Question

An alkene on reductive ozonolysis gives two molecules of \(\mathrm{CH}_{2}(\mathrm{CHO})_{2}\). The alkene is: (a) 2, 4-hexadiene (b) 1,3 -cyclohexadiene (c) 1,4 -cyclohexadiene (d) 1 -methyl-1, 3-cyclopentadiene

Short answer

The alkene is 2,4-hexadiene (option a).

Step-by-step solution

Understanding Reductive Ozonolysis

Reductive ozonolysis of alkenes involves breaking the C=C double bonds and inserting oxygen. This process turns alkenes into aldehydes or ketones. Our task is to deduce the structure of the alkene which gives two molecules of \(\mathrm{CH}_2(\mathrm{CHO})_2\) upon reductive ozonolysis.

Understanding the Desired Product

The given product, \(\mathrm{CH}_2(\mathrm{CHO})_2\), is also known as glyoxal. It signifies that the alkene originally had a structure that underwent cleavage to form two glyoxal molecules. Glyoxal results from the oxidation of ethenic groups in a symmetrical structure.

Structure of Glyoxal

Glyoxal has the structure \(\text{OHC-CHO}\). For it to be formed from two identical parts, the starting alkene must have had symmetrical C=C centers. After breaking the C=C bonds, each part becomes a \(\text{CHO}\) group.

Finding the Alkene

For two molecules of glyoxal to be obtained, the alkene must have two identical ethenic links that can form glyoxal upon cleavage. Among the given options, only option (a) 2,4-hexadiene has a symmetrical structure when broken down, leading to the formation of two molecules \(\mathrm{CH}_2(\mathrm{CHO})_2\).

Verify with the Reaction

Draw the structure of 2,4-hexadiene (\(\mathrm{CH}_3-\mathrm{CH} = \mathrm{CH}-\mathrm{CH} = \mathrm{CH}-\mathrm{CH}_3\)). The ozonolysis cleaves the double bonds, forming glyoxal from each double bond. The structure of 2,4-hexadiene confirms that upon reductive ozonolysis, it indeed produces two glyoxal molecules.

Key concepts

Alkenes

Alkenes are a fundamental class of unsaturated hydrocarbons characterized by at least one carbon-carbon double bond (C=C). This double bond is crucial for the chemical reactivity of alkenes, as it is the site where most chemical reactions occur. Alkenes can be classified into different types based on the number of double bonds they contain and their structural arrangement:

• **Monounsaturated**: Contains only one C=C double bond, like ethene.
• **Dienes**: Has two C=C double bonds, such as 2,4-hexadiene.

Each alkene's physical and chemical properties are influenced by the presence and position of these double bonds.
Alkenes participate in various reactions like addition, polymerization, and oxidation. Among these, ozonolysis is a critical reaction for breaking down alkenes, where the double bond is cleaved, and oxygen is inserted, forming new compounds. This process is especially useful for structure elucidation in organic chemistry.

Glyoxal Formation

Glyoxal is a simple organic compound with the formula \( ext{OHC-CHO}\), consisting of two aldehyde groups attached to the same carbon skeleton. Reductive ozonolysis of an alkene can lead to the formation of glyoxal when the structure of the original molecule allows for it.
In the context of organic synthesis, an alkene undergoing reductive ozonolysis can be cleaved at its C=C double bonds. Oxygen is incorporated as part of this oxidative breakdown, converting each fragment into either aldehydes or ketones.
For example, when an alkene is symmetrical around its double bonds, such as in 2,4-hexadiene, the reaction leads to the formation of two identical molecules of glyoxal. Glyoxal's formation is dependent on the reaction conditions and the symmetry of the starting material. When both sides of the alkene are equivalent, the resulting aldehydes are also symmetric, thus producing two glyoxal molecules directly.

2,4-Hexadiene

2,4-Hexadiene is a specific type of diene, meaning it contains two double bonds within its carbon chain. Its structure can be represented as \(\mathrm{CH}_3-\mathrm{CH} = \mathrm{CH}-\mathrm{CH} = \mathrm{CH}-\mathrm{CH}_3\). This particular arrangement of double bonds in 2,4-hexadiene is what makes it suitable for the formation of glyoxal during reductive ozonolysis.
During reductive ozonolysis, 2,4-hexadiene's double bonds are cleaved by the addition of ozone. This splitting is accompanied by the incorporation of oxygen. The resulting reaction produces two molecules of \(\text{OHC-CHO}\) or glyoxal, due to the symmetry of the molecule that facilitates the formation of identical end products.
This reaction is illustrative of how specific structural features of alkenes determine the outcome of ozonolysis, providing a practical example of designing molecules for specific oxidative products in synthetic organic chemistry.