Question

Which one of the following would, on ozonolysis, yields \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) and \(\mathrm{CH}_{2}(\mathrm{CHO})_{2}\) in the ratio \(1: 1\) ? (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\) \(\mathrm{CH}_{3}\)

Short answer

The correct answer is option (d).

Step-by-step solution

Understand the Ozonolysis Process

Ozonolysis is a reaction where alkenes are cleaved by ozone to form carbonyl compounds. In this case, the C=C double bonds will be split, and oxygen atoms will be inserted to form ketones or aldehydes.

Identify Carbonyl Compounds

The products of ozonolysis given here are acetone \((\mathrm{CH}_3\mathrm{COCH}_3)\) and glyoxal \(\mathrm{CH}_2(\mathrm{CHO})_2\). Each of these is formed from the splitting of a double bond.

Locate Possible Double Bonds in Choices

Evaluate each compound provided in options to determine which one, upon breaking its double bonds, results in the specified products from Step 2. The concerned carbonyl carbons should match the substition pattern.

Analyze Choice (a)

For option (a): \(\left(\mathrm{CH}_3\right)_2 \mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_3\right)_2\), breaking each double bond could theoretically yield different combinations upon analysis, but it does not result in the target ketone and aldehyde.

Analyze Choice (b)

For option (b): \(\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_3\right)_2\), break the double bonds to potentially form the wanted products, but this configuration does not yield \(\mathrm{CH}_3\mathrm{COCH}_3\) or \(\mathrm{CH}_2(\mathrm{CHO})_2\) accurately.

Analyze Choice (c)

For option (c): \(\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_3\right)_2\), ozonolysis also does not yield the desired \(\mathrm{CH}_3\mathrm{COCH}_3\) and \(\mathrm{CH}_2(\mathrm{CHO})_2\) structures.

Analyze Choice (d)

For option (d): \(\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CH}-\mathrm{CH}_2-\mathrm{C}\left(\mathrm{CH}_3\right)=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3\), upon breaking the double bonds and adding oxygen, the first split results in acetone \(\mathrm{CH}_3\mathrm{COCH}_3\) and the second split yields glyoxal \(\mathrm{CH}_2(\mathrm{CHO})_2\). This matches the desired outcome.

Key concepts

Alkene cleavage

Alkene cleavage is a crucial aspect of organic chemistry where the carbon-carbon double bond in alkenes is broken down via specific reactions. Ozonolysis is a common method used for this purpose. During ozonolysis, ozone (O_3) adds to the double bond generating an unstable ozonide intermediate. This intermediate further decomposes to form carbonyl compounds, such as aldehydes and ketones, depending on the structure of the initial alkene.

The reaction itself involves attacking the electron-rich double bond with ozone, splitting apart the carbon-carbon bond into carbon-oxygen bonds. It has profound utility in determining the structure of unknown alkenes, based on the products formed by the cleavage. For instance, ozonolysis can be used to prove the position of double bonds within complex molecules by identifying the resulting carbonyl fragments.

In the context of the given exercise, the cleavage of an alkene by ozonolysis breaks it down into acetone and glyoxal, alkene-specific products, which helps in verifying the structure of the starting compound.

Carbonyl compounds

Carbonyl compounds are organic molecules that contain the carbonyl group (C=O). They are key players in many chemical reactions, including ozonolysis. The carbonyl group itself is polar, with the oxygen atom bearing a partial negative charge and the carbon atom bearing a partial positive charge.

There are two main types of carbonyl compounds: ketones and aldehydes. Ketones have the carbonyl group bonded to two carbon atoms, while aldehydes have it bonded to at least one hydrogen atom. In ozonolysis, the formation of either aldehydes or ketones from alkenes depends on the nature of the substituents on the double-bonded carbons.

In our exercise, acetone (CH_3COCH_3) represents a ketone, whereas glyoxal (CH_2(CHO)_2) represents a di-aldehyde form. These products provide clear insights into the bonds present in the original structure before the application of ozonolysis. Recognizing these products is essential for backwardly determining the original structure of the alkene.

Organic reaction mechanisms

Understanding organic reaction mechanisms is essential to mastering the process of ozonolysis and similar reactions in organic chemistry. A reaction mechanism is a step-by-step description of the electronic rearrangements and molecular transformations occurring throughout a reaction. For every reaction, these steps can provide detailed explanations of how and why certain products are formed.

In ozonolysis, the reaction mechanism begins with the initial formation of the molozonide as the ozonide intermediate quickly rearranges and breaks down, leading to the formation of carbonyl compounds like aldehydes and ketones. Recognizing the stepwise progression during ozonolysis allows chemists to predict reaction outcomes accurately.

Organic reaction mechanisms are not only about the movements of atoms and electrons but also about understanding the stability and reactivity of intermediates, such as the O_3 complexes, during transformations. This knowledge helps in visualizing or even deducing the possible outcomes when analyzing compounds, such as in the challenge of determining which starting alkene led to specific carbonyl products.