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 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

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