Competition
between E2 and E1 reactions:
In an E1 mechanism reaction, the
rate determining step is unimolecular and does not involve the base. The
leaving group depart in the first step, while the proton is removed in the
second step. In contrast, an E2 reaction has a
bimolecular rate determining step requiring the base. Loss of the
leaving group is simultaneous with the removal of a proton by the
base.
Primary alkyl halide undergoes
only E2 elimination reaction. They cannot undergo E1 reactions
because of the difficulty encountered in forming primary carbocations. Secondary
and tertiary alkyl halide undergo both E2 and E1 mechanism
reaction. For those halides that can undergo both E2 and E1 reaction,
an E2 reaction is favoured by the same factor that favour
an SN2 reaction, and an E1 reaction
is favoured by the same factors that favour an SN1 reaction.
Thus an E2 reaction is favoured by the high concentration of a
strong base and an aprotic polar solvent (example DMSO or DMF), whereas
an E1 reaction is favoured by a weak base and protic polar
solvent (example H2O or ROH).
If the attacking reagent is a good
base, it abstracts the β-proton and causes elimination whereas a good
nucleophile attacks the α-carbon causing substitution.
Elimination vs Substitution:
The following factors influence
the extent of elimination and substitution: (i) structure
of the reactant (ii) nature of the base (iii) nature of
the solvent (iv) effect of temperature.
Structure of the reactant: Generally where
elimination and substitution are competing reactions, the proportion of
elimination increases with increase chain branching of the
reactant. One reason for this preference may be ascribed to greater
stability of highly substituted alkene formed from substrates such as
alkyl halides. The other reason is purely steric and is applicable
only to a Unimolecular reaction where Carbocations are the common
intermediates.
Carbocations are planar in structures
with a bond angle of 1200, and with the increase in branching, they
will show more resistance to the decrease in bond angle (1200 to
109.28o) brought about by the substitution. On the other
hand, elimination will be preferred as it does not involve any change in bond
angles.
Nature of the base: A good
nucleophile may not necessarily be a good base or vice versa. The basicity of
some reagents are found to be in the order:
H2O
< I- < Cl- < CH3OO- < SH- <
OH-
Whereas, the nucleophilicity is of the
same reagents follow different order:
H2O < CH3OO- <
Cl- < OH- < I- < SH-
To some extent, the
elimination/substitution ratio depends upon the nature of the base involved. In
bimolecular reaction (E2 and SN2), for example, if we
have a weak base but strong nucleophile like iodine, there will be more
substitution than elimination. On increasing the concentration and strength of
the base, the unimolecular reaction tends to bimolecularility. This
change in molecularity is more pronounced in E1 than in SN1
reaction, leading to a higher proportion of the elimination product. This is
precisely the reason for employing strong bases at high concentration in the
preparative method or alkenes.
Nature of the solvent: A change
in the polarity of the solvent also affects the course of a reaction.
In bimolecular reaction, for example, E2/SN2 ratio
depends on how well the solvent is able to solvate the two transition states of
the same reactant. Usually, a bimolecular reaction is favoured by a decrease in
the polarity of solvents, but this effect is more pronounced in E2 reaction.
Thus a less polar solvent not only favours bimolecular reaction but also E2
reaction over SN2 reactions.
Effect of temperature: Elimination
reactions usually have higher activation energy than the accompanying
substitution reactions. Therefore increase in the temperature of the
reaction increases the extent of elimination.
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