Chemistry Reference
In-Depth Information
Compared to the chloride ion, the iodide ion is less electronegative and more polar-
izable. This polarizability stabilizes the anion as is reflected in the pK
a
value for
hydroiodic acid (
2
10) compared to the pK
a
value for hydrochloric acid (
2
2.2).
Therefore, iodide is the better leaving group.
Compared to nitrogen, oxygen is more electronegative and thus holds
onto its electrons more tightly. This stabilization of the oxygen anion compared
to the amine anion is reflected in the pK
a
values for alcohols (15-19) compared
to the pK
a
value for amines (35). Therefore, the alkoxide is the better leaving group.
When comparing leaving groups where the departing atoms are the same, inductive
effects must be considered. Since fluorine is more electronegative than hydrogen, the
presence of three fluorides pulls electron density from the alkoxide ion, thus stabiliz-
ing the anion. This is reflected in the pK
a
values for trifluoroethanol (11-12) com-
pared to the pK
a
values for ethanol (15-16). Therefore, trifluoroethoxide is the
better leaving group.
Compared to oxygen, sulfur is less electronegative and more polarizable. This
increase in polarizability stabilizes the anion, as is reflected in the pK
a
value for
methanethiol (10.4) compared to the pK
a
values for methanol (15-16). Therefore,
the methylsulfide anion is the better leaving group.
