Substitution and Elimination Reaction Mechanisms
These two different mechanisms explain the difference in reaction rates betweenSN1 and
SN2 reactions.SN1 reactions are dependent on the leaving group disassociating itself from
the carbon. It is the rate-limiting step and thus, the reaction rate is a first-order reaction
whose rate depends solely on that step.
Rate=k[RX]Alternatively, inSN2 reactions, the single step of the nucleophile coming together with
the reactant from the opposite side of the leaving group, is the key to its rate. Because
of this, the rate is dependent on both the concentration of the nucleophile as well as the
concentration of the reactant. The higher these two concentrations, the more frequent the
collisions. Thus the reaction rate is a second-order reaction:
Rate=k[Nu:][RX](where Nu: is the attacking nucleophile)56.1.2 SN2 Reactions
There are primarily 3 things that affect whether anSN2 reaction will take place or not.
The most important is structure. That is whether the alkyl halide is on a methyl, primary,
secondary, or tertiary carbon. The other two components that determine whether anSN 2
reaction will take place or not, are the nucleophilicity of the nucleophile and the solvent
used in the reaction.
Reactivity Due to Structure of SN2CH 3 X > RCH 2 X > R 2 CHX >> R 3 CXThe structure of the alkyl halide has a great effect on mechanism. CH 3 X & RCH 2 X are the
preferred structures forSN2. R 2 CHX can undergo theSN2 under the proper conditions
(see below), and R 3 CX rarely, if ever, is involved inSN2 reactions.
Figure 158 SN2 nucleophilic substitution of bromine with a generic nucleophile
The reaction takes place by the nucleophile attacking from the opposite side of the bromine
atom. Notice that the other 3 bonds are all pointed away from the bromine and towards
the attacking nucleophile. When these 3 bonds are hydrogen bonds, there’s very little
steric hinderance of the approaching nucleophile. However, as the number of R groups
increases, so does the steric hinderance, making it more difficult for the nucleophile to get
close enough to the α-carbon^1 , to expel the bromine atom. In fact, tertiary carbons (R 3 CX)
are so sterically hindered as to prevent theSN2 mechanim from taking place at all.
In the case of this example, a secondary α-carbon, there is still a great deal of steric hin-
derance and and whether theSN2 mechanism will happen will depend entirely on what the
nucleophile and solvent are. SN2 reactions are preferred for methyl halides and primary
halides.