Halogenation and Hydrohalogenation of Alkenes Such as Cyclohexene

Halogenation and Hydrohalogenation of Alkenes

Halogenation is a chemical process in which a halogen atom is swapped out for another material, and the new molecule or compound eventually contains the halogen atom.

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In a hydrohalogenation process, hydrohalic acids like hydrogen chloride or bromide are electrophilically added to alkenes to produce the corresponding haloalkanes. Keep reading for more info.

Halogenation of Alkenes

A bond system undergoes halogenation when halogen atoms are added. For instance, the substituted alkane 1,2 dibromoethane is created when bromine is added to ethene.

Trans addition is used to carry out the reaction. However, a trans product cannot be separated due to the free rotation around the resultant alkane’s single bond. However, a trans-addition product can be isolated if the initial alkene structure has a limited cycle for a reason other than a double bond. For instance, rotation is prohibited in-ring constructions. The carbon backbone is set up in a ring with no starting or ending carbon atoms. When the cyclohexene, which has one double bond, is halogenated, a cycloalkane is produced that is trans substituted.

Nonpolar molecules include alkenes and halogens. However, both types of molecules can produce induced dipoles under the right circumstances. This causes the molecules to experience attractive forces.

In this case, By cyclization into a three-membered ring with a positively charged bromine atom, the mid-reaction bromomethyl carbocation is commonly internally stabilized. (bromonium ion).

The overlap zone of the bromonium ion’s unequal sharing of the electrons in the covalent link between carbon and bromine is closer to the more electronegative bromine. The carbon atoms in the ring receive a partial positive charge ( +). The charge delocalisation stabilizes the ring structure, and the carbon atoms’ consequent partial positive charges draw the nucleophilic bromide ion.

From the side of the carbocation that is opposite the one that has the bromonium ion connected, A partially positive carbon atom must be approached by the second bromide ion. This is because the creation of a link between the two carbon atoms causes the bromonium ion to obstruct access to the carbon atoms along an entire side. Steric hindrances are used to describe this blockage. Only a trans addition is feasible due to the steric barrier.

Hydrohalogenation of Alkenes

Hydrogen halides are polarized compounds that readily form ions, unlike halogens. Hydrogen halides can electrophilically add to alkenes.

Asymmetrically substituted alkenes produce two products when hydrogen halides are added.

According to the Markovnikov rule, when a hydrogen halide is added to an asymmetrically substituted alkene, the hydrogen atom is joined to the double-bonded carbon coupled to more hydrogen atoms. In contrast, the halide ion is added to the other double-bonded carbon, producing the main product. This configuration creates a more stable intermediate carbocation.

Mechanisms for hydrohalogenation. The dissociation of the hydrogen halide is the initial step in adding a hydrogen halide to an alkene. The H + ion is drawn to the alkene’s bond electrons, forming a complex.

The complex disintegrates, forming a single bond between the hydrogen and one of the double-bonded pair’s carbons. The carbon atom transforms into a carbocation when a portion of the bond is lost. Two distinct carbocations are feasible in asymmetrically substituted alkenes. The more stable carbocation yields the major product, whereas the less stable one yields the minor product. 2 Bromopropane is the main byproduct.

An anti-Markovnikov addition of hydrogen bromide to an alkene is also possible. In anti-Markovnikov reserves, the double bond carbon connected to fewer hydrogen atoms gains an extra from the hydrogen atom of the hydrogen halide. Therefore, in the presence of peroxide, the reaction continues by a free-radical mechanism, with the main product formed from the more stable free radical. For this to occur, the response must proceed by a non-carbocation intermediate.

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