Organic Chemistry

158 CHAPTER 4 Reactions of Alkenes

3-D Molecule:

Cyclic bromonium ion of

cis-2-butene

Mechanistic Tutorial:

Addition of halogens to

alkenes

The product of the first step is not a carbocation; rather, it is a cyclic bromonium ion

because bromine’s electron cloud is close enough to the other carbon to engage in

bond formation. The cyclic bromonium ion is more stable than the carbocation would

have been, since all the atoms (except hydrogen) in the bromonium ion have complete

octets, whereas the positively charged carbon of the carbocation does not have a com-

plete octet. (To review the octet rule, see Section 1.3.)

In the second step of the reaction, attacks a carbon atom of the bromonium ion.

This releases the strain in the three-membered ring and forms a vicinal dibromide.

Vicinalindicates that the two bromine atoms are on adjacent carbons (vicinusis the

Latin word for “near”). The electrostatic potential maps for the cyclic bromonium ions

show that the electron-deficient region (the blue area) encompasses the carbons, even

though the formal positive charge is on the bromine atom.

When adds to an alkene, a cyclic chloronium ion intermediate is formed. The

final product of the reaction is a vicinal dichloride.

Because a carbocation is not formed when or adds to an alkene, carbocation

rearrangements do not occur in these reactions.

PROBLEM 15

What would have been the product of the preceding reaction if HBr had been used as a

reagent instead of

PROBLEM 16

a. How does the first step in the reaction of ethene with differ from the first step in the

reaction of ethene with HBr?

b. To understand why attacks a carbon atom of the bromonium ion rather than the

positively charged bromine atom, draw the product that would be obtained if did

attack the bromine atom.

Reactions of alkenes with or are generally carried out by mixing the alkene

and the halogen in an inert solvent, such as dichloromethane (CH 2 Cl 2 ),that readily

Br 2 Cl 2

Br-

Br-

Br 2

Br 2?

1,2-dibromo-3-methylbutane

a vicinal dibromide

3-methyl-1-butene

CH (^2) CH 2 Cl 2
CH 3 CH 3
CH 3 CHCH Br 2 CH 3 CHCHCH 2 Br
Br

• the carbon skeleton
  does not rearrange
  Br 2 Cl 2
  1,2-dichloro-2-methylpropane
  a vicinal dichloride
  2-methylpropene
  CH (^2) CH 2 Cl 2
  CH 3 CH 3
  CH 3 CCHCl 2 3 CCH 2 Cl
  Cl


• 
  

  Cl 2
  Br-
  Br
  Br
  CH 2
  CH 2
  H 2 C
  H 2 C
  H 2 C CH 2

  
  


• 
  

  −

  
  


• 
  

  Br Br
  −
  Br Br
  less stable
  more stable
  sp^2
  cyclic bromonium ion
  of ethene
  cyclic bromonium ion
  of cis-2-butene