Question

Natural rubber is attacked readily by ozone. If the resulting material is treated with \(\mathrm{H}_{2} \mathrm{O}_{2}\) in acetic acid, the major product isolated is levulinic acid \(\left(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{3}\right)\). (a) What is the structure of levulinic acid? (b) If a rubber plant is fed radioactive sodium acetate labeled only at the acetate methyl \(\left({ }^{*} \mathrm{CH}_{3} \mathrm{CO}^{-}{ }_{2} \mathrm{Na}^{+}\right)\), radioactive rubber can be obtained from the latex of the plant after a short time. Degradation of this rubber by ozone yields radioactive levulinic acid. Which specific carbon atoms of the levulinic acid would you expect to be radioactive?

Short answer

The structure of levulinic acid is \(\mathrm{CH_3CH_2C(O)CH_2COOH}\). If the rubber plant is fed with radioactive sodium acetate labeled only at the acetate methyl group, the specific carbon atoms in levulinic acid that would be radioactive are the C1 (methyl) carbon atoms.

Step-by-step solution

Structure of Levulinic Acid

Levulinic acid has the molecular formula C5H8O3. To find the structure, we draw a five-carbon chain, then add oxygen atoms to form a carboxyl group on one end and a ketone group in the middle. The structure of levulinic acid is: \[ \mathrm{CH_3CH_2C(O)CH_2COOH} \]

Radioactive Sodium Acetate and Rubber Plant

The exercise states that the rubber plant is fed with radioactive sodium acetate, labeled only at the acetate methyl group. The formula for this compound is: \[ ^{*}\mathrm{CH_3CO^{-}_{2}Na^{+}} \]

Formation and Degradation of Rubber

After the plant is fed with radioactive sodium acetate, radioactive rubber is obtained from the latex. When this rubber is degraded by ozone, the resulting product is radioactive levulinic acid. We need to identify which specific carbon atoms in levulinic acid would be radioactive.

Identify Radioactive Carbon Atoms

To determine which specific carbon atoms in levulinic acid are radioactive, we must consider that the radioactive label came from the radioactive sodium acetate. After the treatment with ozone and hydrogen peroxide in acetic acid, the radioactive label will appear in the structure of levulinic acid. The carbon atoms in the acetate methyl group are: \[ ^{*}\mathrm{CH_3} \] Based on this, the radioactive carbon atoms in levulinic acid would be the C1 (methyl) carbon, as it is originally part of the labeled methyl group from radioactive sodium acetate: \[ ^{*}\mathrm{CH_3CH_2C(O)CH_2COOH} \] So, the specific carbon atoms in levulinic acid that would be radioactive are the C1 (methyl) carbon atoms.

Key concepts

Levulinic Acid Structure

Levulinic acid is an important compound extensively used in organic chemistry. Its molecular formula is \( C_5H_8O_3 \). This gives us a hint into its structural components, consisting of five carbon atoms, eight hydrogen atoms, and three oxygen atoms. To understand its structure, envision a five-carbon atom chain. At one end of this chain, you have a carboxyl group \( (-COOH) \), which is typical of many organic acids. This is followed by a ketone group \( (-C(O)-) \) located in the middle of the chain. Together, these form the complete structure:\[\mathrm{CH_3CH_2C(O)CH_2COOH}\]By breaking down its structure:

• \( \mathrm{CH_3} \): Methyl group
• \( \mathrm{CH_2} \): Methylene group
• \( \mathrm{C(O)} \): Carbonyl group (Ketone)
• \( \mathrm{CH_2} \): Another methylene group
• \( \mathrm{COOH} \): Carboxyl group

In summary, the combination of these functional groups contributes to the classification of levulinic acid as a keto acid, given its ketone and carboxyl functionalities on the carbon chain.

Radioactive Labeling

Radioactive labeling is a fascinating technique in organic chemistry that helps trace the paths of different atoms through a reaction or biological process. By using a radioactive isotope, chemists can monitor how specific atoms are incorporated or transformed within a chemical reaction. For the exercise in question, the compound used is radioactive sodium acetate. It’s specifically labeled at the methyl group of the acetate ion, denoted as \( ^{*}\mathrm{CH_3CO^{-}_2Na^{+}} \). In this scenario:

• \( ^{*}\mathrm{CH_3} \): The radioactive methyl group
• \( \mathrm{CO^{-}_2Na^{+}} \): Represents the rest of the acetate ion structure

This kind of labeling helps in identifying which parts of the final product, like levulinic acid, will be radioactive. It's a powerful method for understanding metabolic pathways and reaction mechanisms, providing insight into which carbon atoms retain their label through complex reactions.

Ozonolysis

Ozonolysis is a chemical reaction prominently used to break down alkenes using ozone. It’s a form of oxidative cleavage reaction, where the carbon-carbon double bonds of alkenes are cleaved to form two carbonyl groups. When applied to natural rubber, ozonolysis decomposes its polymerized isoprene units. The reaction sequence proceeds as follows:1. Ozone attacks the double bonds in the natural rubber.2. This results in formation of ozonide intermediates. 3. Upon further treatment with peroxides such as \( \mathrm{H_2O_2} \), these intermediates cleave straight into carbonyl compounds. The practical application of ozonolysis in natural rubber leads to a carboxylate product. When combined with hydrogen peroxide in acetic acid, it produces levulinic acid. Ozonolysis is a crucial tool in synthetic organic chemistry, giving chemists the ability to specifically break down complex molecules into more manageable fragments.

Natural Rubber Degradation

Natural rubber, composed mainly of polyisoprene, is highly susceptible to degradation processes such as ozonolysis. When rubber is exposed to environmental oxygen and ozone, its structure breaks down. The double bonds in rubber make it especially vulnerable to these oxidation reactions.Natural rubber degradation can be summarized as follows:

• Exposure to ozone cleaves polymer chains through oxonolysis.
• This results in smaller, reactive fragments.
• Subsequent treatment, like with \( \mathrm{H_2O_2} \) and acetic acid, promotes further breakdown to form simpler molecules like levulinic acid.

Understanding this degradation process is vital for industrial applications. Not only does it inform the synthesis and recycling of rubber, but it also inspires the development of ozone-resistant materials. Rubber degradation thus becomes an essential topic in fields like materials science and environmental chemistry.