Addition Reaction Of Alkenes For Class 11 Chemistry
Addition reaction of alkenes
ELECTROPHILIC ADDITION REACTION
The π bond is localized above and below the C-C σ bond. The π electrons are quite far away from the nuclei and are therefore loosely bound.
nuc: = nucleophile (General mechanism of electrophilic reaction of alkenes)
E+ = electrophile
During Electrophilic addition, the pi electrons attack an electrophile, forming a carbocation (the carbon atom with only 3 bonds and a positive charge), on the most stable carbon. After this, the nucleophile attacks the carbocation to form the product. Alkyl groups have a tendency to “push” electrons away from themselves towards the double bond; this increases the density of electron around the double bond. Therefore, alkenes readily undergo Electrophilic addition reactions.
The simplest addition reaction that alkenes undergo is hydrogenation, i.e., the addition of hydrogen (H2) in the presence of a catalyst such as nickel (Ni) or platinum (Pt).
HYDROGENATION OF ALKENES
The hydrogenation of alkenes is a reduction reaction which is also called catalytic hydrogenation. Hydrogenation of a double bond is a thermodynamically favourable reaction because it produces a more stable (lower energy) product. The energy of the product so produced has lower energy than that of the reactant; thus, it is an exothermic reaction (heat is released). The heat that is released is called the heat of hydrogenation.
The simplest source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not result in any noticeable reaction. Although the overall hydrogenation reaction is exothermic, but due to high activation energy, the reaction does not occur. This barrier of high activation energy is overcome by the use of a catalyst, as shown in the reactions below.
HYDRATION REACTION OF ALKENES
This is another important addition reaction in which, addition of water to alkenes done to form an alcohol. The reaction requires a catalyst which is usually a strong acid, such as sulphuric acid (H2SO4).
Electrophilic hydration of alkenes has practical applications in making alcohols for fuels and reagents for other reaction.
*Markovnikov’s Rule is explained below.
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ADDITION OF HYDROGEN HALIDES TO SYMMETRICAL ALKENES
The alkenes undergo addition reactions with the hydrogen halides. A hydrogen atom attaches to one of the carbon atoms originally in the double bond, and a halogen atom to the other.
For example, with Ethene and hydrogen chloride, chloroethane is formed.
With but-2-ene you get 2-chlorobutane
Alkenes react with hydrogen bromide in the cold.
With cyclohexene, bromocyclohexane is obtained.
Below is its MECHANISM:
The reaction of Hydrogen Bromide with Ethene
Reaction of Cyclohexene with Hydrogen bromide
All other hydrogen halides react in the same way, but the rate of reaction increases as one goes from HF to HCl to HBr to HI. This is because, as the halogen atoms get bigger, the strength of the hydrogen-halogen bond falls.
HF < HCl < HBr < HI
The unsymmetrical alkenes like Propene (where the groups at either end of the carbon-carbon double bond are different), react with hydrogen bromide in the cold. In the case of Propene, the product formed is 2-bromopropane.
Or it can be shown as:
The above product is formed in accordance with Markovnikov’s Rule.
It states that when a compound HX is added to an unsymmetrical Alkene, the hydrogen of HX gets attached to the carbon having the most Hydrogen attached to it already. In this case, the hydrogen becomes attached to the CH2 group, because it has more Hydrogen than the CH group.
The mechanism is shown as-
According to Markovnikov’s Rule
The addition of HBr to Propene, the intermediate carbocation formed, is secondary. This is more stable (and so is easier to form) than the primary carbocation which would be produced if the hydrogen became attached to the center carbon atom and the bromine to the one end. The reaction conditions and the factors affecting the rates of the reaction, in case of symmetrical and asymmetrical alkenes are the same.
The reaction of HX and unsymmetrical alkenes happens under the same conditions as with a symmetrical Alkene, but with the unsymmetrical alkenes is a complicated one because of the hydrogen and the bromine can add in two different ways. The way they add depends on whether there is organic peroxides (or oxygen) present or not.
The HBr adds in the “wrong way around” in the presence of organic peroxides, this is often identified as the peroxide effect or anti-Markovnikov addition.
Now check your understanding about the topic by answering the following questions:
Q1. Why do alkenes prefer to undergo an Electrophilic addition reaction?
Q2. In the presence of peroxide addition of HBr to Propene takes place according to anti-Markovnikov’s rule but peroxide effect is not seen in the case of HCl and HI. Explain.
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Tag- Addition Reaction Of Alkenes / Markovnikov’s Rule