Nucleophilic substitution
“NS”
Lab #4
Substitution reaction:
A reaction in which one atom, ion, or group is substituted
for another.
It is the reaction of an electron pair donor (the nucleophile,
Nu) with an electron pair acceptor (the electrophile).
e.g.
HO:¯ + CH3CH2-Br → CH3CH2-OH + Br:¯
Good nucleophile:
1- have free pair of electrons (-ve charge ).
e.g. ¯OH
2- the nucleophilicity is decrease while the electronegativity
increase.
e.g. ¯ OH > ¯ F ,
I > Br > Cl > F
3-higher molecular weight will be higher nucleophilicity.
e.g. I > Br > Cl > F
Leaving group:

Any group that can be displaced from a carbon atom.

Good leaving groups are those capable of forming stable
ions or molecules upon displacement from the original
molecule.
SN2 mechanism (single step)
there is simultaneous formation of the carbon-nucleophile
bond and breaking of the carbon-leaving group bond
(concerted).
SN2 mechanism (single step)
Energy :
SN2 mechanism (single step)

Energy of the molecules (average energy )

Increase the energy by heating to reach the transition
state(kinetic energy).
Eact= kinetic energy + average energy
SN2 mechanism (single step)
Rate:
depend on
1.
Temperature
2.
Solvent
3.
The concentration of the reactants R + Nu
SN2 mechanism (single step)
Example :
SN2 mechanism (single step)
Example :
CH2CH2CH2CH3 + NaI
Cl
acetone
CH2CH2CH2CH3 + NaCl
I
SN1 mechanism(a multi-step)
In an SN1 there is loss of the leaving group generating an
intermediate carbocation which then undergoes a rapid
reaction with the nucleophile.
SN1 mechanism(a multi-step)
This pathway is multi-step process:
step 1: slow loss of the leaving group, LG, to generate a
carbocation intermediate (rate determining step).
step 2: rapid attack of a nucleophile on the electrophilic
carbocation to form a new bond
SN1 mechanism
Example:
Step1:
Step2:
SN1 mechanism
Energy:
SN1 mechanism

step1: enough energy must be supplied to break the c-x
bond (Eact) & give the carbocation (intermediate).

step2: need lower Eact
SN1 mechanism
Rate :
Depends on
1- the concentration of the reactant R
2-temperature
3-solvent
SN1 mechanism
Reactivity order of -R :
X
X
(CH3)3C- > (CH3)2CH- > CH3CH2- > CH33°
2°
1°
Stability: the key step is the loss of the leaving group to
form the intermediate carbocation. The more stable the
carbocation is, the faster the SN1 reaction will be.
Which substitution mechanism
might operate?
SN1
SN2
1-nucleophile
Nucleophile strength is
unimportant.
Strong nucleophile is
required
2-substrate
3° > 2°
1° > 2°
3-steps
Multi-step
4- Formation of intermediate
carbocation
not 1°
not 3°
Single step
√
X
Nucleophilic substitution of alkyl
halides
Reactions :
SN1:
X= halides (Cl , Br, I )
R=
CH3CH2O- =
---> Leaving group
---> electrophile (e acceptor)
---> nucleophile (e donor)
Nucleophilic substitution of alkyl
halides
Procedure (SN1 reaction):
1- label 5 clean, dry test tubes from 1to5.
2-add 3drops of one of the following halides in test tube
and immediately stopper with each addition:
(1) 2-bromobutane.
(2) 1-chlorobutane.
(3) 2-chloro-2-methylpropane.
(4) 2-bromo-2-methylpropane.
(5) bromobenzene.
Nucleophilic substitution of alkyl
halides
3-in each test tube add 1ml of (1% AgNo3 in ethanol solution) then
immediately stopper the test tube.
4-mix thoroughly and record the times for each reaction to form a
precipitate or cloudiness.
5-after 5 minutes, place any test tubes that do not contain a precipitate
in 100°C water bath, after about 1 minute of heating, cool the test
tubes to room temperature and note whether a reaction has
occurred.
NOTE: consider the alkyl halide to be unreactive if no turbidity or
precipitate appears after 10 min
Nucleophilic substitution of alkyl
halides
Test
tube no.
Alkyl halide
1
2-bromobutane
2
1-chlorobutane
3
2-chloro-2-methylpropane
4
2-bromo-2-methylpropane
5
Bromobenzene
Time
Chemical reaction
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