There
are 2 possible mechanisms: synchronous mechanism of C-Nu bond formation and C-X
bond breakdown (SN2 reaction). Two-stage mechanism: heterolytic rupture of the
C-X bond and formation of a carbocation. Consequent attack of the nucleophile
(SN1 reaction).
SN1
reaction
• The reaction rate depends only on the
concentration of the alkyl halide (monomolecular kinetics).
• The reaction rate increases with increasing
alkyl halide size.
• In the case of a chiral alkyl halide, a
racemic mixture is formed. The SN1 reaction proceeds in two stages: the first
stage determines the overall speed of the reaction and provides for the
formation of an intermediate carbocation:
R-X
? R + + X- (slow stage)
The
second stage of the reaction (fast stage) provides the nucleophile attack to
the carbocation:
R
+ + Nu- ? R-Nu (fast stage)
The
fact that the stage proposed as a slow stage does not involve the nucleophile,
explains the first kinetic order and clarifies why the initial velocity does
not change by changing the nucleophile.
The
formation of carbocation intermediate is the slow stage of the reaction that
conditions and limits the speed of the whole process (it is the
Rate-Determining Step RDS).
When
an optically active alkyl halide is reacted under SN1 conditions, an almost
complete racemization of the final product is observed. This is since a spindle
planar hybridised carbocation is generated as an intermediate product and this
can be attacked by the incoming nucleophile on both sides with equal
probability.
Relative
velocities of SN1 reactions
Let
us now examine the effects of the structure of the R group on the relative
velocities of SN1 nucleophilic substitution reactions. Since a carbocation is
generated in the formation of the transition state, the alkyl halides that
generate the most stable carbocations will react faster. The alkyl residues
tend to stabilize the carbocation both by inductive and hyper conjugative
effect. In this way, a tertiary carbocation is more stable than a secondary
carbocation which in turn is more stable than a primary carbocation. The order
of reactivity of the alkyl halides is, therefore:
tertiary>
secondary> primary> methyl
Transposition
of Wagner-Meerwein
A
further proof that the SN1 mechanism proceeds through a carbocation consists of
the observation that there are rearrangements; the carbocation intermediate
can, in fact, undergo a transposition phenomenon, with a 1,2 shift of an alkyl
or a hydrogen to form a more substituted and therefore more stable carbocation
(Wagner-Meerwein transposition).
Factors
that influence SN1 reactions:
• the reaction takes place according to a
two-stage mechanism. The first stage of the reaction is characterized by the
formation of a carbocation intermediate. This stage is the slowest and
therefore conditions the speed of the entire chemical reaction.
• the kinetics of the reaction is of the
first order; the reaction speed can indeed be accelerated by increasing the
concentration of the alkyl halide alone.
• the reaction passes through formation a
carbocation intermediate with planar trigonal geometry.
• in the second stage of the reaction, there
is the attack of the nucleophile to one of the two sides of the carbocation
intermediate with the formation of the R-Nu bond.
SN2
has the following characteristics:
the
reaction is concerted in the sense that the formation and the breaking of the
link occur simultaneously; the reaction mechanism, therefore, provides a single
stage involving both the reacting molecules (A-Lg and Nu);
the kinetics of the reaction is of the second
order; the reaction speed can indeed be accelerated by increasing the
concentration of only one of the two reagents or both.
the carbon at which the substitution takes
place changes its hybridization from sp3, as it was in the starting product, to
sp2, as it is in the transition state;
in the transition state the incoming group
(Nu) and the outgoing group (Lg) simultaneously overlap on opposite sides to
the orbital p, which is perpendicular to the plane containing the other three
carbon-related residues;
the
carbon at which the substitution takes place undergoes a configuration
inversion (Walden inversion).
In which:
Nu
= nucleophile
Lg = leaving group (outgoing group)
Order
of reactivity in SN2 reactions It has been found experimentally that the
nucleophilic SN2 substitution depends to a considerable extent on the structure
of the alkyl halide, given that the incoming nucleophile has to make its way to
the carbon atom on the opposite side to that which carries the outgoing
halogen, and if they are bound to the carbon itself of the cumbersome groups
these will hinder the entry of the nucleophile. This phenomenon is called
steric impediment to the SN2 reaction. The SN2 reaction is therefore favoured
on less substituted carbon atoms (i.e. with more than bound hydrogen atoms).
The
order of SN2 reactivity is, therefore:
methyl> primary> secondary> tertiary
SN2 steric hindrance the methyl halides are
therefore the fastest in the SN2 substitutions because they have only three
hydrogen atoms projected against the incoming nucleophile. Also, the primary
alkyl halides react quickly because a single linear chain attached to the
carbon to which the inversion occurs can be arranged so as not to give
sensitive interactions. The steric impediment to the nucleophile attack is
greater in the secondary halides but the reaction can also occur at an
appreciable rate. With tertiary halides, steric impediment becomes almost
prohibitive.
Nature
of the nucleophile the nature of the nucleophile also influences the SN2
reaction. SN2 requires strong nucleophiles and to favour the electrophilic
attack the use of aprotic polar solvents is preferred (ie they do not release H
+ ions). The nature of the outgoing group also influences the SN2 reaction. In
fact, a Lg that is too good tends to come out even before the nucleophile’s
attack, favouring an SN1 reaction; the SN2 reaction is therefore favoured by
weak Lg.
Summary, an SN2 reaction is favoured by weak
Lg groups (Lg = leaving group = outgoing group) strong nucleophiles; primary or
secondary alkyl halides.
