Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 105

to show homolytic process. The equation shows that the reaction type is a sub-
stitution in which Cl replaces H.

The mechanism of the reaction is shown in Figure 7.2 and includes three steps:

initiation —the formation of radicals needed to carry out a radical process; n

propagation —the repeated radical process; n

termination —the trapping of radicals to end a radical process.

n

Chapter 5 discussed the reactions in these steps and showed them as homolytic
cleavage and homogenic bond making.

FIGURE 7.2
Stepwise description of radical substitution.

The reaction is not very practical because it gives mixtures of products. This is
even more complex as the length of the alkane chain increases.

7.3 ALKENES
The unsaturated functional group (C ] C) is electron rich between the bonded
carbon atoms. Therefore, as shown in Chapter 5, an alkene is a potential nucleo-
phile. The weaker held pair of π-bond electrons is reactive enough to bond with
suitable electrophiles. This is the characteristic reaction of alkenes. This polar
reaction in Figure 7.3 uses heterolytic bond breaking and making. Because the
rate-determining step is the reaction with an electrophile, this reaction is called
electrophilic addition. See Figure 7.4 .

7.3.1 Electrophilic Addition

FIGURE 7.3
General electrophilic addition sequence.

As defined in Chapter 5 for an addition reaction, the bond order decreases and
C ] C becomes C-C. This characteristic addition to alkenes is clearly seen in the