With dehydrating agent present which dicarboxylic acid is least reactive towards forming an anhydride?

For any dicarboxylic acid, conversion into a cyclic anhydride in the presence of a dehydrating agent (such as $$\text{P}_2\text{O}_5$$ or conc. $$\text{H}_2\text{SO}_4$$) is an intramolecular reaction in which one -COOH group supplies the carbonyl carbon while the other -COOH group supplies the nucleophilic oxygen. The reaction therefore succeeds most readily when the two carboxyl groups can approach each other without creating excessive ring strain. A well-known rule of thumb is that

$$ \text{Ease of anhydride formation } \propto \frac{1}{\text{Ring strain in the product}} $$

The most favourable rings are five-membered; six-membered rings are still quite easy to make; rings containing seven or more atoms (or rings that would be highly twisted) form only with difficulty, if at all. Let us now look at every acid given and work out the ring that would be produced.

1. Phthalic acid. The two -COOH groups occupy adjacent (ortho) positions on a rigid benzene ring, so no conformational adjustment is needed; they are already side by side. Heating alone converts it smoothly to phthalic anhydride, a five-membered ring. Hence its reactivity is extremely high.

2. trans-Cyclohexane-1,2-dicarboxylic acid. Although the -COOH groups are on neighbouring carbon atoms, they sit on opposite faces of the chair conformer. Ring flipping, however, can place one substituent axial and the other equatorial, bringing the two groups reasonably close. When this happens the product is once again a five-membered anhydride, so the reaction does proceed, though somewhat more slowly than in the rigid ortho-aromatic case.

3. Glutaric acid, $$\text{HOOC-(CH}_2)_3\text{-COOH}$$. There are three methylene units between the carboxyl groups. Bending the chain back on itself gives a six-membered ring in the anhydride. Six-membered rings are only slightly less comfortable than five-membered ones, so glutaric acid still dehydrates fairly readily.

4. Succinic acid, $$\text{HOOC-(CH}_2)_2\text{-COOH}$$. Here only two methylene units separate the -COOH groups. To meet, the chain must adopt a gauche conformation in which the two interior C-C bonds are eclipsed. The resulting five-membered ring is therefore under noticeably greater angle and torsional strain than the corresponding rings obtained from the other acids. Consequently, the activation energy for cyclisation is highest and succinic acid is the slowest of the four to give an anhydride, even in the presence of a powerful dehydrating agent.

Comparing all four acids we obtain the qualitative order of reactivity

$$ \text{Phthalic} \;>\; \text{trans-1,2-cyclohexanedicarboxylic} \;>\; \text{Glutaric} \;>\; \text{Succinic}. $$

Thus, succinic acid shows the least tendency to form a cyclic anhydride.

Hence, the correct answer is Option D.