow \ 2HCHO$
$CH_3CH=CH_2\ +\ O_3 \longrightarrow \ CH_3CHO\ +\ HCHO$
$CH_3CH_2CH=CH_2\ +\ O_3 \longrightarrow \ CH_3CH_2CHO\ +\ HCHO$
$CH_3CH=CHCH_3+ \ O_3 \longrightarrow \ 2CH_3CHO$

As we can see, propene on ozonolysis produces formaldehyde and acetaldehyde as the product.
The reaction of ozonolysis of alkene proceeds through the following mechanism:

Alkene reacts with the ozone and forms ozonide then the ozonide is further hydrolyzed in presence of Zn to produce aldehydes, ketone, or carboxylic acid. The five-membered heterocyclic ring formed in the reaction is called ozonide.
Thus, Option (B) is correct

Note: Alkenes are also known as olefins. Ozonides are generally formed by alkenes and they are very reactive species; they decompose rapidly into aldehydes, ketone, and peroxide. During the hydrolysis of ozonide hydrogen peroxide (H₂O₂) is generated to reduce the H₂O₂. Zinc dust is used as a catalyst in the reaction.