Despite very similar skeletal structures, the difference in acid strength between oxalic acid (ethanedioic acid) and malonic acid (propanedioic acid) is quite significant. What is the reason for that?
It seems that the inductive effect plays a role, but what is the explanation for the inductive effect in oxalic acid as there is same group on opposite sides?
How does the inductive effect plays different roles in both acids?
Linear saturated dicarboxylic acids
$n$ = number of $\ce{-CH_2}-$ groups in chain
n pKa1 pKa2 Ka1/Ka2
0 Oxalic acid 1.25 4.14 776
1 Malonic acid 2.83 5.69 724
2 Succinic acid 4.21 5.41 15.8
3 Glutaric acid 4.34 5.41 11.7
4 Adipic acid 4.41 5.41 10.0
5 Pimelic acid 4.50 5.43 8.51
6 Suberic acid 4.526 5.498 9.4
12 Dodecanedioic acid 4.45 5.05 4.0
Aromatic carboxylic acids
pka1 pka2 pka3
benzoic acid 4.202 ---- ----
ortho-phthalic acid 2.89 5.51 ----
meta-phthalic acid 3.46 4.46 ----
para-phthalic acid 3.51 4.82 ----
o,o'-bibenzoic acid
p,p'-bibenzoic acid
benzene-1,3,5-tricarboxylic acid 3.12 3.89 4.70
Linear unsaturated dicarboxylic acids
$n$ = number of $\ce{-CH=CH}-$ trans groups in chain
n pka1 pka2
0 Oxalic acid 1.25 4.14
1 Fumaric acid 3.02 4.38
2 trans,trans-Muconic acid 3.87
3
Straight-chain, saturated carboxylic acids
$n =$ total number of carbon atoms in molecule ($n \geqslant 2$ has terminal methyl group).
n pKa
1 Formic acid 3.77
2 Acetic acid 4.76
3 Propionic acid 4.88
4 Butyric acid 4.82
5 Valeric acid 4.82
6 Caproic acid 4.88
15 Pentadecanoic acid 4.8
most $\mathrm{p}K_\mathrm{a}$ data from Wikipedia.
How does the inductive effect plays different roles in both acids?
First think of what happens when the carboxylic acid group of a straight-chain, saturated carboxylic acids ionizes to form the carboxylate ion. The $\ce{C=O}$ and $\ce{-C-O^{-}}$ bonds of course hybridize. This hybridization occurs faster than hydrogen bonds to the solvent form. The net result is effectively two oxygen atoms with 1/2 a charge each. This increases the ability of the carboxylate ion to solvate, which increases its acidity.
In malonic acid the $\ce{C-C}$ bond "shares" some of that hybridization. This is the "inductive effect." The additional delocalization of the electrons from the carbon atoms makes the carboxylate ions more electronegative than they would be in a long linear saturated dicarboxylic acid. So the first ionization , the second ionization
First, notice that a carboxylic carbon ($\ce{COOH}$) has an oxidation state of $\mathrm{+III}$ (except in formic acid where it is $\mathrm{+II}$). This means, that the carbon is very electron deficient and wishes to draw electrons towards it from sources that are not the two oxygen atoms (because they are stronger). We term this an inductive effect, as we cannot easily draw mesomeric structures to explain it.
Since the inductive effect is mainly based on electrostatic interactions, it gets exponentially weaker with increasing distance. In oxalic acid, there is one one bond separating the two carboxylic groups, so they can exert a strong inductive effect. In malonic acid the effect is weaker due to the separating $\ce{CH2}$ group. And in succinic and subsequent diacids it is almost unmeasurable due to the distance between the groups.
But how do we rationalise each group exerting a $+I$ effect on the corresponding other group? Well — we cannot deprotonate twice immediately (I am introducing this as an axiom, but it does have reasoning that I feel too lazy to explain right now). So we have to pretend one side is going to be inert while the other is deprotonated. And if we do that, we see that the inductive effect of one side is strong in drawing away electron density from the $\ce{COOH}$ group we are deprotonating. Since this means there is a lower effective negative charge, deprotonation happens more easily, i.e. at lower $\ce{pH}$.
We can use this simple picture since the groups are isotopic, i.e. they can be transformed into each other by a $C_2$ rotation of the molecule’s inherent symmetry.
Acidic nature is directly proportional to strength of conjugate base.
In oxalic acid, the hydrogen oxalate ion and the oxalate ion (Which are conjugate bases in first and second dissociations) are resonance stabilized. The negative charge on oxygen atom is in conjugation throughout the molecule (with two pi bonds). Hence it is more stable.
In malonic acid, the conjugate base is relatively less stable, as the conjugation is restricted to only one pi bond.
Resonance effect is more stronger than inductive effect and moreover, resonance effect in the conjugate base of oxalic acid takes place throughout the molecule.(there is a negative charge, pi bond, pi bond, lone pair conjugation ).
Hence oxalic acid is relatively more acidic as compared to malonic acid.
