Sunday, December 21, 2014

IIT JEE Revision - Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)

Reaction mechanism

Reaction takes place in two steps.

Step 1. Attacking molecule gets partially polarised and as it becomes closer to the pi bond of the double bond, the electron cloud of the pi bond repels the electron cloud of the attacking molecule further. As a result, the nearer end of the attacking molecule acquires partial positive charge. At the same time, electromeric effect comes into operation in the double bond and the pi electron pair shifts to one of the carbon atoms making it negatively charged. Thus the partially positively charged atom of the attacking molecule attacks the negatively charged carbon of the alkenes and a new bond is formed. This leaves the other carbon atom with positive charge and also the other atom of the attacking molecule with negative charge.

Step 2: The negatively charged atom of the attacking molecule reacts with positively charged carbon of the alkene to complete the formation of addition product.

Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)

Halogens (particularly chlorine and bromine) react with alkenes in the presence of an inert solvent (e.g. CCl4) to form dihalogen derivatives:

The reaction with flourine is explosive whereas iodine reacts very slowly.

Alkenes react with halogen acids (HCL, HBr, or HI) to form alkyl halides.

In these reactions one part of the molecule attaches itself to one carbon atom of the double bond whereas the other part to the second carbon atom of the double bond.

However,if the alkene is unsymmetrical, then two products are possible depending upon the carbon atom to which the halogen atom is attached.

Markovnikov rule: during the addition across unsymmetrical multiple bond, the negative part of the attacking reagent joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atoms.

Due to fact that the reaction proceeds according to Markow(v)nikov's explanation, addition of HBr to Propene gives 2-Bromopropene as the major product up to 90%.

Exception to Markovnikov rule - Kharasch effect - Peroxide effect: During the addition of HBr to an unsymmetrical alkene in the presence of organic peroxids (e.g., benzoyl peroxide), Br atom will join to the carbon carrying more hydrogen atoms while H atom will go to the other carbon atom.

Only HBr shows peroxide effect. HF, HCl and HI do not exhibit peroxide effect.

Alkenes react with hypohalous acids (HOX) or halogen Cl2 or Br2 in the presence of H2O to give halohydrins. In this reactin, markonikov;s rule is followed and halogen is the positive aprtg and OH is the negative part.

Water adds to alkenes in the presence of mineral acids (catalytic hydration of alkenes). Addition occurs in accordance with Markownikov's rule and we get alcohols from this addition.

Ethene gives ethanol
Propene gives Propan-2-ol as the major product.

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