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 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 of Electrophilic Addition Reactions

• Step 1

– Formation of
carbocation

• Step 2

– Formation of alkyl
halide

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