Chemistry Reference
In-Depth Information
Scheme 4.3 S
N
2 reactions proceed when incoming nuclophiles are more nucleophilic than outgoing
leaving groups.
Scheme 4.4 S
N
2 reactions do not proceed when incoming nuclophiles are less nucleophilic than
outgoing leaving groups.
S
N
2 reaction illustrated in Scheme 4.3 will proceed to completion. Furthermore, as
illustrated in Scheme 4.4, the reverse reaction will not proceed because relative to chloride
ions, methoxide ions are poor leaving groups.
In summary, S
N
2 reactions are defined by the principles surrounding organic acids
and their conjugate bases as discussed in Chapters 2 and 3. Specifically, as discussed in
Chapter 3, relative nucleophilicities can be estimated based on pK
a
values. Finally, relative
pK
a
values can be used to predict whether a reaction will proceed and what component of
the starting material will be the leaving group.
4.3 WHERE CAN S
N
2 REACTIONS OCCUR?
With an understanding of what S
N
2 reactions are, it is now important to understand where,
on a given molecule, such reactions will take place. The answer to this question can be
found by recognizing electronegativity trends. As previously discussed, an atom's
electronegativity relates to how strongly it holds on to its electrons. This translates into
greater partial negative charges residing on more electronegative atoms and smaller
partial negative charges residing on less electronegative atoms. Regarding trends, as
previously discussed, relative electronegativities of atoms are readily identified using the
periodic table of the elements. For example, moving across each row of the periodic table,
we find that electronegativity increases among the atoms, thus indicating that oxygen is
more electronegative than nitrogen and that fluorine is more electronegative than oxygen.
As discussed in previous chapters, pK
a
values are directly related to electronegativities.
This is clearly reflected in the nitrogen-oxygen-fluorine trend discussed above, consider-
ing that amine pK
a
values are approximately 35, alcohol pK
a
values are approximately 15-
19, and the pK
a
value for hydrofluoric acid is approximately 3. The increase in acidity is
related to increased electronegativity because more electronegative atoms are more stable
in their anionic form. Thus, fluorine can stabilize a negative charge better than oxygen,
which, in turn, can stabilize a negative charge better than nitrogen.
