Aliphatic dicarboxylic acids are named by simply adding the suffix -dioic
acid to the root name. The root name comes from the longest carbon chain
containing both carboxyl groups. Numbering starts at the end closest to a
substituent.
O
H
OH
O
O
Br
Br
3,4-Dibromohexanedioic acid
Structure of the carboxyl group
The most stable conformation of a carboxyl group is a planar arrangement
of the molecule. The carbon is sp
2
-hybridized, and the OÀ ÀH bond lies in the
plane, eclipsing the C À À
À À O double bond. This unexpected geometric arrangement can be explained by resonance. The following resonance forms can be
written for a carboxyl group.
C
O
O H
C
O
O H
C
O
O H
..
+
:
..
..
Resonance forms of carboxyl group
..
+
:
..
: : ..
..
Acidity of carboxylic acids
Although carboxylic acids are much weaker acids than the strong mineral
acids, e.g. HCl, H 2 SO 4 and HNO 3 , they can still dissociate in aqueous
solution and form carboxylate ions (RCO 2
À ). The equilibrium constant for
this process is K a ¼$ 10
À5 (pK a ¼$ 5). Carboxylic acids are more acidic
than analogous alcohols. For example, the pK a values of ethanoic acid and
ethanol are, respectively, 4.74 and $15.9.
R C
O
O
R C
O
O H
+ H 3 O +
+ H 2 O
..
..
: : ..
..
: :
..
..
..
Substituent effects on the acidity of carboxylic acids
Any substituent that stabilizes a negative charge enhances the dissociation
process, i.e. increases the acidity. Electronegative elements can enhance the
acid strength through inductive effects. The closer the substituent to the
anion, the more profound the effects are.
4.3 ALKANES, CYCLOALKANES AND THEIR DERIVATIVES
91