Question

The ozonolysis product(s) of the following reaction is(are) $$ \begin{aligned} &\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH} \frac{\text { (i) } \mathrm{O}_{3}}{\text { (ii) } \mathrm{H}_{2} \mathrm{O}} \text { Product }(\mathrm{s}) \\ &\text { (a) } \mathrm{CH}_{3} \mathrm{COCH}_{3} \\ &\text { (b) } \mathrm{CH}_{3} \mathrm{COCH}_{3}+\mathrm{HCHO} \end{aligned} $$ (c) \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{HCOOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{HCOOH}\)

Short answer

Two molecules of acetaldehyde (CH3CHO), also known as ethanal, are formed as the ozonolysis products. Therefore, none of the options (a), (b), (c), or (d) provided are correct.

Step-by-step solution

Understand the ozonolysis mechanism

Ozonolysis of alkenes or alkynes involves cleaving the double or triple bond and forming two carbonyl-containing fragments. In the presence of ozone (O3) followed by water (H2O), the reaction typically splits the carbon-carbon double or triple bonds, and attaches an oxygen to each carbon, giving rise to aldehydes, ketones, or carboxylic acids depending on the substituents attached to the alkene/alkyne.

Apply ozonolysis to the given compound

For the given alkyne, CH3CH2-C≡CH, ozonolysis will cleave the triple bond between the second and third carbon atoms. An oxygen atom is added to each carbon of the former triple bond, generating two separate compounds.

Identify the ozonolysis products

The first and second carbon atoms will form acetaldehyde (CH3CHO) because the first carbon was originally attached to two hydrogens. The third carbon atom, originally part of the triple bond, will also form acetaldehyde, as it was only bonded to an additional hydrogen. Therefore, we expect two molecules of acetaldehyde as the ozonolysis products.

Match the products to the answer options

The product acetaldehyde (CH3CHO) is also known as ethanal. Two molecules of acetaldehyde are formed. There is no further reaction or rearrangement that would lead to the other listed compounds. Therefore, option (b), which suggests the formation of CH3COCH3 (acetone) and HCHO (formaldehyde), is incorrect. Similarly, (c) and (d) list carboxylic acids which are not formed under these reaction conditions. (a) lists only acetone, which is incorrect because we expect two molecules of acetaldehyde, not acetone.

Key concepts

Alkyne Ozonolysis

Ozonolysis is a crucial reaction in organic chemistry for cleaving carbon-carbon multiple bonds, specifically alkenes and alkynes, to form carbonyl-containing compounds. When it comes to alkyne ozonolysis, the reaction takes on a distinctive pathway due to the triple bond's nature found in alkynes. In the provided exercise, the alkyne CH_3CH_2-C≡CH undergoes ozonolysis, a process that breaks down the triple bond and attaches oxygen atoms to the formerly triple-bonded carbons, leading to the formation of two molecules of acetaldehyde (CH_3CHO).

Understanding the mechanism is pivotal to predicting the outcome properly. Ozonolysis of alkynes generally ends with the formation of two carbonyl compounds, often aldehydes or a combination of an aldehyde and ketone, depending on the substituents present at the triple bond. The specific conditions used, such as the reductive workup with water in step two, avoid further oxidation of the aldehyde products which could otherwise transform them into carboxylic acids.

Organic Reaction Mechanisms

Organic reaction mechanisms are step-by-step descriptions of the molecular events that occur during a chemical reaction. A thorough understanding of these mechanisms not only allows chemists to predict the outcomes of chemical reactions, like ozonolysis, but also to explain why certain products are formed. In the ozonolysis mechanism, ozone (O3) acts as the reactant that attacks the alkyne, creating an unstable intermediate known as an ozonide, which eventually decomposes into carbonyl compounds upon reductive workup.

The process includes multiple steps, such as the initial cycloaddition to form the molozonide, followed by rearrangement into the more stable ozonide, and finally, cleavage upon reaction with water. The beauty of learning these steps lies in the predictability they afford, allowing for a clear identification of products given the starting materials and reaction conditions. Thus, a solid grasp of reaction mechanisms is an invaluable tool for students tackling organic synthesis problems.

Carbonyl Compounds Formation

The formation of carbonyl compounds is a common result of ozonolysis. Carbonyl compounds, characterized by a carbon atom double-bonded to an oxygen atom (C=O), include a wide variety of functional groups such as aldehydes, ketones, and carboxylic acids. In the context of alkyne ozonolysis, the reaction specifically yields aldehydes or ketones, depending on the substitution pattern of the alkyne.

The exercise at hand illustrates that two molecules of acetaldehyde are generated when the terminal alkyne is split by ozonolysis. The location of the triple bond and the presence of hydrogen atoms attached to the carbons in the triple bond are crucial for predicting the type of carbonyl compound formed. If the ozonolysis were conducted on an internal alkyne, one could expect ketones as a part of the product mixture. Understanding the nuances of how carbonyl compounds form during these reactions is essential in organic chemistry, as such compounds are versatile intermediates in many synthetic pathways.