Electrophilic Addition Reactions of Alkenes

Introduction

Alkenes, a class of unsaturated hydrocarbons characterized by at least one carbon-carbon double bond, exhibit fascinating chemical reactivity. Their unique pi electrons make them prone to undergo addition reactions, in which an electrophile reacts with the carbon-carbon double bond, resulting in the formation of addition products. This class of reactions is known as electrophilic addition reactions of alkenes. While some of these addition reactions follow a free radical mechanism, others involve electrophilic substitution processes. Alkenes also participate in oxidation and ozonolysis reactions, which further broaden their scope of reactions. In this article, we will delve into the concept of electrophilic addition reactions of alkenes, providing examples and insights into the underlying mechanisms.

Electrophilic addition reactions of alkenes encompass a diverse set of reactions. One prominent example is the addition of hydrogen halides like hydrogen bromide (HBr) and hydrogen chloride (HCl). Among hydrogen halides, the general reactivity trend follows: HI > HBr > HCl. Predicting the final product is relatively simpler for symmetrical alkenes like ethene compared to unsymmetrical alkenes such as propene. For example:

Markovnikov formulated a predictive guideline known as the Markovnikov rule to determine the major products in such scenarios. According to this rule, the negative component of the added molecule attaches itself to the carbon atom with a lower count of hydrogen atoms. Consequently, following this rule, the expected product is 2-bromopropane. This concept can be further elucidated by examining the mechanism of electrophilic substitution reactions in alkenes. The general mechanism is outlined as follows:
Hydrogen bromide generates an electrophile, H⁺, which then proceeds to attack the double bond, resulting in the formation of a carbocation.

Since secondary carbocation is more stable than primary carbocation, secondary carbocation predominates the formation of ions.
Finally, Br^- attacks the carbocation to form alkyl halides.

The oxidation reaction of alkenes: Oxidation of alkenes leads to the formation of ketones and alcohols. For example in the presence of a cool, aqueous solution of potassium permanganate, alkenes oxidize to vicinal glycols.

In the presence of acidic potassium permanganate, alkenes oxidize to ketones or acids.