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Addition Reaction


Unsaturated hydrocarbons have multiple bonds between carbon atoms of parent chain. On the basis of presence of multiple bonds, unsaturated hydrocarbons can be classified as alkene and alkyne. Alkenes or olefins have double covalent bond between parent carbon atoms whereas alkynes have triple covalent bonds. In case of multiple bonds, one bond is always sigma bond and remaining bonds are pi bonds. For example a double covalent bond is formed by one sigma bond and one pi bond. A sigma bond is formed by axial overlapping of hybrid orbitals. It is always placed in the same plane of bonded atoms. 

On the contrary, a pi bond is formed by side way overlapping of un-hybrid orbitals. In a pi bond, the electron density is distributed in two equal halves above and below the plane. A pi bond is always perpendicular to sigma bond and weaker than sigma bond. That is the reason; unsaturated hydrocarbons are more reactive than saturated hydrocarbons. An unsaturated hydrocarbons can easily give addition reactions to form saturated hydrocarbons. Hydrogenation, halogenation, hydration, ozonolysis are some common examples of addition reactions of unsaturated compounds that converts the unsaturated compounds to saturated compounds.

Hydrogenation of Alkene

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Addition of hydrogen over double or triple bonded carbon atoms is called as hydrogenation reaction. We know that alkenes have double bond therefore each double bond can add two hydrogen atoms to convert double bond to single covalent bond. Hydrogenation of alkene and alkynes can be furnish in the presence of metal catalyst such as nickel (Ni), palladium (Pd), platinum (Pt) or rhodium (Rh) at high temperature of about 150°C. 

Hydrogenation of vegetable oils to form saturated margarine is one of the best example of hydrogenation of alkenes. It converts unsaturated fatty acids to saturated one. The mechanism of hydrogenation of alkenes involves below steps;
  • Adsorption of alkene on metal surface: In the first step, alkene molecules adsorb on the metal surface (adhesion or sticking together) that activate the reactant molecules.
  • Activate reactant molecules involve in bond breaking of pi bond between carbon atoms and bond formation between carbon and the surface of the metal.
  • Next step involves adsorption of hydrogen molecules on the nickel surface that breaks the hydrogen-hydrogen bond and new bonds between hydrogen and the catalyst surface are formed. 
  • Addition of hydrogen on carbon atom desorb carbon atoms from metal surface and release alkane molecule.
This is called as heterogeneous catalysis mechanism as alkene and hydrogen exist in gaseous state with solid metal surface as catalyst. 

Addition Reactions of Alkenes

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Addition of atom or group on multi-bonded carbon atoms is called as addition reaction. It leads to formation of saturated hydrocarbons. Ozonolysis of alkenes forms carbonyl compounds (aldehyde and ketones) whereas addition of HOX leads to formation of halohydrines. 

Addition Reaction

We discussed about hydrogenation of alkenes similarly hydrogenation of alkyne also forms alkane but that will be a two steps reactions which first converts alkyne to alkene then to alkane. Hence conversion of alkyne to alkane requires two moles of H2 compare to alkane.

Another good example of addition reaction of alkene is halogenation. It leads to addition of halogen on double bonded carbon atoms to form vicinal dihalides. Halogenation of alkene follows electrophilic addition mechanism that involves attack of electrophile in first step that forms a cyclic intermediate which further reacts with halide ion. It involves the anti-addition of halogen on double bonded carbon atoms.

Another example of addition reaction of alkene is hydrohalogenation reaction that is addition of Hydrogen and halogen on double bonded carbon atom. 

Like halogenation, hydrohalogenation is also an electrophilic addition reaction in which H+ will add on double bonded carbon atom to form carbocation as intermediate.

Carbocation further reacts with halide ion to form haloalkane. For example; hydrobromination of ethene forms bromoethane.

In case of unsymmetrical alkenes like propene, there are two possibilities during hydrohalogenation. Propene can form 1-bromopropane and 2-bromopropane as product in case of hydrobromination. Now which product will form as major product it depends on mechanism followed by reaction that can be explained on the basis of Markovnikov’s rule. 

Markovnikov Rule

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The reaction of HCl with propene forms 2-chloropropane as major product. Reaction follows electrophilic addition mechanism through the formation of more stable secondary carbocation as an intermediate that leads to the formation of 2-chloropropane as major product.  This is called as Markovnikov’s addition on alkene. According to Markovnikov’s rule, the H of HX will add on that carbon atom which has more number of H atoms as it results the formation of more stable carbocation as an intermediate. 

Markovnikov Rule

So does it means there is no possibility to form minor product as major product? No. It is not so. Hydrohalogenation can also proceed with Anti-Markovnikov’s addition.

It involves addition of H-X in just opposite way as of Markovnikov’s addition means Anti-Markovnikov’s addition will form 1-bromopropane as major product during reaction of HBr with propene. This is called as Anti-Markovnikov’s rule or Kharasch effect. 
Anti-Markovnikov’s addition proceeds in the presence of peroxide with free radical mechanism that leads to formation of more stable free radical with Anti-Markovnikov’s addition of H-X on double bonded carbon atoms. 

Diels Alder Cycloaddition

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Diels Alder cycloaddition can be defined as the cycloaddition reaction of diene with dienophile that results the formation of cyclohexene derivatives. Since this reaction involves the cyclic movement of pi bonds and formation of new sigma bonds, therefore reaction named as cycloaddition reaction. It is a kind of pericyclic reaction. 

Diels Alder Reaction

Dienes are electron rich compounds which reacts with dienophile with some electron withdrawing group (EWG). Due to electron withdrawing group on dienophile, it becomes electron deficient and easily pull pi electrons from diene to form cyclic product.