Overview

Introduction

Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.

Reaction Mechanism

In the first step, a π bond from the alkyne acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule, displacing the halide ion and forming a cyclic halonium ion intermediate. In the next step, a nucleophilic attack by the halide ion opens the ring and forms the trans-dihaloalkene. Since the nucleophile attacks the halonium ion from the backside, the net result is an anti addition where the two halogen atoms are trans to each other.

The addition of a second equivalent of halogen across the alkene π bond also proceeds via the formation of a bridged halonium ion to give the tetrahalide as the final product.

For example, the addition of bromine to 2-butyne in the presence of acetic acid and lithium bromide favors anti addition and preferentially forms the trans or (E)-2,3-dibromo-2-butene as the major product. The corresponding cis isomer, (Z)-2,3-dibromo-2-butene, is formed in lower yields. A second addition gives 2,2,3,3-tetrabromobutane.

Reactivity of alkynes and alkenes towards electrophilic addition

Alkynes are less reactive than alkenes towards electrophilic addition reactions. The reasons are twofold. First, the carbon atoms of a triple bond are sp hybridized in contrast to the double bonds that are sp² hybridized. Since the sp hybrid orbitals have a higher s-character and are more electronegative, the π electrons in C≡C are held more tightly than in C=C. As a result, in alkynes, the π electrons are not readily available for the nucleophilic attack, making them less reactive towards electrophilic addition than alkenes.     

Secondly, the cyclic halonium ion formed from alkynes is a three-membered ring with a double bond where the 120° bond angle of an sp² carbon is constrained into a triangle.

Alkyne halonium ion	Alkene halonium ion

In contrast, the cyclic intermediate in alkenes is a three-membered ring with an sp³ hybridized carbon where a bond angle of 109° is constrained into a triangle. Therefore, the larger ring strain associated with the alkyne halonium ions makes them more unstable and hinders their formation. 

Procedure

Electrophilic addition reactions involve the conversion of multiple bonds, such as carbon-carbon double and triple bonds, into other functional groups.

In these reactions, the high electron density around the π bond allows them to function as nucleophiles and attack electrophilic centers. The net result is the addition of a simple molecule across a π bond.

If the simple molecule is a halogen, such as bromine or chlorine, the reaction is called a halogenation reaction. For every mole of the added halogen, one π bond is broken, and two new σ bonds are formed.

Recall that the halogenation of alkenes is a stereospecific reaction that proceeds via an anti addition forming vicinal dihalides.

Halogenation of alkynes follows a similar pattern. However, since alkynes have two π bonds, halogens can add twice across the multiple bonds.

The addition of one equivalent of the halogen forms the trans-dihalide as the major product; another equivalent gives the tetrahaloalkane.

Analogous to alkenes, one of the π bonds in alkynes acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule. 

As this happens, the halogen atom with the partial negative charge leaves as a halide ion, resulting in the formation of a cyclic halonium ion intermediate.

Next, the halide ion attacks either carbon of the halonium intermediate from the backside of the ring, causing the ring to open and form the trans-dihaloalkene.

Further addition of another equivalent of the halogen follows a similar mechanism to yield a tetrahaloalkane.

For example, the addition of one mole of bromine to 2-butyne in the presence of acetic acid and lithium bromide selectively forms E-2,3-dibromo-2-butene. Addition of a second mole of bromine yields 2,2,3,3-tetrabromobutane.

Lastly, alkynes are less reactive to electrophilic additions than alkenes. This is because the π electrons are held more tightly in C≡C bonds than in C=C bonds.

Additionally, the halonium ion formed from alkynes is highly strained and more unstable than the corresponding alkene intermediate.