Question

22.57 Heavy water \(\left(\mathrm{D}_{2} \mathrm{O}\right)\) is used to make deuterated chemicals. (a) What major species, aside from the starting compounds, do you expect to find in a solution of \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{D}_{2} \mathrm{O} ?\) (b) Write equations to explain how these various species arise. (Hint: Consider the autoionization of both components.)

Short answer

The major species are \(\text{CH}_{3}\text{OD}\), \(\text{D}_{3}\text{O}^+\), and \(\text{OD}^-\). The reactions are \(\text{D}_{2}\text{O} \rightleftharpoons \text{D}^+ + \text{OD}^-\), \(\text{D}^+ + \text{D}_{2}\text{O} \rightleftharpoons \text{D}_{3}\text{O}^+\), and \(\text{CH}_{3}\text{OH} + \text{D}_{2}\text{O} \rightleftharpoons \text{CH}_{3}\text{OD} + \text{D}_{2}\text{O}\).

Step-by-step solution

Identify Major Species

When \(\text{CH}_{3}\text{OH}\) and \(\text{D}_{2}\text{O}\) are combined, we need to consider the autoionization of \(\text{D}_{2}\text{O}\) and any potential reactions with \(\text{CH}_{3}\text{OH}\). The major species to consider are \(\text{CH}_{3}\text{OD}\), \(\text{D}_{3}\text{O}^+\), and \(\text{OD}^-\).

Write Autoionization Equations

The autoionization of \(\text{D}_{2}\text{O}\) can be written as: \[ \text{D}_{2}\text{O} \rightleftharpoons \text{D}^+ + \text{OD}^- \] Further combining \(\text{D}^+\) ions: \[ \text{D}^+ + \text{D}_{2}\text{O} \rightleftharpoons \text{D}_{3}\text{O}^+ \]

React \(\text{CH}_{3}\text{OH}\) with \(\text{D}_{2}\text{O}\)

When \(\text{CH}_{3}\text{OH}\) is introduced into the \(\text{D}_{2}\text{O}\) solution, it can undergo exchange reactions, producing \(\text{CH}_{3}\text{OD}\) as follows: \[ \text{CH}_{3}\text{OH} + \text{D}_{2}\text{O} \rightleftharpoons \text{CH}_{3}\text{OD} + \text{D}_{2}\text{O} \] The reaction shifts towards the formation of \(\text{CH}_{3}\text{OD}\) due to the excess \(\text{D}_{2}\text{O}\).

Key concepts

Autoionization

Autoionization occurs when water molecules interact with each other to form ions. In the case of heavy water, \(\text{D}_{2}\text{O}\), this process involves deuterium instead of hydrogen. The autoionization can be expressed as:\[ \text{D}_{2}\text{O} \rightleftharpoons \text{D}^+ + \text{OD}^- \]Here, one \(\text{D}_{2}\text{O}\) molecule splits into a deuterium ion (\(\text{D}^+\)) and a deuteroxide ion (\(\text{OD}^-\)). Then, the deuterium ion can further react with another \(\text{D}_{2}\text{O}\) molecule to form deuterium hydronium ion as follows:\[ \text{D}^+ + \text{D}_{2}\text{O} \rightleftharpoons \text{D}_{3}\text{O}^+ \]Autoionization plays a significant role in understanding the behavior of \(\text{D}_{2}\text{O}\) in solution, especially in reactions involving deuterium exchange, as it produces the ions necessary for further reactions.

Deuterium Exchange

Deuterium exchange refers to the process where deuterium atoms (\(\text{D}\)) are exchanged with hydrogen atoms (\(\text{H}\)) in chemical compounds. When \(\text{CH}_{3}\text{OH}\) is introduced into a \(\text{D}_{2}\text{O}\) solution, a deuterium exchange can occur, producing \(\text{CH}_{3}\text{OD}\) and releasing hydrogen into the solution. This reaction can be described by the following equation:\[ \text{CH}_{3}\text{OH} + \text{D}_{2}\text{O} \rightleftharpoons \text{CH}_{3}\text{OD} + \text{D}_{2}\text{O} \]In this exchange, \(\text{D}_{2}\text{O}\) acts as a source of deuterium, which replaces the hydrogen in methanol, converting \(\text{CH}_{3}\text{OH}\) (methanol) to \(\text{CH}_{3}\text{OD}\) (deuterated methanol). Since \(\text{D}_{2}\text{O}\) is in excess, the equilibrium favors the formation of \(\text{CH}_{3}\text{OD}\). This exchange process is essential in creating deuterated compounds used in various scientific experiments and applications.

Deuterium Oxide

Deuterium oxide, commonly known as heavy water (\(\text{D}_{2}\text{O}\)), is water where both hydrogen atoms are replaced by deuterium. Deuterium is an isotope of hydrogen with an additional neutron, making it twice as heavy as regular hydrogen. This substitution leads to some unique properties:

• Higher boiling and melting points compared to regular water.
• Increased viscosity, which makes it flow differently than normal water.
• Lower reactivity in some chemical reactions due to the stronger \(\text{D}-\text{O}\) bond compared to the \(\text{H}-\text{O}\) bond.

Heavy water is often used in nuclear reactors as a neutron moderator, as it can slow down neutrons effectively without absorbing them. Additionally, it is valuable in research and medical applications, particularly in studying reaction mechanisms and kinetics since the presence of deuterium can be tracked more easily compared to hydrogen.