9.6: Electrophilic Addition to Alkynes: Halogenation

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.

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.