Halogenation

Consider a halogen molecule, such as Br2, which comes into proximity with an alkene’s double bond. The electrons in the double bond exert a repulsive force on the bromine molecule’s electrons, leading to the polarization of the halogen-halogen bond and the creation of a dipole moment. This results in heterolytic bond cleavage, where one halogen acquires a positive charge and acts as an electrophile. The addition reaction that follows is not regioselective but is stereoselective. The stereochemistry of this addition can be elucidated through the reaction mechanism. Initially, the electrophilic halogen (bearing the positive charge) interacts with the pi bond and 2p orbitals of the halogen bond with two carbon atoms, forming a cyclic ion with a halogen as the intermediate. Subsequently, the remaining halide ion (the halogen with the negative charge) targets either of the two carbons in the cyclic ion from the cycle’s rear side, similar to the SN2 reaction. Consequently, the product’s stereochemistry is characterized by the anti-addition of vicinal dihalides.

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During the initial phase of the addition process, the bond between the two bromine atoms undergoes polarization. This leads to heterolytic cleavage, resulting in one of the bromine atoms acquiring a positive charge. This positively charged bromine atom then forms a cyclic intermediate structure with the two carbon atoms from the alkene. In the subsequent phase, the bromide anion targets either carbon atom of the bridged bromonium ion from the rear side of the cyclic structure. The bromine atom within the bromonium ion serves as a sort of barrier, compelling the bromonium anion to initiate its attack from the side opposite to it. This action results in the opening of the ring, positioning the two halogens on diametrically opposite sides. This phenomenon is known as anti stereochemistry, characterized by the two bromine atoms originating from contrasting faces of the double bond. Consequently, the bromine atoms are added in a trans configuration to each other.

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In this particular reaction, the halogens typically employed are Bromine and Chlorine. From a thermodynamic perspective, Iodine is deemed too sluggish for this reaction due to the large size of its atom, while Fluorine is considered too reactive and explosive. The halide ion has the ability to target any carbon from the side opposite to the ring, resulting in a variety of steric products. When the starting material is optically inactive, the products generated are either optically inactive achiral compounds (meso) or a racemic mixture.