Problem 73 - Chemistry: The Central Science

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: Do the \(\mathrm{C}-\mathrm{OH}\) bond of the acid and the \(\mathrm{O}-\mathrm{H}\) bond of the alcohol break in the reaction, or do the \(\mathrm{O}-\mathrm{H}\) bond of the acid and the \(\mathrm{C}-\mathrm{OH}\) bond of the alcohol break? Explain.

Step-by-Step Solution

Answer

In the reaction between acetic acid and methyl alcohol labeled with oxygen-18, the bonds that break are the \(\mathrm{O*}-\mathrm{H}\) bond of the methyl alcohol and the \(\mathrm{C}-\mathrm{OH}\) bond of the acetic acid. The product, methyl acetate, has the structure \(\mathrm{CH}_{3}\mathrm{COO*CH}_{3}\), indicating that the oxygen-18 atom is now bonded to a carbon atom, forming a new \(\mathrm{C}-\mathrm{O*}\) bond. The remaining hydrogen from the methyl alcohol and the hydroxyl group from the acetic acid combine to form water as a byproduct.

Step 1: Write the structures of the reactants and product

Before we analyze the bonds that break during the reaction, we need to write the structures of the reactants and the product. Acetic acid has the formula \(\mathrm{CH}_{3}\mathrm{COOH}\) and methyl alcohol (also known as methanol) has the formula \(\mathrm{CH}_{3}\mathrm{OH}\). The product, methyl acetate, has the formula \(\mathrm{CH}_{3}\mathrm{COOCH}_{3}\). The oxygen-18 label on the methyl alcohol will be indicated by writing an asterisk (*) on its oxygen atom: \(\mathrm{CH}_{3}\mathrm{O*H}\).

Step 2: Analyze the bonds that could break in the reactants

In the reactants, we have the following bonds of interest: the \(\mathrm{C}-\mathrm{OH}\) bond in the acetic acid, the \(\mathrm{O}-\mathrm{H}\) bond in the acetic acid, the \(\mathrm{O*}-\mathrm{H}\) bond in the methyl alcohol, and the \(\mathrm{C}-\mathrm{OH}\) bond in the methyl alcohol. We want to determine which of these bonds break during the reaction to form the methyl acetate, specifically focusing on the fate of the labeled oxygen-18 atom.

Step 3: Analyze the bonds in the product (methyl acetate)

In methyl acetate (\(\mathrm{CH}_{3}\mathrm{COOCH}_{3}\)), the oxygen-18 labeled atom ends up in the molecule. Thus, our product will have the following structure: \(\mathrm{CH}_{3}\mathrm{COO*CH}_{3}\). Analyzing the bonds in the product, we can see that the oxygen-18 labeled atom is bonded to a carbon atom, forming a \(\mathrm{C}-\mathrm{O*}\) bond.

Step 4: Determine which bonds break during the reaction

Comparing the reactants and product, we can notice that the oxygen-18 labeled atom in the product was originally in the methyl alcohol, and now it is bonded to the carbon of the acetic acid. This can only happen if the \(\mathrm{O*}-\mathrm{H}\) bond of the methyl alcohol and the \(\mathrm{C}-\mathrm{OH}\) bond of the acetic acid break during the reaction. The molecules combine, forming a new \(\mathrm{C}-\mathrm{O*}\) bond, and the \(\mathrm{H}\) from the methyl alcohol combines with the \(\mathrm{OH}\) from the acetic acid to form water as a byproduct. Therefore, the \(\mathrm{O}-\mathrm{H}\) bond of the acid and the \(\mathrm{C}-\mathrm{OH}\) bond of the alcohol do not break during the reaction. Instead, the \(\mathrm{O*}-\mathrm{H}\) bond of the methyl alcohol and the \(\mathrm{C}-\mathrm{OH}\) bond of the acetic acid break in the reaction.

Key Concepts

Methyl acetate formation

Methyl acetate, a common ester, is produced when acetic acid reacts with methyl alcohol (methanol). This reaction is an example of esterification, an important process in organic chemistry. During esterification, an acid and an alcohol react to form an ester and water.
In this case, acetic acid (CH extsubscript{3}COOH) and methanol (CH extsubscript{3}OH) are the initial reactants. The crucial step in this reaction involves breaking certain bonds: the \(\text{{O*-H}}\) bond of the labeled methanol (where the \(\text{{O*}}\) signifies oxygen-18) and the \(\text{{C-OH}}\) bond in acetic acid. These bonds break to form methyl acetate (CH extsubscript{3}COOCH extsubscript{3}) and water (H extsubscript{2}O), with the \(\text{{C-O*}}\) bond being newly formed.
This reaction illustrates how acids and alcohols can be transformed into esters, which have numerous applications in fragrances, flavors, and as solvents.

Acetic acid reaction

Acetic acid is a carboxylic acid known for its distinct pungent smell and is a key reactant in this esterification process. In the chemical reaction with methanol, the acetic acid donates a proton, allowing the methanol to bond with the acyl group, forming an ester. This involves breaking the \(\text{{C-OH}}\) bond of the acetic acid.
During the process, the hydroxyl group (OH) of the acetic acid is used to form water, together with the hydrogen from methanol.
These reactions are often performed in the presence of an acid catalyst to increase the rate of reaction.
Acetic acid’s versatile nature makes it a staple in organic synthesis, as well as in household and industrial applications.

Isotope labeling in chemistry

Isotope labeling is a technique used in chemistry to track the path of an atom through a reaction or a series of reactions. It involves replacing an atom in a molecule with an isotope—a variant of an element with a differing number of neutrons. In our exercise, oxygen-18, a stable isotope of oxygen, was used to label methanol (CH extsubscript{3}O*H).
This method of labeling is incredibly useful because it allows chemists to pinpoint changes in molecular structure and follow the movement of atoms during reactions.
Isotope labeling can help clarify which bonds break and form as new molecules are created, providing insight into reaction mechanisms.

• Understanding these pathways aids in designing efficient chemical processes.
• It is also crucial in research fields like biochemistry and pharmacology.

This exercise demonstrates how isotope labeling with oxygen-18 helped us determine the breaking and forming of specific bonds in the formation of methyl acetate.