Anthracene Naphthalene Tetracene

The organic molecules shown here are derivatives of benzene
in which six-membered rings are “fused” at the edges
of the hexagons. (a) Determine the empirical formula of benzene and of
these three compounds. (b) Suppose you are given a sample
of one of the compounds. Could combustion analysis be used to determine unambiguously which of the three it
is? (c) Naphthalene, the active ingredient in mothballs, is a
white solid. Write a balanced equation for the combustion
of naphthalene to CO₂(g) and H₂O(g). (d) Using the Lewis
structure for naphthalene and the average bond enthalpies
in Table 8.3, estimate the heat of combustion of naphthalene
in kJ/mol. (e) Benzene, naphthalene, and anthracene
are colorless, but tetracene is orange. What does this imply
about the relative HOMO–LUMO energy gaps in these molecules?
See the “Chemistry Put to Work” box on orbitals
and energy.