a. When a carboxylic acid is dissolved in isotopically labeled water (H₂O₁₈) and an acid catalyst is added, the label is incorporated into both oxygens of the acid. Propose a mechanism to account for this.

Verified step by step guidance

1. Step 1: Begin by understanding the reaction setup. A carboxylic acid (CH3C=O(OH)) is dissolved in isotopically labeled water (H2O18) in the presence of an acid catalyst (HCl). The goal is to explain how the isotopic label (O18) is incorporated into both oxygen atoms of the carboxylic acid.
2. Step 2: Recognize that the acid catalyst (HCl) protonates the carboxylic acid, making the carbonyl group more electrophilic. This protonation step is crucial for facilitating subsequent reactions. Represent this step as: ${\mathrm{CH}3C}_{=}\left(\mathrm{OH}\right)+H+\to {\mathrm{CH}3C}_{\left(}$
3. Step 3: After protonation, the carbonyl oxygen becomes positively charged, making the carbonyl carbon more susceptible to nucleophilic attack. The isotopically labeled water (H2O18) acts as a nucleophile and attacks the carbonyl carbon, forming a tetrahedral intermediate. Represent this step as: ${\mathrm{CH}3C}_{\left(}$
4. Step 4: The tetrahedral intermediate undergoes proton transfer and exchange with the isotopically labeled water (H2O18). This exchange allows the O18 isotope to be incorporated into the hydroxyl group. Represent this step as: ${\mathrm{CH}3C}_{\left(}$
5. Step 5: Finally, the intermediate undergoes dehydration (loss of water) and reformation of the carbonyl group. During this step, the isotopically labeled oxygen (O18) is incorporated into the carbonyl oxygen. This completes the mechanism, explaining how the O18 isotope is incorporated into both oxygen atoms of the carboxylic acid.

Key Concepts

Carboxylic Acid Structure and Reactivity

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH). This functional group is polar and can participate in hydrogen bonding, making carboxylic acids relatively acidic. Understanding their structure is crucial for predicting their reactivity, especially in nucleophilic addition reactions where the carbonyl carbon is susceptible to attack by nucleophiles.

Isotopic Labeling in Chemical Reactions

Isotopic labeling involves using isotopes, such as oxygen-18 (O-18), to trace the movement of atoms through a chemical reaction. In this context, the incorporation of O-18 into the carboxylic acid indicates that the water molecules are participating in the reaction mechanism. This technique helps chemists understand reaction pathways and mechanisms by tracking how specific atoms are transformed during the reaction.

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction, often by protonating a reactant to make it more electrophilic. In the case of carboxylic acids, the addition of an acid catalyst like HCl can facilitate the formation of a more reactive intermediate, allowing for the incorporation of water molecules into the product. This mechanism is essential for understanding how the isotopic label is transferred to the carboxylic acid.