Difference Between SN1 & SN2
Reaction
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SN2
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SN1
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R-x
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Methyle < Primary...
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Tertiary < Secondary…
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Nucleophile
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Strong Nucleophile
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Poor Nucleophile
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Mechanism
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One step
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Two steps
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Rate limiting step
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Bimolecular T.S
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Carbon cation
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Rate Law
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Rate=k[R-X][Nu]
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Rate=k[R-X]
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Sterochemistry
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Inversion of configuration
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Mixed configuration
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Solvent
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Polar a protic
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Polar protic
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Nucleophilic substitution
Nucleophilic substitution at
carbon is of broad synthetic utility and has received exceptionally detailed
mechanistic study by organic chemists. The goal developing a coherent mechanistic
interpretation was first undertaken by С. К. Ingold and E. D. Hughes in England in the 1930s. Their studies laid
the basis for current understanding. Nucleophilic substitution reactions may
involve several different
combinations of charged and uncharged
species as reactants of.
Aliphatic Substitution
In nucleophilic aliphatic
substitution the attacking (electron donating) reagent(the nucleophile) brings
an electron pair to the substrate, using this pair to form the new bond, and
the leaving group (the nucleofuge) comes away with an electron pair:
This equation says nothing about
charges. Nuclephile Y may be neutral or negatively charged.
Mechanism of Aliphatic Substitution
Several distinct mechanisms are
possible for aliphatic nucleophilic substitution reactions, depending on the
substrate, nucleophile, leaving group, and reaction conditions. In all of them,
however, the attacking reagent carries the electron pair with it, so that the
similarities are greater than the differences.there for these reactions can be
classified as bellow :
The SN2 Mechanism
The designation SN2 stands for substitution
nucleophilic bimolecular. In this mechanism, there is backside attack, the
nucleophile approaches the substrate from a position 180 away from the leaving
group. The reaction is a one-step process with no intermediate The C_Y bond is
formed as the C_X bond is broken to generate transition state(I):
The energy necessary to break the
C_X bond is supplied by simultaneous formation of the C_Y bond. The position of
the atoms at the top of the curve of free energy of activation is represented
as transition state I. Of course, the reaction does not stop here since this is
the transition state. The group X must leave as the group Y comes in, because
at no time can the carbon have more than eight electrons in its outer shell.
When the transition state is reached, the central carbon atom has gone from its
initial sp3 hybridization to an sp2 state with an approximately perpendicular p
orbital. One lobe of this p orbital overlaps with the nucleophile and the other
with the leaving group.
The SN1 Mechanism
The most ideal version of the SN1
mechanism (substitutional nucleophilic unimolecular) consists of two steps,
(once again, possible charges on the substrate and nucleophile are not shown):
The effect of Concentration [R-X]
and [Nu] on SN1&SN2(Knetics for the machanisms):
1. SN2 mechanism:
The reaction between [-SMe] (as
Nucleophile ) and Me-I were indeed SN2 as we would expect:
MeS-Na + Me-I------ <MeS-Me + NaI
First we would keep the concentration
of [SMe] constant and very that of [MeI] and see what happened to the rate
.Then we would keep the concentration of [MeI] constant and very that of [MeS]
and see what happened to the rate.If the reaction is indeed SN2 we should get
linear relationship in both cases. The first graph tells us that the rate is
proportional to [MeI] , that is rate= k1[MeI] and the second graph that is
proportional to [MeSNa] , that is rate =k2[MeSNa] .
The Question Why are the slopes different?
If you look at the rate equation
for the reaction you will see that we have incorporated a constant
concentration of one of the reagents into what appears to be the rate
Constant for the reaction .the
true rate equation is :
Rate
=k2[MeSNa][MeI]
If [MeSNa] constant the equation
becomes:
Rate =k1[MeI]
where k1=k2[MeSNa]
If [MeI] constant the equation
becomes:
Rate =k2[MeSNa]
where k2=k1[MeI]
Slop1=k1=k2[MeSNa],but
slope2 =k2=k2[MeI].
We can easily measure the true
rate constant k2 from these slope because we know the constant values for [MeSNa] in the first experiment and the
for[MeI] in the second .
2. SN1 mechanism:
In this mechanism the starting
material tertiary alkyl halide [t-BuBr] with base [NaOH] the mechanism
following bellow :
He formation of Cation is the rate
determining and the the Nucleophile attach the carbon cation .The rate of
disappearance of t-BuBr is simply the rate of slow step is called the (the rate
determining ) step. It’s a Unimolecular reaction with simple rate of equation :
Rate =k1[t-BuBr]
They look like this when we vary
[t-BuBr] at constant [NaOH] and then vary[NaOH] at constant [t-BuBr].The slope
of the first graph is simply the first order constant because :
Rate =k1[t-BuBr]
But the second graph is zero, the
rate determining step does not involve NaOH adding more of it does not speed up
the reaction. The reaction shows first order kinetics .
References
1. Advanced Organic Chemistry ,fourth edition ,(Francis
A.Carey &Richard J.Sudberg),University of Virginia.
2. Michael B. Smith and Jerry March March’s Advanced
Organic Chemistry: Reactions, Mechanisms, and Structure, Sixth Edition.
3. Morrison and Boyd Organic Chemistry, Sixth Edition.
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