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9.6: Adición electrofílica a alquinos: Halogenación
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Adición electrofílica a alquinos: Halogenación
 
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9.6: Adición electrofílica a alquinos: Halogenación

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.

Figure1

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.

Figure2

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.

Figure3

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.

Figure4

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.

Figure5a Figure5b
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. 

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