8.2 Halogenation of Alkenes: Addition of \(X_2\)

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Objectives

After completing this section, you should be able to

write the equation for the reaction of chlorine or bromine with a given
identify the conditions under which an addition reaction occurs between an alkene and chlorine or bromine.
draw the structure of the product formed when a given alkene undergoes an addition reaction with chlorine or bromine.
write the mechanism for the addition reaction that occurs between an alkene and chlorine or bromine, and account for the stereochemistry of the product.

Key Terms

Make certain that you can define, and use in context, each of the key terms listed below.

anti stereochemistry
bromonium ion
syn stereochemistry

Study Notes

In the laboratory you will test a number of compounds for the presence of a carbon-carbon double bond, using the procedure described on pages 264 and 265 of the textbook. The mechanism shown at the top of page 265 gives you an idea of how the reaction between an alkene and a halogen occurs. However, as you read this section, you will find that much more is involved than is represented in this scheme. In an examination, you would be awarded no marks if you reproduced the reaction on page 265 in response to the question: “Write down the mechanism for the addition of bromine to an alkene.” Full mechanistic details are given later in the course. Additional evidence in support of the bromonium ion mechanism comes from the results obtained when an alkene (such as cyclopentene) is reacted with bromine in the presence of sodium chloride (see Figure 8.2, below).

Figure 8.2.1: Reaction of an alkene with bromine in the presence of sodium chloride

Once formed, the bromonium ion is susceptible to attack by two nucleophiles—chloride ion and bromide ion—and, in fact, a mixture of two products (both produced by anti attack) is formed.

Halogens can act as electrophiles to attack a double bond in alkene. Double bond represents a region of electron density and therefore functions as a nucleophile. How is it possible for a halogen to obtain positive charge to be an electrophile?

Introduction

As halogen molecule, for example Br₂_, approaches a double bond of the alkene, electrons in the double bond repel electrons in bromine molecule causing polarization of the halogen bond. This creates a dipolar moment in the halogen molecule bond. Heterolytic bond cleavage occurs and one of the halogens obtains positive charge and reacts as an electrophile. The reaction of the addition is not regioselective but stereoselective.Stereochemistry of this addition can be explained by the mechanism of the reaction.In the first step electrophilic halogen with a positive charge approaches the double carbon bond and 2 p orbitals of the halogen, bond with two carbon atoms and create a cyclic ion with a halogen as the intermediate step. In the second step, halogen with the negative charge attacks any of the two carbons in the cyclic ion from the back side of the cycle as in the S_N2 reaction. Therefore stereochemistry of the product is vicinial dihalides through anti addition.

\[\ce{R_2C=CR_2 + X_2 \rightarrow R_2CX-CR_2X} \tag{8.2.1}\]

Halogens that are commonly used in this type of the reaction are: \(Br\) and \(Cl\). In thermodynamical terms \(I\) is too slow for this reaction because of the size of its atom, and \(F\) is too vigorous and explosive. Solvents that are used for this type of electrophilic halogenation are inert (e.g., CCl₄) can be used in this reaction.

Because halogen with negative charge can attack any carbon from the opposite side of the cycle it creates a mixture of steric products.Optically inactive starting material produce optically inactive achiral products (meso) or a racemic mixture.

Electrophilic addition mechanism consists of two steps.

Before constructing the mechanism let us summarize conditions for this reaction. We will use Br₂ in our example for halogenation of ethylene.

Nucleophile	Double bond in alkene
Electrophile	Br₂_, Cl₂
Regiochemistry	not relevant
Stereochemistry	ANTI

Step 1: In the first step of the addition the Br-Br bond polarizes, heterolytic cleavage occurs and Br with the positive charge forms a intermediate cycle with the double bond.

Step 2: In the second step, bromide anion attacks any carbon of the bridged bromonium ion from the back side of the cycle. Cycle opens up and two halogens are in the position anti.

Mechanism 2 a.bmp

Summary

Hallogens can act as electrophiles due to polarizability of their covalent bond.Addition of halogens is stereospecific and produces vicinial dihalides with anti addition.Cis starting material will give mixture of enantiomers and trans produces a meso compound.

References

Vollhard,K.Peter C., and Neil E.Schore.Organic Chemistry:Structure and Function.New Yourk: W.H.Freeman and Company 2007
Chemestry-A Europian Journal 9 (2003) :1036-1044