Reactivity: How and Why - Organic Chemistry Concepts: An EFL Approach - page 72

Curly arrows are an important tool to describe mechanisms. They show details
of what happens to the nucleophiles, electrophiles, and radicals that are formed
in the reaction. They allow us to follow all bond breaking and making, and help
us to accurately count electrons. Remember that curly arrows are specific and are
always drawn from the electron-rich source to the electron-poor center.

For our purposes, we will keep all data of the reaction rates as qualitative . Quali-
tative means that we do not need exact values. We only use relative terms, such
as fast or slow, to describe the ease or rate of a reaction step. The slowest step
in a stepwise reaction controls the overall rate of reaction. This is called the
rate-determining step . Changes in stereochemistry during reactions can occur.
Further details are shown in Appendices 8 and 9.

5.6 REACTION ENERGY
Qualitative reaction energetics uses the basic concepts of:

reaction rate -how fast a reaction occurs;

n

reaction equilibrium -shows the overall direction of a reaction. n

For a reaction to be spontaneous and give off energy, it must have an equilib-
rium constant >1. Then the reaction is described as exothermic , and the heat of
reaction Δ H is negative.

If energy must be added to make a reaction occur, the equilibrium constant
is <1. Then the reaction is described as endothermic , and Δ H is positive. This
qualitative description ignores the relatively small entropy contribution.

Energy diagrams , as shown in Figure 5.10 , are simple qualitative descriptions of
the energy changes during a reaction. The vertical axis shows the energy of the
reacting system, and the horizontal axis shows the progress of reaction.

FIGURE 5.10
Typical reaction energy diagrams.

As Figure 5.10 shows, a concerted , single-step process has a single activation
energy ( E A ) that must be provided for reaction to occur. This leads to the energy
maximum of the transition state (TS). In the example shown, Δ H is negative and