Elimination reaction involves the loss
of two groups are atoms from a Molecule. The reactions are classified under 2
general headings β- elimination (1, 2 elimination), the most
common elimination reaction, in which groups on adjacent atoms are
eliminated with the formation of an unsaturated bond. β- elimination
includes acid catalysed dehydration of alcohol, solvolytic and base-
induced elimination reaction from sulphonates, alkyl halides and the Hofmann
elimination from quaternary ammonium salts.
The 2nd mode of elimination involves
two groups departing from the same atom. 1,1- or α-elimination are used
for generating the reactive intermediate called carbenes. β- elimination
reaction occurs through 3 mechanistic pathway out of which E1 and
E2 are the most common. These processes are closely related to the
SN2 and SN1 mechanism of substitution. The 3rd mechanism is designated as E1cB
(elimination, Unimolecular of the conjugate base) which involves a
carbanion intermediate and the substrate must contain substituents which
stabilize it. The substrate undergoing E1cB elimination has a leaving
group which is β placed to the carbanion stabilizing group. A good
example is of Knoevenagel reaction. If a substrate undergoing elimination
as a poor leaving group, the transition state of otherwise E2 elimination gains
E1cB character.
The Bimolecular mechanism for elimination
E2 process:
E2 mechanism:
The formation of ethylene on the
treatment of ethyl Bromide with sodium ethoxide is an example of this
type. The rate of alkene formation is proportional to the concentration
of ethyl Bromide as well as that of sodium ethoxide.
In this process substrate and the base
participate in a single step transition state from which the removal of a Proton
β to the leaving group is concerted with the leaving group.
The structure of the organic compound
undergoing elimination and the strength of the base used for the E2 elimination
reflex on the extent of these three Bond changes at the transition state.
This structure gains individual importance which is based on several factors.
Structure (iii) would gain
importance when L- is the for leaving group or if the
negatively charged carbanion is adjacent to the group when -I effect.
Structure (iv) gains importance
when on the other hand L- is a good leaving group while B:
base is a weak base.
Structure (ii), i.e., the
alkene-like character of the transition state becomes significant when L-
is a good leaving group and B: is a strong base.
The transition state (iii)
will have considerable carbanion character when the leaving group is poor,
example NMe3 in Hoffman elimination.
The rate-determining step involves the
breaking of the C-H bond has been shown by Kinetic isotope effect.
Changing H to D can affect the rate of the reaction only if that H (or D)
is involved in the rate-determining step.
It is known that the C-D bond is
stronger than the C-H bond and thus requires more energy to be broken.
The rate of elimination in one should be much faster than 1a which has
indeed been found to be the case.
The direction of elimination in E2
reaction:
With several substrates, the
elimination can take place in more than one way. Generally, the more
substituted alkene is formed as the major product. This generalisation is known
as the Saytzeff rule. When 2 bromobutane reacts with a base 2
elimination products are expected since in the transition state both the C-H as
well as the C-Br bond is breaking. The transition state has alkene like
structure and the factors which stabilize an alkene also stabilizes the
transition state. Thus 2-butane is formed as the major product.
The relative reactivity of alkyl
halide in an E2 mechanism follows the order:
Tertiary alkyl halide > secondary alkyl
halide > primary alkyl halide
This is due to the
predominant-formation of a more substituted alkene.
An exception to the saytzeff rule
is observed from base-induced elimination from quaternary ammonium salts
and from sulphonium salts which gives predominantly the less substituted alkene
(Hofmann rule).
It can be understood that this
difference between the elimination from an alkyl Bromide (saytzeff rule)
and from a quaternary ammonium ion (Hofmann rule) on the basis of the
poor leaving group tendency of amine compared to Br-.
Thus in the E2 elimination of a
quarterly ammonium ion, the C-H bond is almost fully broken in the
transition state and consequently, the structures 1 and 2 may be
considered as important contributors to the transition state. The structure
1 is much more significant since and alkyl group destabilizes an adjacent
negative charge, therefore, the less substituted alkene predominates. In
case of an alkyl Bromide, the transition state will be more alkene-like.
Further, it is seen that with an alkyl
Bromide itself, the Hoffman orientation predominates in case the proton removed
the saytzeff orientation is in a sterically hindered environment. In such
a case the use of sterically hindered base may lead to Hoffman orientation.
The transition state is the
hybrid of structures that help in explaining the orientation observed in the
four 2-halohexane. With X = F the orientation is
largely Hoffman while with X=I the orientation is predominantly
saytzeff. Two factors may be considered, firstly the bond strengths
lie in the order C-I < C-Br< C-Cl <C -F and
secondly the electron withdrawing effect of X follows
the order F > Cl > Br > I. In the fluorine case, the
primary hydrogen is preferentially extracted by base since it allows the
negative charge to develop on a primary carbon which can best accommodate it to
give Hoffman orientation.
The Rate of E2 reactions and
other aspects:
The rate of a reaction increases with
the increasing strength of the base. It also increases with a good
leaving group and the leaving group ability parallels the stability of the
anion. Ethers and alcohols do not undergo E2 elimination reaction since
alkoxide and Hydroxide ions are relatively high energy species while a
sulphonate displays this reaction since sulphonate anion is very stable (the
conjugate base of a strong acid).
The stabilization energy associated
with conjugation in the product formed is partly developed at the transition
state. Thus CH2=CH-CH2-CH2-Br eliminates HBr to give
butadiene more readily when compared with 2-bromobutane.
It has been seen that during E2
elimination saytzeff rule predict the formation of a more substituted alkene
and the exceptions are when the leaving group is poor. In this case, negative
charge will build upon the Carbon from which the proton is lost and therefore
the carbanion stability determines the major alkene product. Moreover, in
several eliminations, the less stable alkene predominates. Example: when
the base is bulky and sterically hindered, the conjugate alkene is more stable
even though it may not be the most substituted alkene. It is more stable than a
non-conjugate product due to resonance. The transition state in such
cases has a partial development of conjugation which provides it with enough
stabilization. Thus the elimination gives the conjugated product as the
major alkene though it is not highly substituted.
Dehalogenation:
An elimination reaction is not only
confined only to those reactions in which one of the leaving group is hydrogen.
Reactive metals such as zinc and magnesium are capable of removing
halogens from 1,2-dihalides to yield alkenes. Like other E2 reactions,
these reaction also proceed by a concerted trans elimination process.
Thus dl-and meso-2,3-dibromobutane yield cis and trans-2-butenes, respectively
on treatment with zinc.
A similar treatment of
Trans-2,3-dibromo-2-butene with zinc results in the formation of
dimethylacetylene.
0 comments:
Post a Comment