Question

Discuss Different Types of Reactions of Alkenes giving at least two examples for each type. Your explanation should include relevant mechnisms and references to actual uses in life.

Answer 1

Alkenes has a double bonds (pi electrons) in it. These are stable compounds, but more reactive than alkanes because of the presence of C=C double bond. Mostly alkenes undergo addition reaction to form a new single (sigma) bond. Now we will discuss few such reactions.

1) Electrophilic addition reactions

In this reaction, the electrophile reacts with alkene, and these are added to the double bond. Here, the formation of most stable carbocation takes place during the reaction. Example and mechanism shown below.

Example 1: Addition of HCl to alkene: This reaction follows Markovnikov's rule that is, nucleophile added to the more highly substituted carbon.

Mechanism: In the first step the formation of most stable carbocation takes place because of the addition of a hydrogen to the double bonded carbon. Then the attack of chloride on the carbocation and gives a additional product.

CH CH H3C Нас H3C 2-methylbut-2-ene hydrogen chloride Tertiary carbocation H2C C H3C H2c 2-chloro-2-methylbutane

Example 2: Addition of water also follows same mechanism mentioned above. Here -OH^- acts as nucleophile which attacks on the carbocation. And hence it forms a alcohol as product.

он ethanol ethene

2) Reduction of alkenes:

Addition of hydrogen across C=C bond to form a alkane (newly formed sigma bond) in the presence of a catalyst. This reaction also called as catalytic hydrogenation.

Example 1 Addition of hydrogen (reduction of limonene) to limonene CH CH PtO2 (catalyst), H2 H2C CH H3C CH p-Menth-1(2)-ene Limonene Example 2: Addition of hydrogen (reduction of limonene) to selective of alkene using palladium catalyst Pd /H2 NC NC CH CH2 4-ethenylbenzonitrile 4-ethylbenzonitrile

3) Oxidation of alkene

There are many type of oxidation of alkene such as epoxidation, dihydroxylation, reductive workup (gentle cleavage) and strong cleavage (oxidative workup). Which depends on the type of reagent we take.

Example: Oxidation of Oleic acid in the presence of KMnO4 in the presence of sodium hydroxide gives 1,2 diol as product.

H2C OH Oleic acid KMnO4 NaOH HO OH H2C OH

Answer 2

Alkene give ploymerization reaction in which small organic molecules (monomers) combine to form large molecule (polymer) Example .ethene molecule combine to form polyethylene molecule Condition ,400°C 100atm pressure treaces amount of oxygen 0.1%