Now, let’s move to kinetics. Look again at the energy diagram for exergonic reaction: although it is ‘downhill’ overall, it isn’t a straight downhill run.
First, an ‘energy barrier’ must be overcome to get to the product side. The height of this energy barrier, you may recall, is called the ‘activation energy’ (ΔG‡). The activation energy is what determines the kinetics of a reaction: the higher the energy hill, the slower the reaction. At the very top of the energy barrier, the reaction is at its transition state (TS), which is the point at which the bonds are in the process of breaking and forming. The transition state is an ‘activated complex’: a transient and dynamic state that, unlike more stable species, does not have any definable lifetime. It may help to imagine a transition state as being analogous to the exact moment that a baseball is struck by a bat. Transition states are drawn with dotted lines representing bonds that are in the process of breaking or forming, and the drawing is often enclosed by brackets. Here is a picture of a likely transition state for our simple SN2 reaction between hydroxide and chloromethane:
The SN2 reaction involves a collision between two molecules: for this reason, we say that it has second order kinetics (this is the source of the number ‘2’ in SN2). The rate expression for this type of reaction is:
rate = k[reactant 1][reactant 2]
. . . which tells us that the rate of the reaction depends on the rate constantk as well as on the concentration of both reactants. The rate constant can be determined experimentally by measuring the rate of the reaction with different starting reactant concentrations. The rate constant depends on the activation energy, of course, but also on temperature: a higher temperature means a higher k and a faster reaction, all else being equal. This should make intuitive sense: when there is more heat energy in the system, more of the reactant molecules are able to get over the energy barrier.
This study of reaction rates and the reaction coordinate diagram turns out to be invaluable for determining the mechanism for a reaction. The mechanism tells us how the bonds are broken and made, and shows what intermediates (if any) are formed along the way.
- The rate determining step is the slowest one – the one with the highest energy barrier
- The rate of the rate determining step is equal to the rate for the overall reaction
Enzymes – nature’s catalysts
- 6.3: Enzymatic catalysis - the basic ideas. Authored by: Tim Soderbergu00a0(University of Minnesota, Morris). Located at: https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry/Book%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)/Chapter_06%3A_Introduction_to_organic_reactivity_and_catalysis/6.3%3A_Enzymatic_catalysis_-_the_basic_ideas. Project: Chemistry LibreTexts. License: CC BY-NC-SA: Attribution-NonCommercial-ShareAlike