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NUCLEOPHILIC SUBSTITUTION SUBSTITU TION IN ALKYL HALIDES
In nucleophilic substitution of alkyl halides, the halide group is replaced by a nucleophile. R-X
+
Nu
Alkyl Alkyl halide
R-Nu
X-
+
Nucleophile
ACi
Nucleophili Nucleophilicc substitution in alkyl alkyl halides halides is of two types. 1) S N2 - Bimolecular Nucleophilic Substitution hem stry 2) S N1 - Unimolecular Nucleophilic Substitution
1) SN2 - Bimolecular Nucleophilic Substitution E.g., Substitution of Methyl bromide by hydroxyl ion.
CH 3 -Br
+
OH
-
CH 3 -OH
-
+
Br
Hydroxyl ion by Pre Prepared pared by
Methyl bromide
* The rate of substitution is proportional to the concentration of both alkyl alkyl halide halide and nucleophile.
V. A d i t ya Var d h an or
rate rate
[CH [CH 3 -Br] Br] [OH - ]
rate rate
k[CH k[CH3 -Br] Br] [OH [OH - ]
Rate law indicates the involvement of both alkyl halide h alide and nucleophile nucleoph ile in the rate determining transition state.
o ccurs in one step. A transition state is formed during during this t his Mechanism : The reaction occurs step in which the bond making and bond breaking occurs simultaneously.
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Br - CH3
.. . .OH..
[Br - - - - CH3 - - - - OH]
-
-
Br
CH3-OH
Transition state
[Br - - - - CH 3 - - - - OH] y g r e n e e e r F
-
ACi G
Br - CH3
+
.. . .OH ..
†
CH3-OH
Progress of reaction
-
+ Br
hem
stry
* The presence of bulky ‘R’ groups around the halogen containing carbon, hinders the attack of nucleophile. Due to this steric hindrance, the rate of nucleophilic substitution by S N2, decreases with increase in number of ‘R’ groups. Decreasing order of rates in S N2 is as follows. CH3-X > Primary halides > Secondary halides > Tertiary halides
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* Stereochemistry: The nucleophile always attacks from the opposite side of the C-X bond. It is called back side attack. Hence the stereochemistry of the chiral carbon is always inverted leading to the formation of optically pure compound. R R'
R
ACi X
+
R''
hem
OH
-
X
R
R' R'
-
CH
OH
R''
R''
stry
OH
configuration is inverted
Transition state
* The rate of the reaction also depends on nature of halide group and the nucleophile in S N2 mechanism. In S N2, decreasing order of rate of reaction with different leaving groups is as follows. R-I > R-Br > R-Cl > R-F And the influence of nucleophile on the rate is as follows. Prepared RS- > I- > CN- > CH3O- > Br - > NH3 > Clby > F2) SN1 - Unimolecular Nucleophilic Substitution E.g., Substitution of tert-Butyl bromide by hydroxyl group in water.
V. Aditya Vardhan CH3
H3C
C
CH3
+
Br
H2O
H3C
CH3
C
+
OH
HBr
CH3
tert - Butyl bromide
tert - Butyl alcohol
www.adichemistry.com * The rate of substitution is proportional to the concentration of alkyl halide only. rate
[tert Butyl bromide]
or rate k[tert Butyl bromide] Rate law indicates the involvement only alkyl halide in the rate determining step. Mechanism: S N1 reaction occurs in two steps as follows.
CH3
1)
H3C
C
CH3 Br
CH3
C+ CH3
Slow step - rate determining
Br
tert - Butyl carbocation
CH3
CH3 H3C
+
C+
CH3
tert - Butyl bromide
2)
ACi
H3C
-
..
H2O ..
H3C
C
+
O
CH3 H
+
-H
H
hemCH stry 3
H3C
C
OH
fast step
CH3
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Prepared by V. Adit ya vardhan
m o c . y r t s i m e h c i d a . w w w t a r o h t u A e h t
-
ACi
y g r e n e e e r F
hem
-
CH3
stry
H3C
CH3 H3C
C
CH3
CH3 H3C
C
O
+
U p d a t e s & k e y s w i t h
CH3
H H3C
CH3 H
Br
C
OH
CH3
CH3
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Progress of reaction Prepared by
* The rate of reaction increases with increase in the stability of carbocation formed in the rate determining step. The stability of carbocations increases with the number of alkyl groups. Alkyl groups stabilize the carbocation by +I inductive effect. Hence the order of rates with different halides in S N1 reaction is as follows. Tertiary halides > Secondary halides > Primary halides > CH 3-X Benzyl and allylic halides react more faster than tertiary halides due to resonance stabilization of benzyl and allyl carbocations.
V. Aditya Vardhan
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CH2
+
CH2
CH2
CH2
+
+
Resonance structures of benzyl cation
* Stereochemistry: The nucleophile may attack from either side of the plane of carbocation formed. Hence a racemic mixture is formed when optically active alkyl halide is substituted by S N1 mechanism.
X
R R'
R
ACi R
R'
R'
+ R''
OH
R''
R''
+
HO
R R' R''
Racemic mixture
OH
-
hem
stry
Attack on carbocation from both the sides is possible
* The rate of the reaction also depends on nature of halide group in S N1 mechanism. In S N1, decreasing order of rate of reaction with different leaving groups is as follows. R-I > R-Br > R-Cl > R-F * There is no effect of nature of nucleophile on S N1 reaction as it does not participate in the rate determining step.
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