Electrophilic Addition Reaction

Electrophilic Addition Reaction

Contents

• Electrophilic addition reaction

• Free radical addition reaction

• 1, 2 – Shift

• Driving force – stability

• 1, 2 – hydride shift

Learning Objectives

At the end of this lecture, student will be able to

• Classify electrophiles

• Explain the mechanism involved in electrophilic addition reactions

• State Markovnikov’s rule

• Explain mechanism involved in Markovnikov’s addition

Addition reaction

• A reaction in which two atoms or ions react with a double bond, forming a compound with the two new groups bonded to the carbons of the original double bond

• Reactions at carbon-carbon double bond

Reactions of alkenes

• Electrophilic addition reaction

• Free radical addition reaction

• Reactive intermediates common in organic chemistry

Addition of Halogens

• Electrophilic addition reaction

• Examples

 

• Addition of Br2 – detection of unsaturation

Mechanism

• Step 1

– Formation of halonium ion

Addition of hydrogen halides

• Electrophilic addition reaction

Regioselective reaction

• An addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed

• Unsymmetrical alkene – regioselective reaction

Markovnikov’s rule

• Markovnikov’s rule, Markovnikov’s addition

       Markovnikov’s rule

       In the addition of HX, H2O, or ROH to an alkene, hydrogen adds to the carbon of the double bond having the greater number of hydrogens

       This regioselectivity was noted by Vladimir Markovnikov who made the generalization known as Markovnikov’s rule

       Provides a way to predict the products of many alkene addition reactions

       It does not explain why one product predominates over other possible products

Problem-01

       Name and draw a structural formula for the product of each alkene addition reaction

Mechanism

• Step 1

– Formation of carbocation


• Step 2

– Formation of alkyl halite

Problem-02

Arrange these carbocations in order of increasing stability

Carbocation (a) is 2°, (b) is 3°, and (c) is 1°. In order of increasing stability they are c < a < b

Carbocation Rearrangements

Rearrangement

A change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material

1,2-Shift

A type of rearrangement in which an atom or group of atoms moves with its bonding electrons from one atom to an adjacent electron deficient atom

• Driving force – stability

       Rearrangement- Either an alkyl group or a hydrogen migrates, each with its bonding electrons, from an adjacent atom to the electron-deficient atom

• 1, 2 – hydride shift

• 1, 2 – methyl shift

       Step-1: Proton transfer to alkene gives 20 carbocation

       Step-2: Migration of methyl group with bonding electrons gives a more stable 30 carbocation 

       Step-3: Reaction of 30  carbocation with chloride ion

1,2 – hydride Shift, Driving force – stability

Summary

• π electrons in the structure of double bond are readily available to electrophilic reagents and free radicals

• Alkenes readily undergo electrophilic addition and free radical addition reactions

• Catalytic hydrogenation involves addition of hydrogen to alkenes in the presence of metal catalyst

• Electrophilic addition reactions of alkenes leads to the synthesis of alkyl halides, vicinal dihalides, halohydrins

• Synthesis of dihalides and halohydrins – takes place through cyclic carbonium ion

• Markovnikov’s addition involves addition of hydrogen halides to alkenes to form alkyl halides

• Markovnikov’s rule states that hydrogen adds to the sp2 carbon of the alkene bonded to the greater number of hydrogens

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