Question

Methyl acetate \(\left(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\right)\) is formed by the reaction of acetic acid with methyl alcohol. If the methyl alcohol is labeled with oxygen-18, the oxygen-18 ends up in the methyl acetate: \(\begin{array}{l}{\text { (a) Do the } \mathrm{C}-\mathrm{OH} \text { bond of the acid and the } \mathrm{O}-\mathrm{H} \text { bond }} \\ {\text { of the alcohol break in the reaction, or do the } \mathrm{O}-\mathrm{H} \text { bond }} \\ {\text { of the acid and the } \mathrm{C}-\mathrm{OH} \text { bond of the alcohol break? }}\end{array}\) \(\begin{array}{l}{\text { (b) Imagine a similar experiment using the radioisotope }^{3} \mathrm{H} \text { , }} \\ {\text { which is called tritium and is usually denoted T. Would the }} \\ {\text { reaction between } \mathrm{CH}_{3} \mathrm{COH} \text { and } \mathrm{TOCH}_{3} \text { provide the same }} \\ {\text { information about which bond is broken as does the above }} \\ {\text { experiment with } \mathrm{H}^{18} \mathrm{OCH}_{3} ?}\end{array}\)

Short answer

In the reaction between acetic acid (CH3COOH) and methyl alcohol (CH3OH), the O-H bond of the alcohol breaks, as evidenced by the presence of the oxygen-18 label in the methyl acetate product. Using tritium (T) as a radioisotope would provide the same information as it also reveals that the O-H bond of the alcohol is broken.

Step-by-step solution

(a) Identifying Broken Bonds

To understand which bonds are breaking, let's write the balanced chemical equation for this reaction. The reaction between acetic acid (CH3COOH) and methyl alcohol (CH3OH) can be written as: \(CH_3COOH + CH_3OH \rightarrow CH_3COOCH_3 + H_2O\) Now, in the given exercise, it is given that the oxygen-18 label ends up in the methyl acetate. Let's denote oxygen-18 with O* for simplicity. So, when the methyl alcohol is labeled with oxygen-18, it will be written as CH3O*H. The reaction becomes: \(CH_3COOH + CH_3O*H \rightarrow CH_3COO*CH_3 + H_2O\) Since the oxygen-18 labeled atom is present in the methyl acetate product, this suggests that the O-H bond of alcohol is being broken.

(b) Using Tritium as a Radioisotope

Now, let's consider the reaction between CH3COH and TOCH3, where T is the tritium isotope: \(CH_3COH + TOCH_3 \rightarrow CH_3COTCH_3 + H_2O\) In this case, the tritium isotope label ends up in methyl acetate in the same manner. Comparing this with the previous labeled oxygen-18 experiment, we can infer that the information about which bond is broken is the same for these two reactions, revealing that the O-H bond of the alcohol is broken in both cases.

Key concepts

Chemical Reaction

The synthesis of methyl acetate involves a chemical reaction known as esterification, where an alcohol and an acid react to form an ester and water. In the provided exercise, acetic acid (\textsc{CH}\(_3\)\textsc{COOH}) reacts with methyl alcohol (\textsc{CH}\(_3\)\textsc{OH}) to yield methyl acetate (\textsc{CH}\(_3\)\textsc{COOCH}\(_3\)) and water (\textsc{H}\(_2\)\textsc{O}).

To ensure the concept of a chemical reaction is clear, it's important to note that it involves breaking and forming chemical bonds. This transformation leads to one or more new substances with different properties from the reactants. Additionally, a balanced chemical equation represents the stoichiometry of the reaction, which means the number of atoms of each element is conserved on both sides of the reaction.

Isotope Labeling

Isotope labeling is a technique used to track the movement of atoms through a chemical reaction. In this exercise, methyl alcohol is labeled with oxygen-18, a stable isotope of oxygen. This method allows us to observe that, after the reaction, the oxygen-18 appears in the methyl acetate molecule, not in the by-product water. This information is crucial as it highlights which specific bond breaks during the reaction—namely, the bond between oxygen and hydrogen in the alcohol.

Importance of Isotope Labeling

Isotope labeling helps in understanding intricate details of reaction mechanisms. In research, it can shed light on reaction pathways and the rate at which individual atoms or groups of atoms are involved in a reaction. Through exercises like this one, students can visualize and comprehend how isotope labeling aids in the study of chemical processes.

Reaction Mechanism

The reaction mechanism provides insight into the step-by-step sequence of elementary reactions by which overall chemical change occurs. In the case of methyl acetate synthesis, the reaction mechanism involves a nucleophilic attack by the oxygen of the methyl alcohol on the carbonyl carbon of acetic acid, followed by the elimination of water and the formation of the ester bond.

Understanding Reaction Intermediates

Reaction mechanisms often involve intermediates—species that appear in the course of the reaction but are not present in the final products. By using isotopically labeled reactants, such as oxygen-18 or tritium, the fate of particular atoms can be monitored, confirming or refuting proposed reaction mechanisms. As such, these techniques provide vital clues to the precise steps that constitute the reaction and offer a window into the complex transformations that compounds undergo during chemical reactions.