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