Four dicarboxylic acids are shown beside the options (the drawn compound is glutaric acid, for reference). Which structure has the lowest pKa1?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

In oxalic acid the two carboxyl groups are directly bonded, so each strongly withdraws electrons from the other. pKa1 values: oxalic about 1.25, malonic 2.83, succinic 4.2, adipic 4.4. The inductive effect falls off rapidly with the number of CH2 groups separating the acids.

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