organic chemistry

1. Factors affecting the rate of elimination
Elimination reactions are affected by following factors:-
1.Attacking base
2.Leaving group
3.Reaction medium
These are explained further as follows…
1. Attacking Base
In E1 reactions, attacking base has much a smaller
effect because external base is generally not
required; the solvent itself acts as a base. When
external base is added, reaction proceeds through
E2 mechanism.
A strong base is favourable for E2 pathway. E2
reactions always involve a strong base.
They need chemicals strong enough to pull off a
weakly acidic hydrogen atom. Usually
charged bases like C₂H₅O⁻ and (CH₃)₃CO⁻ are
strong enough for E2 mechanism.
Uncharged bases like H₂O, C₂H₅OH and (CH₃)₃COH
are too weak to remove weakly acidic β H atom.
The order of basicity of Halides is F−>Cl−>Br−>I−.

2. Which means that Fluoride ion is the most basic of all
and it will be most precise for attacking the alkyl halide in
E2 pathway. General concept is The more basic the
attacking base, the more faster the E2 reaction.
2. Leaving Group
The role of leaving group in elimination reactions is
similar to that in the substitution reactions. Reaction
speeds up with a good leaving group. This is because the
leaving group is involved in the rate-determining step. A
good leaving group wants to leave so it breaks the C-
Leaving Group bond faster. Once the bond breaks, the
carbocation is formed and the faster the carbocation is
formed, the faster the attacking base can come in and
the faster the reaction will be completed.
A good leaving group is a weak base because weak
bases can hold the charge. They're happy to leave with
both electrons and in order for the leaving group to
leave, it needs to be able to accept electrons. Strong
bases, on the other hand, donate electrons which is why
they can't be good leaving groups. As you go from left to
right on the periodic table, electron donating ability
decreases and thus ability to be a good leaving group
increases. Halides are an example of a good leaving

3. group whose leaving-group ability increases as you go
down the column.
Since carbocation is formed during E1 reactions, so when
a good L.G is present it will leave the substrate easily,
forming a carbocation faster and meanwhile promoting
the E1 mechanism. Whereas a poor L.G favours the E2
mechanism.
3. Reaction Medium
The most simple concept is that when there is a
polar solvent, it will definitely promote the species
that is more charged or polar. Because a polar
solvent has the ability to stabilize the charged
carbocation intermediate, therefore, a polar
solvent will favour the E1 mechanism as it
involves an ionic intermediate i.e; carbocation.
The highly positive and highly negative parts
interact with the substrate to lower the energy of
the intermediate. Since the carbocation is
unstable, anything that can stabilize this even a
little, will speed up the reaction. Since there is no

4. such intermediate involved in E2 mechanisms,
only a transition state is present, therefore, these
will be promoted by rather a non-polar solvent.
Examples of polar protic solvents are: acetic acid,
isopropanol, ethanol, methanol, formic acid,
water, etc.