Physical Properties:
- Dicarboxylic acids are crystalline solids, their melting points being much higher than the corresponding monocarboxylic acid.
- The lower member dissolve readily in water but the solubility falls with the increase of molecular weight.
- The melting points of dicarboxylic acids show ‘alternation’ or ‘oscillation’ from one member to the other. The melting point of an ‘even acid’ (having even number of carbon atoms) is always higher than that of the ‘odd acid’ lying immediately above and below it.
Acidity. The dicarboxylic acids ionise in two steps:
The electron-withdrawing effect of the second COOH group enhances the acidity of the first COOH. Therefore, the value of the dissociation constant K1 is higher than that of acetic acid (Ka=1.5×10-5). The inductive effect becomes weaker as the two COOH groups are further separated by CH2 group and K1 decreases as we ascend the series.
As evident from the Table 21.2 the value of K2 is smaller than Ka of acetic acid (with the exception of oxalic acid) because the removal of proton from the second COOH is more difficult. This is so because the electrostatic repulsion due to negative charges in the dianion, ōOC-COō, destabilizes the dianion. That is why the value of K2 is much less than K1.
Chemical Properties:
Dicarboxylic acids give the same reactions as those of monocarboxylic acids in duplicate. Thus they give mono- and di-derivatives such as salts, esters, amides and acid halides. However, in the lower members in which the two COOH groups are closer together , the possibilities of mutual interactions increases. From this point, the action of heat on Dicarboxylic Acids is very interesting and needs a special mention.