Question

The oxygen in water is primarily \((99.8 \%)^{16} \mathrm{O},\) but water enriched with the heavy isotope \(18 \mathrm{O}\) is also available. When an aldehyde or ketone is dissolved in \(18 \mathrm{O}\) -enriched water, the isotopic label becomes incorporated into the carbonyl group. Explain. $$ \mathrm{R}_{2} \mathrm{C}=\mathrm{O}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{R}_{2} \mathrm{C}=\mathrm{O}+\mathrm{H}_{2} \mathrm{O} \quad \text { where } \mathrm{O}={ }^{18} \mathrm{O} $$

Short answer

Oxygen exchange occurs via reversible hydration, incorporating \( ^{18} \mathrm{O} \) into the carbonyl group.

Step-by-step solution

Understanding the Reaction

An aldehyde or ketone structure, represented as \( \mathrm{R}_{2} \mathrm{C}=\mathrm{O} \), consists of a carbonyl group \( \mathrm{C}=\mathrm{O} \). When this compound is dissolved in \( ^{18} \mathrm{O} \)-enriched water, a reversible reaction occurs by which the \( ^{18} \mathrm{O} \) can be incorporated into the carbonyl group of the aldehyde or ketone.

Mechanism of Isotope Exchange

The reaction involves the exchange of the oxygen atom in the carbonyl group with the oxygen atoms from water through a reversible hydration mechanism. This means the double bond of the carbonyl group temporarily shifts to form a gem-diol (a hydrate), where both R groups remain bonded to a single carbon now attached to two OH groups. Subsequently, dehydration returns the carbonyl double bond, potentially leaving an \( ^{18} \mathrm{O} \) atom in place if the exchange occurred with an \( ^{18} \mathrm{O} \)-containing water molecule.

Final Explaining

Since water enriched with \( ^{18} \mathrm{O} \) participates in the hydration-dehydration process, the reaction will repeatedly allow for the opportunity of \( ^{18} \mathrm{O} \) incorporation into the carbonyl group. With each cycle, there's a chance that the carbonyl group will re-form with an \( ^{18} \mathrm{O} \) from the water.

Key concepts

Carbonyl Group

A carbonyl group is a common and essential functional group in organic chemistry. It consists of a carbon atom double-bonded to an oxygen atom, written as \( \mathrm{C} = \mathrm{O} \).
This group is found in many different functional classes of molecules, including aldehydes and ketones. The carbonyl group is highly reactive because of the polar nature of the C=O bond.
Pertaining to the interaction with water, the electronegative oxygen atom in the carbonyl group can form transient interactions, which are crucial in many chemical processes, including isotope exchange.

Hydration Mechanism

The hydration mechanism refers to the reversible reaction where a carbonyl compound, such as an aldehyde or ketone, interacts with water. During this process, the carbonyl group can transform into a hydrate, or gem-diol, by transiently interacting with water.
1. **Addition of Water**: Initially, water adds to the carbonyl group. The lone pair of electrons on the oxygen atom in water attacks the electrophilic carbon in the C=O bond.
2. **Formation of Gem-Diol**: The double bond in the carbonyl group temporarily shifts, forming a gem-diol, with two hydroxyl groups attached to a single carbon.
3. **Reversible Nature**: This process is reversible, allowing for the oxygen in the carbonyl group to exchange with the oxygen in water, making it a crucial step in the incorporation of heavy isotopes like \(^ {18} \mathrm{O} \).

Aldehydes and Ketones

Aldehydes and ketones are two types of organic compounds that contain the carbonyl group. They are pivotal in both biological systems and industrial applications.
**Aldehydes**: These compounds have at least one hydrogen atom attached to the carbonyl group. Formaldehyde is a simple aldehyde, widely used and studied.
**Ketones**: In contrast, ketones have two organic groups attached to the carbonyl carbon. The simplest example is acetone, an industrial solvent. Both these types of compounds undergo similar reactions like hydration, occurring during interactions with enriched water.

Heavy Isotopes

Heavy isotopes are different forms of an element with additional neutrons, such as \(^{18} \mathrm{O} \) in water. These isotopes have slightly different physical properties from their lighter counterparts.
The presence of heavy isotopes, like \(^{18} \mathrm{O} \), allows for tracking and studying various chemical processes, such as the hydration and isotope exchange reactions. In the isotope exchange process, the use of \(^{18} \mathrm{O} \)-enriched water enables scientists to track the movement and incorporation of the isotope into the carbonyl group during the hydration-dehydration cycles.
This ability to follow changes in isotope distribution can provide insights into reaction mechanisms, helping us understand fundamental chemical processes at a deeper level.