Question

Identify (Y) in the following reaction: \(\mathrm{CH} \equiv \mathrm{CH} \quad \stackrel{\mathrm{O}_{3}}{\longrightarrow} \quad(\mathrm{X}) \stackrel{\mathrm{Zn} / \mathrm{H}_{3} \mathrm{OH}}{\longrightarrow}(\mathrm{Y})\) (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{2} \mathrm{OH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

Short answer

(Y) is formaldehyde, corresponding to none of the options a, b, c, or d.

Step-by-step solution

Understand the Reaction Type

The reaction sequence given involves ozonolysis followed by reduction. Ozonolysis is a process where ozone (\(\mathrm{O}_{3}\)) cleaves carbon-carbon triple bonds, forming ozonides.

Apply Ozonolysis to \(\mathrm{CH} \equiv \mathrm{CH}\)

Ozonolysis of \(\mathrm{CH} \equiv \mathrm{CH}\) will break the triple bond to form an ozonide. The cleavage of this bond generally yields two carbonyl groups. In this case, the intermediate (X) will actually be an ozonide, which upon reductive work-up will yield two molecules of \(\mathrm{HCHO}\) or formaldehyde.

Perform Reduction of Ozonide to Identify (Y)

Upon treating the ozonide with \(\mathrm{Zn/H}_{3}\mathrm{OH}\), it is reduced to form formaldehyde molecules, \(\mathrm{HCHO}\). Since formaldehyde is the smallest aldehyde, it cannot condense further and represents the simplest breakdown product from the original terminal alkyne.

Identify the Product (Y)

Upon completion of the reduction, the product (Y) is \(\mathrm{HCHO}\), which corresponds to formaldehyde. This outcome is based on the expected result of the ozonolysis and reduction of acetylene.

Key concepts

Reduction Reaction

In chemistry, a reduction reaction is a type of process where a molecule gains electrons. During the reduction, the oxidation state of the molecule decreases. This is a fundamental concept as it often pairs with oxidation reactions in what is known as redox reactions.
When we look at the context of the given exercise, the reduction occurs after ozonolysis. Ozonolysis initially creates ozonides, which are unstable compounds. By applying a reducing agent like zinc (Zn) in the presence of an acid or alcohol, such as

• Zn/H₃OH (zinc in acidic or alcoholic media)

these ozonides get converted into stable smaller molecules, such as aldehydes or ketones. In our specific reaction with acetylene, ozonolysis creates an ozonide that is then reduced to form simple molecules of formaldehyde (HCHO).
This is a crucial step as it finalizes the breakdown of the initially strong and unstable triple-bonded structure into recognizable and stable end products.

Carbon-carbon triple bond

A carbon-carbon triple bond is a type of covalent bond found between two carbon atoms, often represented as

• R–C≡C–R'

This triple bond forms the basis for alkynes, which are hydrocarbons featuring this bonding type. Alkynes are characterized by their linear structure and high reactivity due to the bond's energy-rich nature.
In the original exercise, we start with acetylene, which is the simplest form of an alkyne:

• CH≡CH

The presence of this triple bond makes acetylene highly reactive and suitable for reactions, such as ozonolysis. The ozonolysis process specifically targets the carbon-carbon triple bond, leading to its cleavage.
Ozonolysis effectively transforms these energy-rich bonds into more stable forms like aldehydes or ketones, facilitating further reactions such as reduction.

Formaldehyde Formation

Formaldehyde is a simple aldehyde with the chemical formula

• HCHO

It is notable for being the smallest aldehyde and has widespread applications both in industry and research. The process of its formation in the original reaction is insightful because it demonstrates how complex organic structures can be broken down into simple, practical compounds.
During the exercise, ozonolysis is applied to acetylene, which is subsequently treated with a reducing agent to yield formaldehyde molecules. The breakdown can be seen as:

• Cleavage of carbon-carbon triple bonds into ozonides
• Reduction of these ozonides into formaldehyde

The formation of formaldehyde from acetylene's triple bond via ozonolysis and reduction highlights the efficiency of these reactions. They showcase how transformations of complex molecular structures lead to the synthesis of key chemical building blocks.