Chemistry Reference
In-Depth Information
Scheme 3.11 Protonated carbonyl-based functional groups are susceptible to reaction with
nucleophiles.
associated carbon atom (Scheme 3.10). The net result renders the carbon atom highly sus-
ceptible to reaction with nucleophiles (Scheme 3.11).
3.2 WHAT ARE NUCLEOPHILES?
As alluded to in Section 1.3 and in the previous section, heterolytic reactions generally
involve species known as nucleophiles and complementary species known as electrophiles.
By definition, a nucleophile is a compound that has an affinity for a positive charge. By
analogy, an electrophile is a compound that has an affinity for a negative charge.
Nucleophiles generally present themselves as either neutral species bearing available
lone pairs of electrons or as anions (negatively charged ions). When a nucleophile is
an anion, the anion is generally the conjugate base of an acid. Figure 3.2 lists common
conjugate bases used as nucleophiles along with their parent acids and associated
pK
a
values.
In considering the information presented in Figure 3.2, it is important to become
familiar with the general trends that influence the degree of nucleophilicity associated
with the conjugate bases of various acids. From our discussions of acids, we know that
as the pK
a
increases, acidity decreases. Furthermore, as acidity decreases, the basicity
associated with conjugate bases increases. Since, by definition, bases attract protons
and since protons are, by definition, positively charged, we can translate this relationship
to infer that bases exhibit affinities for positive charges. Since nucleophiles are
defined as substances that have affinities for positive charges, we can understand the state-
ment from the previous paragraph equating nucleophiles with conjugate bases of acids.
Taking this discussion to the next level, the weaker the acid (higher pK
a
), the stronger
the conjugate base. Furthermore,
the stronger the conjugate base,
the stronger the
nucleophile.
While the general trend relating basicity and nucleophilicity stands, we cannot simply
rely on the pK
a
values listed in Figure 3.2 as a guide for these trends. In fact, with
respect to overall nucleophilicity, there are relevant factors other than basicity. Among
these are polarizability of the nucleophilic atom, electronegativity of the nucleophilic
atom, steric factors, and solvent effects. For our purposes, solvent effects will be discussed
in the context of polarizability.
