Energy Changes Accompany Each Elementary Step
progress of the reaction
free energy
(a)
(b)
(c)
(e)
(d)
A Reaction Coordinate Diagram
for a One-Step Process
(a) reactants
(b) transition state
(c) products
(d) ΔG˚ Gibbs standard free energy change
(e) ΔG≠ free energy of activation
We have already encountered a useful tool
that allows us to track energy changes
along a complex reaction pathway – the
tool was the reaction coordinate diagram
used to describe the ring flipping process
of cyclohexane. Although cyclohexane’s
“reaction” did not involve electron
reconfiguration, the minimum energy
pathway (MEP) correlated energy and
progress of change for a complex set of
conformations. Not only does the reaction
coordinate diagram and its MEP serve as a
way to track energy changes for
conformational processes, but it also is a
tool for tracking energy of reactions that
involve electron reconfiguration.
We are interested only in the
minima and maxima on reaction
coordinate diagrams. By knowing
the energy at these positions, either
qualitatively or quantitatively, we
will be able to judge the
equilibrium position of the reaction,
and how fast each step of the
reaction takes place.
The essential components of a reaction coordinate diagram for a one-step reaction are shown above. Most important is
the MEP on this diagram. The particular MEP on the above diagram might, for example, apply to the proton transfer
from phenol to hydroxide. The reactants (a) are higher in energy than the products (c), so there is a favorable driving
force (d) to promote the change. This driving force is the Gibbs standard free energy and it’s directly related to the
equilibrium constant (Keq) by ΔG˚ = -RT ln Keq where T is the temperature in Kelvin and R is the gas constant.
The transition state (b) is a transient structure caught in the act of undergoing electron reconfiguration. The
symbol TS≠ represents the transition state structure. Notice that the energy associated with TS≠ is higher than the
reactants. This maximum on the energy profile represents a barrier that resists the transformation of reactants to
products. The higher the barrier is, the slower the rate of the reaction. The coefficient, k, is proportional to the
reaction rate (k and ΔG≠ are related by k = 2.084x1010·T·e-(ΔG≠/1.986T)).
MEP on a 1-step reaction
coordinate diagram
