9.6: Electrophilic Addition to Alkynes: Halogenation
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.
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.