Organic Chemistry

• Initiation Steps.The first step is an initiation stepbecause it creates radicals.
  The second step is also an initiation step because it forms the chain-propagating
  radical


• Propagation Steps.Steps 3 and 4 are propagation steps. In step 3, a radical
  reacts to produce another radical. In step 4, the radical produced in the first
  propagation step reacts to form the radical that was the reactant in the first
  propagation step. The two propagation steps are repeated over and over. Hence,
  the reaction is called a radical chain reaction. A propagation step is a step that
  propagates the chain.


• Termination Steps.Steps 5, 6, and 7 are termination steps. In a termination
  step, two radicals combine to produce a molecule in which all the electrons are
  paired, thus ending the role of those radicals in the radical chain reaction. Any
  two radicals present in the reaction mixture can combine in a termination step, so
  radical reactions produce a mixture of products.

PROBLEM 26

Write out the propagation steps that occur when HBr adds to 1-methylcyclohexene in the

presence of a peroxide.

An alkyl peroxide is a radical initiatorbecause it creates radicals. Without a per-

oxide, the preceding radical reaction would not occur. Any reaction that occurs in the

presence of a radical initiator, but does not occur in its absence, must take place by a

mechanism that involves radicals as intermediates. Any compound that can readily un-

dergo homolysis—dissociate to form radicals—can act as a radical initiator. Examples

of radical initiators are shown in Table 28.3.

Whereas radical initiators cause radical reactions to occur,radical inhibitorshave

the opposite effect: They trap radicals as they are formed, preventing reactions that

take place by mechanisms involving radicals. How radical inhibitors trap radicals is

discussed in Section 9.8.

A peroxide has no effect on the addition of HCl or HI to an alkene. In the presence

of a peroxide, addition occurs just as it does in the absence of a peroxide.

Why is the peroxide effectobserved for the addition of HBr, but not for the addi-

tion of HCl or HI? This question can be answered by calculating the for the two

propagation steps in the radical chain reaction (using the bond dissociation energies in

Ta b l e 3. 1 ).

¢H°

CH 3 CH CH 2 HCl CH 3 CHCH 3

peroxide

+

Cl

CH 3 CH 3

CH 3 CCH 2 HI CH 3 CCH 3

peroxide

+

I

(Br–)

(Br–)

(Br–).

Br Br

Br

Br

Br Br

CH 3 CCH 2 Br

CH 3 CH 3

+ CH 3 CCH 2 Br

+

2 CH 3 CCH 2 Br BrCH 2 C

CH 3 CH 3 CH 3

CH 3 CH 3

CCH 2 Br

termination steps

5.

6.

7.

170 CHAPTER 4 Reactions of Alkenes