What are Bimolecular Nucleophilic substitution(SN2)
Reactions?
Discuss factors affecting the rate of these reactions.
Bimolecular Nucleophilic substitution(SN2)
Reactions:
The abbreviation SN2 conveys the information,
substitution-nucleophilic-bimolecular. In SN2 reactions only one
step is involved without the formation of intermediate. Nucleophile attacks the
substrate from the side opposite to the leaving group, i.e. from the backside. This
results in an inversion process for
the other groups on carbon center under attack, rather like an umbrella turning inside out in a
violent gust of wind.
The bond formation between the nucleophile and substrate
and the bond cleavage resulting in the removal of leaving group occur
simultaneously, i.e. the reaction is concerted
one.
As the nucleophile starts making bond with the central
carbon atom of the substrate, the leaving group starts leaving it and in the
transition state both the nucleophile and the leaving group are loosely bound
to the carbon atom such that at no time the carbon atom has more than eight
electrons in its outer shell.
The hybridization of the central carbon atom changes from
sp3 to sp2 in the transition state and again to sp3
in the product. The nucleophile-carbon-leaving group bonding is linear, and
three groups around carbon are in planar array. This is the neutral arrangement
to avoid steric interactions. Unhybridized p orbital is perpendicular to the
plane and having lobe on either side of the plane; one lobe of the p orbital is
engaged by the nucleophile and the other by the leaving group.
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| SN2 reaction mechanism |
Molecularity:
Since the two molecules, i.e. the nucleophile and the
substrate are involved in the formation of the transition state in the
rate-determining step(the only step of the reaction), this reaction is said to
be bimolecular.
Kinetics of SN2 reaction:
In SN2 mechanism in rate determining step both
nucleophile and substrate are involved, the rate of reaction should therefore
depend upon the concentration of both, i.e. reaction should follow second-order kinetics.
Rate =
k[CH3Cl][OH-]
However if nucleophile is used in large access, e.g. the
solvent acts as a nucleophile the mechanism will still be bimolecular but
kinetics will appear as first order. The rate will only depend on the
concentration of substrate, such a reaction is said to be pseudo-first-order.
Stereochemical study of SN2 reaction:
Since nucleophile attacks from the backside , inversion of
configuration should take place. In the reaction of radioactive iodide ion with
optically active (+)-R-2 –iodobutane, the substitution of I in the substrate by
I- wil produce enantiomer of the substrate, i.e. (-)-S-2-iodobutane.
The nucleophile iodide is the same as the leaving group. Therefore inversion of
configuration merely converts (+) isomer into (-) isomer. As a result the
optical activity gradually disappears and becomes zero as mixture becomes
racemic form.
We are never going to get complete conversion of (+) isomer
into (_) isomer because reverse reaction also may occur.
Factors affecting SN2 reactions:
Effect
of substrate: The SN2 reactions proceed through a
transition state that involves five groups attached to the central carbon atom
and rather becomes over crowded. The attack of nucleophile from the
backside to a tertiary carbon atom is
sterically more hindered than to a primary carbon atom. Order of reactivity for
alkyl halides is methyl halide>
primary halide> secondary halide> tertiary halide.
Effect
of nucleophile: The SN2 reaction require a
nucleophile to push off the leaving group in the rate-determining step. The
rate of this reaction therefore greatly depends on the nucleophile, e.g. the
rate of hydrolysis of methyl bromide that proceeds by SN2 mechanism
increases by more than 5000 times when the nucleophile is changed from H2O
to OH-.
A good Lewis base is a good nucleophile. The order of
nucleophilicity of RO->HO->ArO->H2O is the same as the order of their basicity.
However in case of halides the order of nucleophilicity is I->Br->Cl->F-
opposite to the order of their basicity.
The more the atoms are polarizable
the more is the nucleophilicity. Similarly less sterically hindered specie are strong nucleophile, e.g.
tert-butoxide, although its basicity is the same as methoxide ion, it is a weak
nucleophile.
Effect
of solvent: Nucleophilic reactions usually take place in
solution. In SN2 reaction a charge may be created, dispersed or
destroyed as the reaction proceeds toward the transition state. The reaction in
which charge is created in the transition state is favoured by increasing the
polarity of the solvent, whereas in the reaction where charge is dispersed or
destroyed in the transition state is favoured by decreasing the polarity of the
solvent.
A polar solvent may decrease the nucleophilicity of the nucleophile
by solvation.
Energy Diagram for SN2 reaction:
The following energy diagram shows energy changes during
the reaction between methyl chloride and OH-. It is a single step
process so the diagram has only one curve.
Some Important MCQs:
1.
2.
For details on SN1
reactions visit (Unimolecular nucleophilic substitution reactions SN1 )
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