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REACTION MECHANISMS
transition
state
Energy
Reaction coordinate
activation energy
∆G˚ standard free energy
change for reaction
reactants
products
Figure 5.1 Energy profile diagram: transition state
Energy
Reaction coordinate
reactants
products
∆G˚ standard free energy
change for reaction
activation energy 2
activation
energy 1
intermediate
transition
state 1
transition
state 2
Figure 5.2 Energy profile diagram: intermediate
activated complex. This material cannot be isolated,
or even detected.
In an alternative scenario, again with a negative
energy change, the energy profile may appear different, as shown in Figure 5.2. In this case, there is
again an activation energy required to set the reaction
off, but this energy maximum is then followed by an
energy minimum. The energy minimum represents an
intermediate in the reaction pathway. It is converted
into the products by overcoming a second activation
energy, though this is likely to be considerably less
than the first activation energy. Because the intermediate is at an energy minimum, this material may
be stable and can be isolated, or it may be reactive
and short-lived, but nevertheless detectable. The two
energy maxima represent different transition states.
The energy diagrams shown here are merely generalized examples. We shall meet some specific
examples as we consider various reaction mechanisms.
5.5 Types of reaction
At first glance, there appear to be an infinite number
of different chemical reactions, all of which will
have to be remembered. A cursory look through
any textbook of organic chemistry does little to
dispel this fear. However, the beauty and strength of