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THE SN2 REACTION: FACTORS
AFFECTING SN2 REACTION
CHM2210L.010U20.51410
June 15, 2020
Introduction:
Substitution 1 (SN1), Substitution 2 (SN2), Elimination 1 (E1), and Elimination 2 (E2) are
all common ways of denoting the nature of a chemical reaction. In this experiment, a series of
SN2 reactions were performed to assess the factors that affect the rate of SN2 reactions as well as
the kind of affect each factor has. Substitution is the process in a chemical reaction by which a
group that is part of a molecule gets replaced by another group (Weldegirma, 2018). Typically,
alkyl species are the ones that have the leaving group as part of them. An ion that donates its lone
pair of electrons would form the new bond with the alkyl species and becomes to nucleophile,
which is the name for the group that replaces the original group (Weldegirma, 2018). The nature
of this replacement process will determine whether the reaction was a substitution 1 or a
substitution 2 reaction. Substitution 1 is defined as a reaction where, between the detachment of
the leaving group and the attachment of the nucleophile, the alkyl species exists as a carbocation
(Weldegirma, 2018). This is allowed to happen when the nucleophile is not strong enough to
“attack” the alkyl species and replace the leaving group right as the group is leaving in a single,
symphonic step. However, when the nucleophile is, indeed, quite strong, the nucleophile’s
affinity to the alkyl species causes the leaving group to detach (Weldegirma, 2018). This reaction
is the definition of a substitution 2 reaction. The strength of the nucleophile, as well as the
properties of the leaving group, influence the rate of such reactions. Since two molecules are
involved in the determination of the rate of the process, these reactions are also known as
bimolecular processes (Weldegirma, 2018). Other factors, such as the order of the alpha carbon,
which is the carbon attached to the leaving group, and the solvent used in the reaction also affect
the rate of SN2 reactions. Triethylamine is a much better nucleophile than ethyldiisopropylamine
for the reason that steric hinderance in the nucleophilic molecule is unfavorable in substitution 2
reactions (www.chem.ox.ac.uk). Similarly, if the substrate, which is the molecule that is changed
by the reaction, is iodomethane, then the reaction will go a lot faster than if the substrate was 2-
bromopropane. The reasons for this are that iodomethane contributes less steric hindrance than 2-
bromopropane, and Iodine is a better leaving group than bromine, due to its large atomic radius
(www.chem.ox.ac.uk).
Figure 1
To illustrate SN2 reactions, in figure 1, a substitution 2 reaction between triethylamine
and 1-iodopropane is illustrated with the arrows representing electron transfer. The
triethylamine, due to its lone pair of electrons, is the nucleophile, while the 1-iodopropane, due
to its halide leaving group, is the substrate. The nucleophile in this reaction is strong enough to
cause SN2. The lone pair of electrons on the amine are transferred to the carbon that is, at first,
attached to the iodine. The backside attack, meaning the attack from the opposite orientation of
the halide group, causes the iodine to hog the electrons that made up its bond and leave
(Weldegirma, 2018). The transition state of the reaction is shown in brackets, where both the
amine and the iodine ion feel partial charges while still feeling affinity from the alkyl species,
with the amine having a partial positive charge, and the iodine having a partial negative charge.
The transition state is succeeded by the products on the right. The amine ends up having a
positive formal charge, and the iodine ends up having a negative charge (Weldegirma, 2018).
Figure 2
When another stable combination of products is possible, some of the reaction goes in the
other direction. This is called a side reaction since it is not the desired reaction that the
experimenters hope for (Weldegirma, 2018). One such side reaction that is possible for the SN2
reaction described in figure 1 is illustrated in figure 2. The amine, instead of attacking the alpha
carbon with its lone pair, attacks a proton on a beta carbon. This causes beta carbon to use the
lone pair to form a second bond with carbon, while the iodine leaves with the lone pair from the
