Chemistry Reference
In-Depth Information
Figure 2.10 Representative functional groups with associated acidic protons.
As shown in Figure 2.10, carboxylic acids (compounds that dissociate into resonance-
stabilized carboxylate anions) have pK
a
values ranging from 4 to 6. However, alcohols
(compounds that dissociate into alkoxide anions possessing no resonance stabilization)
have pK
a
values ranging from 15 to 19. A comparison of the resonance capabilities of car-
boxylic acids compared to alcohols is illustrated in Scheme 2.8. As shown, the resonance
capabilities of the carboxylate anion are due to the presence of a carbonyl group adjacent to
the OH. This carbonyl group imparts additional partial charges that attract the negative
charge and distribute it over multiple centers. In the case of an alcohol, the deprotonated
alkoxide anion has no place to distribute its charge and the charge remains entirely on the
oxygen. Because alcohols have no resonance capabilities, their pK
a
values are higher than
those of carboxylic acids. Nevertheless, both carboxylic acids and alcohols are considered
organic acids.
Having illustrated how resonance effects influence the relative acidities of different func-
tional groups, it is important to understand why the same functional groups in different
compounds can possess different acidities. In order to address this, we must move from
the general representation of compounds presented above to a treatment of specific com-
pounds. For example, the carboxylic acid represented in Scheme 2.2 possesses an “R”
group. In organic chemistry, R groups are commonly used to represent regions of com-
pounds that are variable. To illustrate this point, Figure 2.12 lists several common
carboxylic acids and their respective pK
a
values.
As illustrated in Figure 2.12, formic acid (R
¼
H) has a pK
a
of 3.75. However, if R is
changed to an electron-withdrawing group such as CF
3
, the anion resulting from dis-
sociation becomes more stabilized resulting in a lower pK
a
compared to formic acid.
Alternately, if R is changed to an electron-donating group such as CH
3
, the anion resulting
