Chemistry (Organic Reaction Mechanism and Synthetic Applications)
Organic Reaction Mechanism
UNIT-8
and Synthetic Applications
1. ORGANIC REACTION MECHANISM
A detailed and stepwise description of pathway by which reactant is converted into product is
called reaction mechanism.
Types of Reaction Mechanism :
Mechanism
Addition Substitution Elimination
Reactions Reactions Reactions
Addition Reaction
Addition reaction are the characteristics property of the multiple bond. (e.g. alkenes and alkynes).
A B + X Y A B
X Y
The addition reaction can be divided into four types i.e
1. Electrophilic additions
2. Nucleophilic additions
3. Free Radical additions
4. Concerted additions
1. Free Radical Addition : In general, the addition of halogen acid to unsymmetrical alkenes give
Markownikoff's addition product.
Markownikoff's Addition : When an unsymmetrical reagent added to an unsymmetrical multiple
bond (alkene or alkyne) the negative part of reagent attached to that carbon atom which has lesser no.
of H atom.
CH3 CH CH 2+ HBr CH3 CH CH3
(Markowinkoff's
addition)
Propene Br
(Iso-propyl bromide)
However, this is not always the case. Addition of HBr to unsymmetrical alkenes in presence of
a peroxide gives a product which is different from the Markownikoff's product this is known as
Antimarkownikoff's addition. This is also known as peroxide effect or Kharasch effect.
Peroxide
CH3 CH CH 2 + HBr CH3 CH2 CH2 Br
Propene (Antimarkowinkoff's addition) (n-propyl bromide)
Free radical addition reactions occurs either in the gas phase or in inert non polar solvents in the
presence of UV light or sunlight, heat or catalytic amount of radical initiators such as peroxide, azo
compounds like AIBN (Azo isobutyronitrile) etc.
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Following steps are involved in the free radical mechanism.
(1) Chain Initiation Step : Peroxide dissociates into two free radicals called alkoxy radicals.
Dissociation
RO : OR 2R O
Peroxide Alkoxy radical
R O + H : Br ROH + Br
(2) Chain Propagation Step : Bromine radical adds to a double bond, forming a new free radical
with the odd electron on the carbon atom.
Br + H 2C CH CH3 Br CH2 CH CH3
Bromine Propene 2º free radical
free radical (more stable)
Since secondary free radical is more stable than the primary free radical, only the secondary free
radical is formed.
The alkyl free radical (secondary free radical) reacts with an HBr molecule to generate another
bromine radical.
Br — CH2 — CH — CH3 + HBr Br — CH2 — CH2 — CH3 + Br
(3) Chain Termination Step : Termination occurs by any or all of the reactions which use up
species involved in the propagation steps.
(A) Br + Br Br2
CH3 CH CH2 Br + Br CH2 CH CH3 CH3 CH CH CH3
(B)
CH2Br CH2Br
(C) CH3 CH CH2 Br + Br
CH3 CH CH2 Br
Br
Only HBr gives free radical addition reaction in the presence of Peroxide. HF, HCl and HI
do not gives free radical addition. This is because HCl and HF have large bond energy
so does not gives free radicals.
While HI have minimum bond energy so it is very reactive.
HI H + I
2I I2
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Examples :
X
X2
R CH CH2 X
CCl4
R CH CH2
Cl CCl3
CHCl3
R CH CH2
H CCl3
R CH CH2 RSH
R CH2 CH2 SR'
Peroxide
H2 S
R CH2 CH2 SH
ClCOCl
R CH CH2COCl
O
Cl
R' C H
R CH2CH2COR'
2. Nucleophilic Addition : Such reaction are usually encountered in compound containing Polar
functional groups e.g., carbonyl, – C N, C = S etc.
The most important examples are encountered in compounds containing carbonyl group.
S+ S–
+
O O– or O
The heteroatom i.e. oxygen in the case of aldehyde or ketone, reduce the electron density on
carbonyl carbon.
Thus, attack of nucleophile takes place at carbonyl carbon.
OE
–
O + E Nu
Nu
Mechanism :
–
Slow O E OE
–
O + E Nu
R.D.S. Nu fast Nu
(i) Addition of HCN to Carbonyl Group : The addition is catalysed by acid or base. The base
increases the concentration of CN– So the base catalysed nucleophilic additions are common.
–
HCN + OH – HO
2 + CN
O–
Slow
C O + CN C
R.D.S.
CN
–
O OH
fast
C + HO
2
C + HO
–
CN CN
(Cyanohydrin)
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Acid catalysed addition of HCN
fast
C O+H C OH
CN
slow
C — OH + CN C
OH
(cyanohydrin)
(ii) Bisulphite Addition : In the case of Bisulphite, the effective nucleophile in this case is SO32–
The Product is Bisulphite Crystalline adduct
+ + 2–
NaHSO3 Na + H + SO3
–
O + OH
H
O + SO 32– C
SO3
– SO3Na
(salt)
Examples
CH3 CH3 OH
O + NaHSO3 C
H H SO3Na
CH3 OH
CH3
O + NaHSO3 C
CH3 CH3 SO3Na
(iii) Addition of Grignard Reagents : Addition of Grignard reagent to Carbonyl compound gives
the adduct, which can be hydrolyzed to alcohols.
S– S+ O Mg X H2O/H+ OH
O + R Mg X
R H
(iv) Addition of Alcohol and Thioalcohol :
OR
R CH O + ROH/H
R CH
OR
(Aldehyde) (Alcohol) (Acetal)
SR
+ RSH/H R CH
SR
(thioalcohol) (thioacetal)
R R OR
O + ROH/H
R R OR
(Ketone) (Ketal)
R R SR
O + RSH/H
R R SR
(thio Ketal)
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Organic Reaction Mechanism
UNIT-8
and Synthetic Applications
1. ORGANIC REACTION MECHANISM
A detailed and stepwise description of pathway by which reactant is converted into product is
called reaction mechanism.
Types of Reaction Mechanism :
Mechanism
Addition Substitution Elimination
Reactions Reactions Reactions
Addition Reaction
Addition reaction are the characteristics property of the multiple bond. (e.g. alkenes and alkynes).
A B + X Y A B
X Y
The addition reaction can be divided into four types i.e
1. Electrophilic additions
2. Nucleophilic additions
3. Free Radical additions
4. Concerted additions
1. Free Radical Addition : In general, the addition of halogen acid to unsymmetrical alkenes give
Markownikoff's addition product.
Markownikoff's Addition : When an unsymmetrical reagent added to an unsymmetrical multiple
bond (alkene or alkyne) the negative part of reagent attached to that carbon atom which has lesser no.
of H atom.
CH3 CH CH 2+ HBr CH3 CH CH3
(Markowinkoff's
addition)
Propene Br
(Iso-propyl bromide)
However, this is not always the case. Addition of HBr to unsymmetrical alkenes in presence of
a peroxide gives a product which is different from the Markownikoff's product this is known as
Antimarkownikoff's addition. This is also known as peroxide effect or Kharasch effect.
Peroxide
CH3 CH CH 2 + HBr CH3 CH2 CH2 Br
Propene (Antimarkowinkoff's addition) (n-propyl bromide)
Free radical addition reactions occurs either in the gas phase or in inert non polar solvents in the
presence of UV light or sunlight, heat or catalytic amount of radical initiators such as peroxide, azo
compounds like AIBN (Azo isobutyronitrile) etc.
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Following steps are involved in the free radical mechanism.
(1) Chain Initiation Step : Peroxide dissociates into two free radicals called alkoxy radicals.
Dissociation
RO : OR 2R O
Peroxide Alkoxy radical
R O + H : Br ROH + Br
(2) Chain Propagation Step : Bromine radical adds to a double bond, forming a new free radical
with the odd electron on the carbon atom.
Br + H 2C CH CH3 Br CH2 CH CH3
Bromine Propene 2º free radical
free radical (more stable)
Since secondary free radical is more stable than the primary free radical, only the secondary free
radical is formed.
The alkyl free radical (secondary free radical) reacts with an HBr molecule to generate another
bromine radical.
Br — CH2 — CH — CH3 + HBr Br — CH2 — CH2 — CH3 + Br
(3) Chain Termination Step : Termination occurs by any or all of the reactions which use up
species involved in the propagation steps.
(A) Br + Br Br2
CH3 CH CH2 Br + Br CH2 CH CH3 CH3 CH CH CH3
(B)
CH2Br CH2Br
(C) CH3 CH CH2 Br + Br
CH3 CH CH2 Br
Br
Only HBr gives free radical addition reaction in the presence of Peroxide. HF, HCl and HI
do not gives free radical addition. This is because HCl and HF have large bond energy
so does not gives free radicals.
While HI have minimum bond energy so it is very reactive.
HI H + I
2I I2
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Examples :
X
X2
R CH CH2 X
CCl4
R CH CH2
Cl CCl3
CHCl3
R CH CH2
H CCl3
R CH CH2 RSH
R CH2 CH2 SR'
Peroxide
H2 S
R CH2 CH2 SH
ClCOCl
R CH CH2COCl
O
Cl
R' C H
R CH2CH2COR'
2. Nucleophilic Addition : Such reaction are usually encountered in compound containing Polar
functional groups e.g., carbonyl, – C N, C = S etc.
The most important examples are encountered in compounds containing carbonyl group.
S+ S–
+
O O– or O
The heteroatom i.e. oxygen in the case of aldehyde or ketone, reduce the electron density on
carbonyl carbon.
Thus, attack of nucleophile takes place at carbonyl carbon.
OE
–
O + E Nu
Nu
Mechanism :
–
Slow O E OE
–
O + E Nu
R.D.S. Nu fast Nu
(i) Addition of HCN to Carbonyl Group : The addition is catalysed by acid or base. The base
increases the concentration of CN– So the base catalysed nucleophilic additions are common.
–
HCN + OH – HO
2 + CN
O–
Slow
C O + CN C
R.D.S.
CN
–
O OH
fast
C + HO
2
C + HO
–
CN CN
(Cyanohydrin)
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Acid catalysed addition of HCN
fast
C O+H C OH
CN
slow
C — OH + CN C
OH
(cyanohydrin)
(ii) Bisulphite Addition : In the case of Bisulphite, the effective nucleophile in this case is SO32–
The Product is Bisulphite Crystalline adduct
+ + 2–
NaHSO3 Na + H + SO3
–
O + OH
H
O + SO 32– C
SO3
– SO3Na
(salt)
Examples
CH3 CH3 OH
O + NaHSO3 C
H H SO3Na
CH3 OH
CH3
O + NaHSO3 C
CH3 CH3 SO3Na
(iii) Addition of Grignard Reagents : Addition of Grignard reagent to Carbonyl compound gives
the adduct, which can be hydrolyzed to alcohols.
S– S+ O Mg X H2O/H+ OH
O + R Mg X
R H
(iv) Addition of Alcohol and Thioalcohol :
OR
R CH O + ROH/H
R CH
OR
(Aldehyde) (Alcohol) (Acetal)
SR
+ RSH/H R CH
SR
(thioalcohol) (thioacetal)
R R OR
O + ROH/H
R R OR
(Ketone) (Ketal)
R R SR
O + RSH/H
R R SR
(thio Ketal)
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