Chemistry Reference
In-Depth Information
Figure 3.3 Relationship between nucleophilicity, electronegativity, and basicity as illustrated using
first-row elements.
Electronegativity, discussed in Chapter 1, is a measure of an atom's affinity for electrons.
Thus, as electronegativity increases, affinity for electrons increases. Furthermore, as affinity
for electrons increases, so does acidity. This is reflected in the decreasing pK
a
values
moving from methane to methylamine to methanol to hydrofluoric acid (Fig. 3.3). In
this sequence, the trend relating increasing basicity of conjugate bases to increasing
nucleophilicity holds true. Furthermore, this relationship holds true for each row in the
periodic table of the elements moving from left to right.
Polarizability refers to the ability of an atom to become polarized in the presence
of external influences such as solvent effects. In general, polarizability increases as elec-
tronegativity decreases. Another way of looking at this relationship involves atomic
size. Essentially, the larger an atom, the more diffuse its outer shell of electrons. As
this electron shell becomes more diffuse, it also becomes more susceptible to polarizing
influences. Furthermore, these polarizing influences can dramatically impact the order
of nucleophilicity among atoms represented in any given column of the periodic table
of the elements. In fact, polarizability can override the relationship between nucleophili-
city and pK
a
. This effect is illustrated using the relative nucleophilicities of Cl
2
,Br
2
,
and I
2
. If we refer to the pK
a
values listed in Figure 3.2, we would expect the order of
nucleophilicity among these halide ions to be Cl
2
.
Br
2
.
I
2
. This is, in fact, the
case in the presence of polar aprotic solvents (solvents not possessing a dissociable
proton) such as dimethylformamide. However, in the presence of polar protic solvents
(solvents possessing a dissociable proton) such as water or alcohols the order of nucleo-
philicity is I
2
.
Br
2
.
Cl
2
. This effect, shown in Figure 3.4, illustrates that relative
nucleophilicities are not absolute.
Another factor influencing nucleophilicity and related to polarizability is the hardness
or softness of the nucleophilic base. Specifically, a hard base is high in electronegativity
and low in polarizability. Alternatively, a soft base is low in electronegativity and high
in polarizability. Using these definitions, F
2
is considered a hard base because it is high
in electronegativity, small in size and holds its electrons very tightly. On the other hand,
I
2
is considered a soft base because its large size causes it to hold its electrons loosely
and renders it highly polarizable.
The relationship between hard bases and soft bases now relates back to solvent effects. if
we consider a hard base in a polar solvent, we find that the concentrated electron density
