pKaand AcidityThis will all be discussed in greater detail as the topics of specific reactions and reaction
mechanisms are covered. In the meantime, try to bear in mind that nucleophiles are basic
and electrophiles are acidic.
11.5 pKaand Acidity
The acid dissociation constant of a substance is commonly called its pKa, and it is a measure
of the negative log of the K value of an acid dissociation reaction. (The K value refers to
the equilibrium calculations you learned how to perform in general chemistry -- if you have
forgotten your K’s and Q’s, now would be a good time to refresh your memory^1 on the
topic.)
pKa=−log(Ka)The lower the pKavalue is, the more acidic (and consequently, less basic) a substance is.
There is also a pKbvalue for all relevant substances, but it is common in organic chemistry
to use pKaexclusively, even when discussing bases. This is because extremely high pKa
values correlate exactly to extremely low pKbvalues, so there is no need to use both kinds of
measurements. Any pKavalue higher than seven means that a substance is not acidic when
placed in water, butit does not mean that substance cannot be an acid. Alcohols
are a good example of this: they can donate a hydrogen ion in chemical reactions but they
do not do so readily, which makes them acidic but only very weakly so. Many of the acids
in organic chemistry are considerably weaker than acids used for inorganic chemistry, so
discussion of acid-base chemistry in organic reactions may not necessarily relate well to your
previous understanding of the topic.