In this reaction, alkene reacts with HX where X can be Br, Cl or I. To predict the product easily, you have to turn double bond into single and add X (Cl, Br or I) to the most substituted carbon from the double bond carbons. This follows Markovnikov’s Rule which states that halogen (or another electrophile) is added to the MOST substituted carbon from the double bond. The most substituted carbon is the carbon that is attached to the most other carbon atoms. Hydrogen is added to the other, less substituted carbon from the double bond.
We do need to watch out for rearrangements in this reaction which I will cover later.
Addition Of Hydrogen Halides To Alkenes (Hydrohalogenation) Mechanism
In this mechanism, there are two steps. In the first step, the double bond attacks the hydrogen, and the bond between the hydrogen and halogen breaks, transferring electrons to the halogen. The resulting product is an intermediate, where a carbocation (C⁺) forms on the most substituted carbon from the double bond, while the other carbon gains the hydrogen. In the second step, the halide ion attaches to the carbocation. Since the carbocation is planar, the halogen can attach from either the top or bottom, resulting in two different stereochemistries and possible products (halogen attached on a wedge or a dash).
Practice Predicting the Products of Hydrohalogenation
Carbocation Rearrangements
Any mechanism that has a carbocation intermediate can possibly have a carbocation rearrangement.
A rearrangement will occur if there is a MORE substituted carbon neighboring the carbocation. If possible, a hydrogen will shift. If there is no hydrogen, an alkyl group, such as methyl, will shift.
In the hydride shift example, the initially formed carbocation is secondary because it is attached to only two carbons. However, the neighboring carbon to its left is more substituted, because it is tertiary. The hydrogen from the carbon on the left will attach to the carbocation, creating a new more substituted carbocation. After this rearrangement is done, chlorine attaches to the carbocation.
In the methyl shift example, the initially formed carbocation is secondary because it is attached to only two carbons. However, the neighboring carbon to its left is more substituted because it is attached to four carbons. One of the methyl groups from the carbon on the left will attach to the carbocation, creating a new more substituted carbocation. After this rearrangement is done, chlorine attaches to the carbocation.
Please note that alkyl groups other than methyl can rearrange as well.
Summary
LINKS
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References:
Organic Chemistry by David Klein
Organic Chemistry by Leroy G. Wade, Jr
Organic Chemistry by Maitland Jones, Jr
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