Alkenes belong to the group of unsaturated hydrocarbons that is one molecule of alkene contains at least one double bond. Due to the presence of pi electrons they show addition reactions in which an electrophile attacks the carbon-carbon double bond to form the addition products. These reactions are known as electrophilic addition reactions of alkenes. Sometimes these addition reactions follow free radical mechanism too. Oxidation and ozonolysis reactions are also some prominent reactions of alkenes. Some of these reactions are discussed below:
Electrophilic addition reactions of alkenes: Alkenes exhibit wide range of electrophilic addition reactions. Addition of hydrogen halides such as hydrogen bromide and hydrogen chloride is an example of electrophilic addition reactions of alkenes. The general trend of hydrogen halide is given as: HI >HBr> HCl. For symmetrical alkenes such as ethene it is quite easier to predict the end product in comparison to unsymmetrical alkenes such as propene. For example:
\(CH_2\) = \(CH_2~+~H-Br~\rightarrow~ CH_3-CH_2-Br\)
Markovnikov proposed a rule called Markovnikov rule for the prediction of major product in such cases. Markovnikov rule states that negative part of the adding molecule gets attached to that carbon atom which possesses lesser number of hydrogen atoms. Thus, 2-bromopropane is the expected product by this rule. This can further be explained with the help of mechanism of electrophilic substitution reactions of alkenes. The general mechanism is explained below:
An electrophile, H+ is generated from hydrogen bromide which attacks on double bond to form carbocation.
Since, the secondary carbocation is more stable than the primary carbocation, the secondary carbocation predominates the formation of ions.
Finally, Br– attacks the carbocation to form alkyl halides.
Oxidation reaction of alkenes: Oxidation of alkenes leads to the formation of ketones and alcohols. For example in the presence of cool, aqueous solution of potassium permanganate, alkenes oxidize to vicinal glycols.
In the presence of acidic potassium permanganate, alkenes oxidize to ketones or acids.
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