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Which Alkene Gives Acetone Only on Ozonolysis?

Ozonolysis is a powerful oxidation reaction in organic chemistry, particularly in the study of alkenes. One intriguing question that often arises is: which alkene gives acetone only on ozonolysis? This article aims to explore the reaction in detail, providing clarity on which specific alkene undergoes ozonolysis to yield acetone as the sole product.

Understanding Ozonolysis

Ozonolysis is the process of cleaving the carbon-carbon double bond of an alkene using ozone (O₃). This reaction is typically carried out in two steps:

1. The alkene reacts with ozone to form an ozonide intermediate.
2. The ozonide is then reduced, often using a reducing agent like zinc in acetic acid or dimethyl sulfide, resulting in the cleavage of the double bond and the formation of carbonyl compounds (aldehydes or ketones).

The outcome of this reaction depends largely on the structure of the alkene. Specifically, the nature of the substituents attached to the double bond will determine the products formed after ozonolysis.

Acetone as a Product of Ozonolysis

Acetone, a simple ketone, is formed from alkenes that are symmetrically substituted. To produce acetone exclusively, the alkene must be structured in a way that both sides of the double bond yield identical fragments after the bond is cleaved. Thus, the question arises: which alkene gives acetone only on ozonolysis?

The Ideal Alkene: 2,3-Dimethyl-2-butene

The answer to this question lies in the structure of 2,3-dimethyl-2-butene, also known as tetramethylethylene. This alkene is composed of a carbon-carbon double bond flanked by two methyl groups on each side:

       CH₃
        |
   CH₃—C=C—CH₃
        |
       CH₃

When 2,3-dimethyl-2-butene undergoes ozonolysis, the double bond is cleaved, resulting in two identical fragments, each of which is a molecule of acetone (CH₃COCH₃). Because of its symmetry, 2,3-dimethyl-2-butene is the alkene that gives acetone only on ozonolysis. No other side products are formed, making this reaction a clean and predictable process.

Reaction Mechanism for Acetone Formation

Let's delve deeper into the mechanism to understand why 2,3-dimethyl-2-butene gives only acetone upon ozonolysis. When ozone reacts with this alkene, the first step involves the formation of a five-membered cyclic ozonide. Upon reduction, this ozonide splits into two molecules of acetone. The symmetry of the molecule ensures that each fragment after the cleavage is identical, hence producing two acetone molecules as the sole products.

This reaction can be summarized as follows:

1. Ozone adds across the double bond to form the ozonide.
2. The ozonide is reduced, cleaving the C=C bond.
3. The result is the formation of two acetone molecules.

Why Symmetry Matters

The symmetry of 2,3-dimethyl-2-butene is critical for the exclusive production of acetone. If the alkene were asymmetrically substituted, ozonolysis would yield a mixture of products, possibly including aldehydes and different ketones. For example, an unsymmetrical alkene like propene would produce acetaldehyde and formaldehyde, rather than a single product like acetone.

Therefore, when considering which alkene gives acetone only on ozonolysis, it is crucial to choose a symmetrical alkene, like 2,3-dimethyl-2-butene, to ensure that the reaction yields a single type of product.

Conclusion

To summarize, the answer to the question which alkene gives acetone only on ozonolysis is 2,3-dimethyl-2-butene. This symmetrical alkene undergoes ozonolysis to produce two molecules of acetone with no other byproducts. The key to this reaction lies in the symmetry of the molecule, which ensures that the cleavage of the carbon-carbon double bond results in identical carbonyl fragments, both of which are acetone. Understanding this reaction provides valuable insight into the selective production of specific compounds through ozonolysis in organic synthesis.