Chemistry Reference
In-Depth Information
Figure 2.12 Common carboxylic acids and their respective pK
a
values.
also occur when electropositive groups push electron density toward acidic centers,
rendering these centers less acidic. The concept of electronegativity was introduced in
Section 1.2 and referred to the periodic table of the elements as a resource for calibration.
In comparing formic acid to acetic acid, the CH
3
group of acetic acid is electron-donating
while the H of formic acid is not. This means that there is greater electron density present
in the carboxylate anion of acetic acid than in the carboxylate anion of formic acid.
An increase in electron density associated with a carboxylate anion lowers the stability
of the anion and raises the pK
a
. Thus, as mentioned at the end of Section 2.1, these
observations demonstrate that inductive effects work in concert with resonance effects to
alter pK
a
values.
While carboxylic acids are among the most acidic of all organic compounds, we are
more frequently interested in removing protons that are not directly associated with the
carboxylic acid functional group. Additionally, reliance upon removal of protons from
molecules containing functional groups other than carboxylic acids is common. In this
context, esters represent the next functional group we will study.
Esters, as functional groups, are simply oxygen-alkylated carboxylic acids (see Fig. 1.3).
As such, we cannot remove a proton from the oxygen as we are able to do with carboxylic
acids. However, as shown in Scheme 2.9, the proton in the position
a
(adjacent) to the ester
