Addition to c=c multi bonds

Good Morning to You All
Welcome
Dr. V Alex Ramani
SJC, Trichy 2

ADDITION TO CARBONCARBON
MULTIPLE BONDS
Introduction
Mechanisms
Orientation and Reactivity
Examples
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Introduction

Two ways of Addition based on the Substrate

They are given as follows
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Introduction

Four ways of Addition based on the Reagents

Three are twostep processes and the fourth one is a
onestep process. They are given as follows
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Electrophilic Addition

Electrophilic Reagent approaches the double or triple
bond substrate to give positive intermediate

The positive intermediate is attacked by the
nucleophilic species

This reaction may be uni , bi or termolecular

Let us look at the mechanisms
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The stereochemistry of the addition reaction may be
either 'syn' or 'anti' type

Mostly openchain intermediates favours 'syn' addition

Cyclic intermediate favours more preferentially the
'anti' addition

Let us look at these stereochemical aspects
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Year 1911: McKenzie and Fischer independently proved
the antiaddition in case of cyclic intermediate formation

Addition bromine to maleic acid and fumaric acid to give
dlpair and meso forms of 2,3dibromosuccinic acid,
respectively

Later, chemists proved the formation of 70% of trans
isomer when dicarboxyacetylene was brominated
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

It has been found that the addition of bromine is not
stereospecifically favouring antiaddition product

The anti addition depends on the polarity of the solvent

Solvent of low dielectric favours 90100% antiaddition

Solvent of high dielectric favours nonstereospecific
addtion
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Nucleophilic Addition

Nucleophilic Reagent approaches the double or triple
bond substrate to give negative intermediate

The positive intermediate is attacked by the
electrophilic species

This addition reaction is favoured when the substrate
has an electron withdrawing group

Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Dr. V Alex Ramani
SJC, Trichy 2
Note:
When the substrate contains a good leaving group
substitution reaction is more favoured
When electronwithdrawing group is present in the
substrate, addition reaction is more favoured

MULTIPLE BONDS.....
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Michael reaction is a type of nucleophilic addition

This Michael type of reaction can be reversed by
heating either with or without alkali
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The nucleophilic addition is nonstereospecific and
stereoselective

The products are formed based on the favourable
conformation, assumed by the carbanion
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The nucleophilic addition of some reagents – EtOD or
Me3
CSD to transethyl but2enoate shows anti
addition product predominantly

Nucleophilic addition to CC triple bond systems
mostly yield antiaddition product as that of
electrophlic addition
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Free Radical Addition

Free Radical addition is a type of chain reaction

There are three stages – initiation, propagation and
termination of the reaction
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Free Radical addition is nonstereospecific

But, these reactions are mostly stereoselective. Let us
look at some examples
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Free Radical addition is becoming stereospecific at low
temperature in some cases
– The stereospecificity is disappeared at room tempt.
– The addition products are 78% d,l and 22% of meso isomes

It is due to the formation of bridged intermediate
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Cyclic Mechanisms

Some addition reactions go via fourcentre, five
centre or sixcenter intermediate mechanisms showing
a closed ring intermediate / transition state
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Addition to Conjugated Systems

Addition to conjugated double bonded systems shows
1,2addition and 1,4addition

The addition reaction goes via openchain or cyclic
intermediate

Sometimes the reaction is a concerted one
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....


Electrophilic addition via openchain intermediate
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Potential energy diagram of the 1,2 and 1,4addition
reactions
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Formation of some uncommon intermediates during
the addition mechanism
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Nucleophilic addition to conjugated systems follows
the mechanism that resemble the electrophilic
addition mechanism
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

Freeradical addition to conjugated systems follows
the mechanism that resemble the electrophilic
addition mechanism
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Theory of Reactivity of Addition

The reactivity of the substrate towards addition
reaction is influenced by the nature of substituent
present
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The effect of substituent is so great that there is a
generalization as given here.
– simple olefines do not react by nucleophilic
mechanism, and polyhalo or polycyano olefine
do not react by electrophilic mechanism
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The reactivity of the substrate towards electrophilic
addition reaction is shown with an example
Dr. V Alex Ramani
SJC, Trichy 2
Bromination
Hydration

MULTIPLE BONDS.....

The reactivity of alkenic and alkynic substrates have
been observed.
– The alkenes more preferred to show electrophilic
addition and the alkynes show nucleophilic addition
more readily
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

been observed.
– The Free radical additions can occur in any type of
substrate
– The nucleophilic radicals behave like nucleophiles and
the rate is enhanced in presence of electron
withdrawing groups
– The reverse is true for electrophilic radicals
– The deciding factor is the presence of a radical
initiator(If
)
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

been observed.
– Steric influences have profound effect on the reactivity
of these multiple bonded substrates
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Theory of Orientation of Addition

The orientation of reagents in alkenic and alkynic
substrates have been observed.
– When unsymmetrical alkenes or alkynes undergo
addition, the electrophilic part of the reagent goes to
the carbon having more number of hydrogens
– This observation was proposed as a rule ~
Markovnikov's rule. This addition takes place favouring
the formation of a better stabilised intermediate
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

– Presence of electron donating substituents in the
substrate favours Markovnikov's addition
– Presence of electron withdrawing substituents in the
substrate favours antiMarkovnikov's addition
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

– The nucleophilic attack is studied to a very little extent
and it is done only in Michael type of addition
– Presence of electron withdrawing substituents in the
substrate favours nucleophilic attack
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

– The freeradical attack is influenced very much by the
steric factor
– The inductive effect has less impact on this reaction
Dr. V Alex Ramani
SJC, Trichy 2Stability order of the free radical: 3o
> 2o
> 1o

MULTIPLE BONDS.....

The orientation in 5,6 alkenic substrates have been
observed to be intramolecular
– Cyclic products are preferred and Smaller ring types
are more favoured than big rings
– It obeying the Baldwin's rule – ExoTrig addition is
more preferred than the EndoTrig addition
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The orientation in conjugated alkenic substrates have
been observed to be at the tailend part
– The attack by the electrophile, nucleophile or free
radical will be at the terminal carbon of the conjugated
system
– It favours the formation of a stable intermediate
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The orientation in allenic substrates have been
observed to be at the middle carbon
– The attack by the electrophile will be at the middle
carbon of the allenic system
– It favours the formation of a stable intermediate,
though it is an antiMarkovnikov's addition.
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

The stereochemistry of orientation
– For cyclic compounds, the addition may be syn or anti
– In syn addition to unsymmetrical cyclic olefins, the two
groups approach from the lesshindered side.
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

– For cyclic compounds, the addition may be syn or anti
for both electrophilic and nucleophilic reagents
– In synface addition to unsymmetrical cyclic olefins,
the groups approach from the side of electron
withdrawing group.
– In antiface addition, the groups approach from the
side and opposite of electrondonating group
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....

– The addition of Br2
or HOBr are often anti because of
the formation of cyclic intermediate
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Birch Reduction

Aromatic rings undergo reduction showing 1,4
addition of hydrogen to give nonconjugated dienes
– It follows radical ion mechanism
– Electron donating substituents decrease the rate of the
reaction. Electron withdrawing groups increase the
rate of the reaction.
– The radical ion is proved by the epr studies
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Birch Reduction

Aromatic rings undergo reduction showing 1,4
addition of hydrogen to give nonconjugated dienes
– Ordinary olefines do not undergo this reduction
– Phenylated olefines, internal alkynes and conjugated
olefines show this Birch Reduction.
– Benzene shows this reduction to give 1,4diene and not
1,3diene.
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Hydroboration

Olefines on hydrobortion give alcohols, ultimately
– Borane always goes to carbon having more number of
hydrogens.
– It follows markovnikov's addition. Boron is more
electropositive than hydrogen
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Hydroboration

– Borane BH3
or its substituted derivative – BHR2
attacks
the olefinic carbon that is sterically less crowded
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Hydroboration

– Enantioselective hydroboration using
diisopinocampheylborane
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Michael Reaction

Olefines undergo the following addition
– It is a nucleophilic addition
– Olefines must have an electron withdrawing group.
Otherwise, this reaction is not possible
– The nucleophilic reagent also has the electron
withdrawing substituents like COOR, C=O, CN, NO2
,
S=O, etc.
– The reaction is carried out with malonates,
cyanoacetates, acetoacetates, other βketoesters, etc.
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Michael Reaction

Olefines undergo the following addition
– Michael reaction has been carried out traditionally in
protic solvents, with catalytic amount of a base.
Dr. V Alex Ramani
SJC, Trichy 2

MULTIPLE BONDS.....
Michael Reaction

Alkynes undergo the following addition
– Michael reaction has been shown by some alkynic
substrates having an electronwithdrawing group
– In some cases, the Michael reaction is possible under
acidic conditions
Dr. V Alex Ramani
SJC, Trichy 2

Addition to c=c multi bonds

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