Nucleophilic Substitution Reactions (SN1, SN2) AND Elimination Reactions (E1, E2)
Nucleophilic Substitution Reactions (SN1, SN2) AND Elimination Reactions (E1, E2)
Nucleophilic Substitution Reactions (SN1, SN2) AND Elimination Reactions (E1, E2)
1. SN1
2. SN2
1. SN1
The "1" represents the fact that the rate-determining stepis unimolecular. Thus, the rate
equation is often shown as having first-order dependence on electrophile and zero-
order dependence on nucleophile. This relationship holds for situations where the
amount of nucleophile is much greater than that of the carbocation intermediate.
Instead, the rate equation may be more accurately described using steady-state
kinetics. The reaction involves a carbocation intermediate and is commonly seen in
reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under
strongly acidic conditions, with secondary or tertiary alcohols.
MECHANISM
An example of a reaction taking place with an SN1 reaction mechanism is
the hydrolysis of tert-butyl bromide with water forming tert-butanol:
EXAMPLES
An example ofthe Sn1 Mechanism
2.SN2
In the SN2 reaction, the addition of the nucleophile and the departure of the leaving
group occur in a concerted(taking place in a single step) manner, hence the name SN2:
substitution, nucleophilic, bimolecular. In the SN2 reaction, the nucleophile approaches
the carbon atom to which the leaving group is attached. As the nucleophile forms a
bond with this carbon atom, the bond between the carbon atom and the leaving group
breaks. The bond making and bond breaking actions occur simultaneously. Eventually,
the nucleophile has formed a complete bond to the carbon atom and the bond between
the carbon atom and the leaving group is completely broken.
1. The rate of the reaction depends on the concentration of both the nucleophile
and the molecule undergoing attack. The reaction requires a collision between
the nucleophile and the molecule, so increasing the concentration of either will
increase the rate of the reaction.
2. Since the unique geometry of back side attack is required, the most important
factor in determining whether an SN2 reaction will occur is steric effects. Steric
effects refer to the unfavorable interaction created when atoms are brought too
close together. In effect, if the nucleophile or the molecule undergoing attack
have too many substituents or substituents which are too bulky, the reaction
cannot occur since the nucleophile will be unable to get close enough to the
molecule to do a backside attack.
3. With primary and secondary alkyl halides, the SN2 reaction occurs
MECHANISM
In the example of the SN2 reaction, the attack of Br− (the nucleophile) on an ethyl
chloride (the electrophile) results in ethyl bromide, with chloride ejected as the leaving
group.:
. In the following example, the hydroxide ion is acting as the nucleophile and bromine is
the leaving group:
Because of the backside attack of the nucleophile, inversion of configuration occurs.
ELIMINATION REACTIONS
there are two mechanisms of substitution (SN2 and SN1), there are two mechanisms of
elimination (E2 and E1).
The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation,
analogous to the SN2 and SN1 mechanisms. E2 and SN2 reactions have some
features in common, as do E1 and SN1 reactions
1) E1 MECHANISM
E1 is a model to explain a particular type of chemical elimination reaction. E1 stands
for unimolecular elimination and has the following specificities.
SALIENT FEATURES OF E1
a) Identity of R group –
More substituted halides react faster Rate: R 3CX > R 2CHX > RCH 2 X
c) Leaving group
d) Type of solvent –
Favored by polar protic solvents, which can stabilize the ionic intermediates
EXAMPLES
:
E2 MECHANISM
During the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of
chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination. The
reaction involves a one-step mechanism in which carbon-hydrogen and carbon-
halogen bonds break to form a double bond (C=C Pi bond).
SALIENT FEATURES OF E2
E2 is a single step elimination, with a single transition state.
It is typically undergone by primary substituted alkyl halides, but is possible with
some secondary alkyl halides and other compounds.
The reaction rate is second order, because it's influenced by both the alkyl halide
and the base (bimolecular).
Because the E2 mechanism results in the formation of a pi bond, the two leaving
groups (often a hydrogen and a halogen) need to be antiperiplanar.
An antiperiplanar transition state has staggered conformation with lower energy than
a synperiplanar transition state which is in eclipsed conformation with higher energy.
The reaction mechanism involving staggered conformation is more favorable for E2
reactions (unlike E1 reactions).
MECHANISM
E2 typically uses a strong base. It must be strong enough to remove a weakly acidic
hydrogen.
In order for the pi bond to be created, the hybridization of carbons needs to be
lowered from sp3 to sp2.
The C-H bond is weakened in the rate determining step and therefore a
primary deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
E2 competes with the SN2 reaction mechanism if the base can also act as a
nucleophile (true for many common bases).
For instance, the base-induced elimination of "HX" (dehydrohalogenation) of an alkyl
halide gives rise to an alkene (illustrated below for the conversion of tert- butyl
bromide to isobutylene).
E2 eliminations, in contrast to E1 reactions are promoted by strong base. The
base vital to the reaction; it is directly involved in the rate-determining step. The
reaction is bimolecular--that is, it involves "second-order kinetics--because two
molecules must come together for the reacti on to occur. The mechanism of an E2
elimination reaction is shown below:
Notice that the hydrogen that is removed is on the carbon atom that is adjacent to
the one bearing the halogen. For some reason, beginning students are often
confused on this point, although it is mysterious as to why they should be. Carbon-
carbon double bonds, by definition,exist between two adjacent carbon atoms.
Likewise, the "H" and the "X" atoms that are eliminated during the
dehydrohalogenation of an alkyl halide must be on the carbon atoms.
There are close parallels between E2 and SN2 mechanisms in how the identity of the
base, the leaving group and the solvent affect the rate.
a. Strenght of bases
The base appears in the rate equation, so the rate of the E2 reaction increases as the
strength of the base increases. E2 reactions are generally run with strong negatively
charged bases like OH E2 − reactions are generally run with strong, negatively charged
bases like OH and OR−
b. solvents
c. Leaving group
There is a partial breaking of the bond to the leaving group in the transition state. So,
the better the leaving group the faster the E2 reaction.
d. Rate of reaction Rate of reaction follows the order Rate of reaction follows the
order, R−I > R−Br > R−Cl > R−F
EXAMPLES
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