Which part of $\mathrm{acetylsalicylic} \mathrm{acid}$ is most rigid due to restricted rotation around its bonds?

• A. The $\mathrm{methyl} \mathrm{group}$
• B. The $\mathrm{ester} \mathrm{linkage}$
• C. The $\mathrm{carboxylic} \mathrm{acid} \mathrm{group}$
• D. The $\mathrm{benzene} \mathrm{ring}$

Answer: (D)

Verified step by step guidance

1. Identify the different parts of acetylsalicylic acid: the methyl group, the ester linkage, the carboxylic acid group, and the benzene ring.
2. Recall that single bonds generally allow free rotation, while double bonds and conjugated systems can restrict rotation due to partial double bond character or resonance.
3. Understand that the methyl group is attached via a single bond, allowing free rotation, so it is not rigid.
4. Recognize that the ester linkage and carboxylic acid group have some resonance stabilization, which can restrict rotation to some extent, but this restriction is localized and less extensive.
5. Note that the benzene ring is a planar, conjugated cyclic system with delocalized pi electrons, which creates a rigid structure with very restricted rotation around the bonds within the ring, making it the most rigid part of acetylsalicylic acid.