Question

(a) What is a kinetic isotope effect?

(b) Do compounds of hydrogen exhibit a relatively large or small kinetic isotope effect? Explain.

(c) Carbon compounds also exhibit a kinetic isotope effect. How do you expect it to compare in magnitude with that for hydrogen compounds? Why?

Short answer

1. Deuterium is isolated using the isotope effect.
2. The mass ratio of the isotopes is larger for H than for any other atom, hydrogen compounds exhibit a relatively large kinetic isotope effect.
3. It is stronger and harder to break. So, carbon compound exhibit relatively low kinetic isotope effect.

Step-by-step solution

Definition of Concept

Oxidation is the process by which a molecule, atom, or ion loses an electron in a chemical reaction.

Reduction is the process by which a molecule, atom, or ion gains electrons from one of the atoms in a chemical reaction.

Subpart (a)

Different bond energies result from the mass difference between two atoms, which affects reactivity. As a result, H-atom bonded to a given atom vibrates at a higher frequency than a D atom, resulting in a higher energy bond to H. As a result, it is weaker and easier to defeat break.

Therefore, any reaction that includes breaking a bond to hydrogen in the rate-determining step occurs faster with H than with D. This phenomenon is called a kinetic isotope effect; No other element displays a kinetic isotope effect nearly as large.

Subpart (b)

${}^{\text{2}}\text{H}$ has two times the mass of ${}^{\text{1}}\text{H}$ , whereas ${}^{\text{13}}\text{C}$ has only $\text{1.08}$ times the mass of ${}^{\text{12}}\text{C}$. Since the mass ratio of the isotopes is larger for H than for any other atom, hydrogen compounds exhibit a relatively large kinetic isotope effect.

Subpart (c)

${}^{\text{13}}\text{C}$ has only $\text{1.08}$ times the mass of ${}^{\text{12}}\text{C}$. The mass difference leads to different bond energies, which affect reactivity. As a result, ${\text{a}}^{\text{12}}\text{C}$ atom bonded to a given atom vibrates at a lower frequency than ${}^{\text{13}}\text{C}$ atom does, so the bond to ${}^{\text{12}}\text{C}$ is lower in energy. Thus, it is stronger and harder to break. Hence, carbon compounds exhibit a relatively low kinetic isotope effect.