Topic 11
Organic Chemistry
Note 9
Nucleophilic Substution
A nucleophile is any chemical species that is attracted to a centre of positive
charge. Nucleophiles include
o
o
o
o
Negative ions
Negative ends of polar bonds.
Pi electrons
Lone pairs
The reactions of halogeno alkanes with NaOH provide a good illustration of
nucleophilic substitution.
R
Cl
+
OH-
R
OH
+
Cl-
The chlorine is substituted by the OH- and the halogenoalkane is turned into an
alcohol.
The reaction occurs because in an halogenoalkane, the carbon atom which
carries the halogen has a slightly positive charge the carbon –chlorine bond is
polar.
H
H
H
C
C
H
H
The polarity of the C
C
Cl
X bond increases in the order
I < Br < Cl < F.
F Bonds are very stable (un-reactive), for example CFC’s are stable.
Two types of reaction are possible between an halogenoalkane and a
nucleophile. These two types are distinguished by their rates of reaction and are
designated as first order nucleophilic substitutions and second order
nucleophilic substitutions
The symbols SN1 and SN2 are sometimes used for this. S stands for substitution,
N stands for nucleophilic and 1 and 2 stand foe first and second order.
SN1
Substitution
First order
Nucleophilic
SN2
Second order
First order nucleophilic substitutions take place in two steps:
1. The rate determining step is the slow dissociation of the
halogenoalkane. A relatively stable carbocation is formed. This process
is first order since it only involves one reactant (the halogenoalkane).
2. This then reacts with the nucleophile in a second fast reaction.
Halogenoalkane + OH-
For SN2 Reactions:
Alcohol
Rate = k [halogenoalkane] [OH-]
The halogenoalkane reacts directly with the OH- in a single step reaction.
The rate of the reaction is therefore dependent on the concentrations of both the
OH- and the halogenoalkane.
Second order nucleophilic substitution is found in primary halogenoalkanes.
For SN1 Reactions:
Rate = k [halogenoalkane]
First order nucleophilic substitution is found in tertiary halogenoalkanes. There
are two steps. First the halogenoalkane splits up into two parts forming a halide
ion and a carbocation:
Step 1 (slow ) decomposes into a relatively stable carbocation and the
halide ion.
CH3
H3C
C+
X-
CH3
CH3
H3C
C
+
X
-
CH3
Then the OH- bonds with the carbocation. The curly arrow shows the lone
pair on the oh- being used to bond with the carbocation. This step is fast.
Step 2 (fast)
Nucleophile bonds to the carbocation.
CH3
H3C
C
:Nu
CH3
CH3
H3C
C
CH3
Nu