A younger student is contemplating taking Advanced Organic Chemistry CHM 338 next year and asks you about the course. To help your friend capture the essential aspects of Isotope Effects (primary and secondary), clearly illustrate in 200-300 words with appropriately drawn diagrams, mechanism that help clarify your explanation.
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An isotopic substitution will greatly modify the reaction rate when the isotopic replacement is in a chemical bond that is broken or formed. In such a case, the rate change is termed a primary isotope effect. When the substitution is not involved in the bond that is breaking or forming, one may still observe a smaller rate change, termed a secondary isotope effect. Thus, the magnitude of the kinetic isotope effect can be used to elucidate the reaction mechanism. Isotope effects are most easily observed when they occur in the rate-determining step of a reaction. If other steps are partially rate-determining, the effect of isotopic substitution will be masked.
Thus we can summarise as:
Primary Kinetic Isotope Effect - a rate change due to isotopic substitution at a site of bond breaking or bond making in the rate determining step of a mechanism
Secondary Kinetic Isotope Effect - a rate change due to isotopic substitution at other than a site of bond breaking or bond making in the rate determining step of a mechanism
For example, replacing hydrogen (H) in a molecule with its isotope deuterium (D), which is heavier by one neutron, can slow down a reaction by up to 20 times. The reason isotopes H and D behave differently relates to their "dissociation energy," or the amount of energy required to break their bonds with other atoms. Generally, the lighter the atom, the less energy is needed to excite its bond to its breaking point, and the lower the "dissociation energy". Consider, for example, two bonds, a carbon-hydrogen (C-H) and a carbon-deuterium (C-D) bond. The atoms at each end of the bond can be viewed as two balls vibrating back and forth on two ends of a spring. The lighter hydrogen atoms are like Ping-Pong balls, and the heavier deuterium atoms like golf-balls. The golf-balls are harder to get moving than the ping pong balls just because they are heavier, and more energy is required vibrate them to the point where they will separate "dissociate". In this example then, the C-D bond will take more energy to break than the C-H bond.
The potential energy diagram below visualizes the when comparing C-D and C-H. Here potential of C-D is lower so it is more stable than C-H bond.