NUCLEOPHILIC SUBSTITUTION AND
ELIMINATION OF ALKYL HALIDES
TABLE OF CONTENTS FOR THIS
SUBSTITUTION AND ELIMINATION:GENERAL
- In contrast to addition, in which two molecules add
together, substitution (Symbol S)is a reaction type in which in a single
molecule, one group or atom replaces another.
- In the example given below, a hydroxyl group replaces a
chlorine atom, thus converting an alkyl halide into an alcohol.
- Whereas alkenes undergo addition to their pi bonds, because
they can add another group, saturated carbon compounds (which have only tetrahedrally
hybridized carbon atoms) normally do not undergo addition, but rather substitution.
- Most commonly, this occurs by a nucleophilic substitution
mechanism , i.e., in which the organic compound reacts with a nucleophile.
To do this, the organic molecule must have a good leaving group, which can
depart with and stabilize the electron pair of its former bond to carbon.
Good leaving groups are relatively stable anions (such as bromide or chloride
ions( or, even better, neutral molecules like water or nitrogen. This is necessary
because the reacting nucleophile, by definition, brings its own electron pair
to bond formation.
- The mechanistic symbol is SN. There are two specific
types of SN mechanisms.
- Elimination (E) is a reaction type which is the
reverse of addition. In this process a single molecule splits up into two
- Most eliminations are base catalyzed, and they also
require a good leaving group such as a halide ion, but also a proton beta
to the leaving group, which is pulled off by the base.
- Since elimination is the reverse of addition, it forms
alkenes (e.g., from alkyl halides) rather than using alkenes as reactants.
MECHANISMS OF NUCLEOPHILIC
THE SN2 MECHANISM
In this mechanism,a nucleophile
reacts directly with an organic substrate such as an alkyl halide
in a concerted (one step) reaction. The nucleophile (Nu) provides
the electron pair for the new bond, and the leaving group (L) departs
with the pair of electrons which previously formed the C-L bond.
THE SN1 MECHANISM
- In this mechanism, the formation of the new bond and the
breaking of the old one are not concerted, but stepwise. In particular,
the old bond is first broken, to give a carbocation (remember?
a carbocation is a species having a positively charged, trivalent carbon atom;
it is trigonally hybridized (planar), and has a vacant 2p orbital).
- The first step is the rds, since is is obviously highly
- The subsequent formation of the new bond to the carbocation
does not involve the breaking of any bonds, so it is hgihly exothermic and
- The RATE LAW for a reaction is a description of the rate
of the reaction as it depends upon the concentrations of various species.
It includes a rate constant, k(small k, not an equilibrium constant, K), which
is equal to the rate of the reaction at unit concentrations of all reagents.
As noted previously, it is a measure of the inherent rate of the reaction,
independent of concentrations.
- A first order rate law is one which depends upon
the concentration of a single reactant taken to the first power, i.e.,
the rate is linearly dependent upon the concentration of that reagent.
The order of the rate law is the sum of the exponents of all concentration
expressions in the rate law.
- The rate law for the SN1 reaction is first order,
depending only upon the concentration of the alkyl halide, and not at all
upon the concentration of the nucleophile. That is because the first step
is the rate determining one. The subsequent step which involves the nucleophile
is very fast.
- The "1" of SN1 is the molecularity
(the number of molecules present) of the rate determining step, in which only
the alkyl halide is involved.
- In contrast, the SN2 reaction is second order,
depending upon the concentrations of both the alkyl halide and the nucleophile,
each linearly. The rate determining step (indeed the only step) is bimolecular,
so that the mechanistic designation is SN2.
- The reason the rate of this reaction is dependent upon the
concentrations of both reactants is simply that they must collide in
order to react. The number of collisions between the two reagents is directly
proportional to the concentrations of both.
Stereochemistry of the SN2
An especially interesting, and useful,
aspect of the SN2 reaction is that the new bond does not directly
replace the old one in space. Instead, the nucleophile approaches from
the opposite side (or face) of the molecule from which the leaving group
departs, resulting in what we call INVERSION OF CONFIGURATION.
There are two other hypothetically possible
stereochemical results for substitution which could have occurred, but
do not. One is RETENTION OF CONFIGURATION, in which the nucleophile approaches
from the same side as the leaving group departs. This would have resulted
in the formation of (S)-2-butanol.
A third hypothetically possible result
is racemization, in which both retention and inversion occur to
the same extent, giving racemic 2-butanol.
Stereochemistry of the SN1
Since a carbocation is formed as
an intermediate in this reaction, and since the carbocation is planar
and therefore achiral, the result is racemization. The carbocation
can be seen to be achiral because it has a plane of symmetry, which
is the trigonal plane.
Mechanistically, we can see how racemization
arises. Since the carbocation 2pz AO has two equivalent lobes,
the nucleophilie can react at either lobe equally rapidly, to give both
enantiomers of the product in equal amounts. You should be able to illustrate
this on an exam.
TRANSITION STATE MODELS
TS FOR THE SN2 REACTION
The carbon atom which is undergoing substitution
is sp2 hybridized, with the leaving group bonding to
one lobe of a 2p orbital and the nucleophile bonding to the other.
The TS does not have carbocation, radical,
etc. character but is characterized as having pentacovalent (or pentacoordinate)
Pentacovalent carbon has more groups around
it than normal for carbon, so these groups are more proximate (closer)
than usual. Therefore, pentacovalent carbon is especially sensitive to
TS for the RDS of the SN1
The first step of the SN1 reaction
is rate limiting, so we are interested only in the TS of this step for
Using resonance theory in the usual way,
we can derive (and you should be able to derive) the transition
state shown, with a partial carbon-L bond, and positive partial charge
on carbon (negative on the leaving gorup).
This TS has carbocation character.
Since the breaking of a strong bond without the formation of any other
bond is bound to be highly endothermic, the Hammond postulate indicates
that the TS resembles the products. Therefore, the TS has extensive
THE EFFECT OF STRUCTURE ON
THE RATES OF REACTION
THE SN1 REACTION
We consider the transition state energies
for reactions involving the generation of methyl, primary, secondary,
and tertiary carbocation character. As a specific example, it might be
methyl, ethyl, isopropyl, and tert-butyl chlorides. We know that
the order of carbocation stability is tertiary>secondary>primary>methyl
and that the differences are large. The same order avails for transition
states which have extensive carbocation character.
Thus, the SN1 reaction of tert-butyl
chloride or any other tertiary halide would be much faster that for a
secondary halide, than for a primary halide, than for a methyl halide.
THE SN2 REACTION
We note that the order of reactivity is
now exactly the reverse for this mechanism, i.e., the methyl substrate
is fastest, followed by the primary, the secondary, and finally the tertiary
This is because of the steric sensitivity
of the SN2 TS. In the case of the methyl substrate, the
groups attached to the pentacovalent carbon (other than the nucleophile
and the leaving group) are all hydrogens (smallest atom), whereas in the
tert-butyl case, they are all methyl groups. This gives rise to
substantial steric repulsions between the five groups around carbon in
MECHANISTIC TENDENCIES BASED
As a result of the opposing rate tendencies
in the two mechanisms, it turns out that tertiary systems, which
are the fastest at the SN1 and slowest at the SN2,
virtually always can be relied upon to undergo reaction via the SN1
mechanism, i.e., via a carbocation mechanism.
In contrast, methyl and primary
systems which are good at SN2 and slow at SN1 virtually
always react via the SN2 mechanism.
Secondary systems are more difficult
to predict, but the main factor here is the strength of the nucleophile.
Remember that the nucleophile is present in the SN2 transition
state but not in the SN1 TS. The stronger the nucleophile,
the faster the SN2 reaction, but increasing the nucleophilic
strength has no effect on the SN1 reaction. So, with weak
nucleophiles like water or methanol, secondary systems prefer the SN1
mechanism, but with strong ones like hydroxide or other negatively charged
ions they will proceed via an SN2 mechanism.
halides undergo the SN1 reaction very rapidly, because
allylic cations, which are formed in the rds are highly resonance stabilized.
The transition state has allylic cation character. Allylic systems and
tertiary systems undergo these reactions at comparable rates.
under the the SN2 reaction very rapidly if the
halide is attached to a primary carbon atom. Thus, the simplest
allylic halide, 3-chloropropene (allyl chloride), undergoes both SN
reaction mechanisms extremely rapidly.
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