Organic Chemistry

9.7 Reactions of Cyclic Compounds

Cyclic compounds undergo the same reactions as acyclic compounds. For example,

cyclic alkanes, like acyclic alkanes, undergo radical substitution reactions with chlo-

rine or bromine.

Cyclic alkenes undergo the same reactions as acyclic alkenes.

In other words,the reactivity of a compound usually depends solely on its functional

group, not on whether it is cyclic or acyclic.

PROBLEM 14

a. Give the major product(s) of the reaction of 1-methylcyclohexene with the following

reagents, ignoring stereoisomers:

1. 
   
   
   
   2. 
      
      
      
      3. HBr 4.
         b. Give the configuration of the products.
      
      
      
      
   
   
   
   

NBS>¢>peroxide Br 2 >CH 2 Cl 2 HBr>peroxide

HBr

NBS

+

∆

peroxide

Br

Br

+

bromocyclohexane

3-bromocyclopentene

+ Cl 2 + HCl

h

Cl

+ Br 2

Br

chlorocyclopentane

bromocyclohexane

h

+ HBr

350 CHAPTER 9 Reactions of Alkanes • Radicals

CYCLOPROPANE

Cyclopropane is one notable exception to the gen-

eralization that cyclic and acyclic compounds un-

dergo the same reactions. Although it is an alkane, cyclopropane

undergoes electrophilic addition reactions as if it were an alkene.

Cyclopropane is more reactive than propene toward addition of

acids such as HBr and HCl, but is less reactive toward addition

+ HBr CH 3 CH 2 CH 2 Br

FeCl 3

+ Cl 2 ClCH 2 CH 2 CH 2 Cl

Ni

• H (^280) °C CH 3 CH 2 CH 3
  of and so a Lewis acid ( or ) is needed to
  catalyze halogen addition (Section 1.21).
  It is the strain in the small ring that makes it possible for
  cyclopropane to undergo electrophilic addition reactions
  (Section 2.11). Because of the 60°bond angles in the three-
  membered ring, the orbitals cannot overlap head-on; this
  decreases the effectiveness of the orbital overlap. Thus, the
  “banana bonds”in cyclopropane are considerably weak-
  er than normal bonds (Figure 2.6 on p. 92). Conse-
  quently, three-membered rings undergo ring-opening reactions
  with electrophilic reagents.
  XY H 2 CCH 2
  XY
  CH 2


• 
  

  C¬Cs
  C¬C
  sp^3
  Cl 2 Br 2 , FeCl 3 FeBr 3