Because SN1 reactions pass through a planar intermediate before the nucleophile attacks, the
product will usually be a racemic mixture. The incoming nucleophile can attack the carbocation
from either side, resulting in varied stereochemistry.
SN2 Reactions
Bimolecular nucleophilic substitution (SN 2 ) reactions contain only one step, in which the
nucleophile attacks the compound at the same time as the leaving group leaves. Because this
reaction has only one step, we call it a concerted reaction. The reaction is called bimolecular
because this single rate-limiting step involves two molecules.
In SN2 reactions, the nucleophile actively displaces the leaving group in a backside attack. For this
to occur, the nucleophile must be strong, and the substrate cannot be sterically hindered.
Therefore, the less substituted the carbon, the more reactive it is in SN2 reactions. Note that this is
the opposite of the trend for SN1 reactions. The one-step mechanism is shown in Figure 4.6.
Figure 4.6. Mechanism of SN2 ReactionThe single step of an SN2 reaction involves two reacting species: the substrate (often an alkyl halide,
tosylate, or mesylate) and the nucleophile. Therefore, the concentrations of both have a role in
determining the rate: rate = k[Nu:][R–L]
SN2 reactions are accompanied by an inversion of relative configuration. Much like an umbrella
being turned inside out on a blustery day, the position of substituents around the substrate carbon
will be inverted. If the nucleophile and leaving group have the same priority in their respective
molecules, this inversion will also correspond to a change in absolute configuration from (R) to (S) or
vice-versa. This is an example of a stereospecific reaction, one in which the configuration of the
reactant determines the configuration of the product due to the reaction mechanism.
MCAT Concept Check 4.2: