Question

Arrange oxalic acid, succinic acid and adipic acid in the increasing order of their acidity:

• A. Oxalic acid < succinic acid < adipic acid
• B. Oxalic acid < adipic acid < succinic acid
• C. Adipic acid < succinic acid < oxalic acid
• D. None of the above

Answer 1

The correct option is C Adipic acid < succinic acid < oxalic acid

All are diacids and it is important to know the common names as they are often used in chemistry and biology/biochemistry.
Oxalic acid is by far more acidic than the other two. It is just $HOOC-COOH$ and both the functional groups are electron withdrawing. So the charge density on the $-CO{O}^{-}$ anion will be stabilized by the adjacent electron-withdrawing group $HOOC-$. The more the number of methyl groups in between the two COOH groups, the lower the acidity (the ease with which a compound furnishes $H^{+}$ ions:
$\begin{array}{cccc}Name& Structure& p{K}_1& p{K}_2\\ Oxalic& H{O}_{2}CC{O}_{2}H& 1.2& 4.2\\ Succinic& H{O}_{2}CC{H}_{2}C{H}_{2}C{O}_{2}H& 4.2& 5.6\\ Adipic& H{O}_{2}C(C{H}_2{)}_{4}C{O}_{2}H& 4.4& 5.4\end{array}$
Trend:
1. The inductive effect of groups propagates through sigma bonds. In oxalic acid the EWG of one carboxylic acid makes the other highly acidic.
2. Through the carbon chain the inductive effect weakens