Which of the following elements is not commonly involved as a heteroatom in heterocycles?

Which of the following heteroatoms is involved in the structure of thiophene?

In a heterocycle containing both oxygen and nitrogen, which heteroatom should be prioritized in naming according to IUPAC rules?

How does the structure of benzimidazole relate to that of imidazole?

Which of the following correctly describes Benzofurane and Benzothiophene?