The carbon atoms are not explicitly drawn but occupy the positions where the bonds join together.
This is a common convention in organic chemistry. Notice how the molecular formulas for cyclic
compounds do not follow the generic formulas given above for alkanes, alkenes, and alkynes: The
extra carbon-carbon bond formed in making a ring upsets the ratio of carbon to hydrogen atoms.
One final warning is that as Lewis structures, the drawings do not accurately reflect the three-
dimensional appearance of the compounds; cyclohexane, for example, is not a planar hexagon but
instead adopts a “chairlike” conformation in its most stable state.
AROMATICS
Certain unsaturated cyclic hydrocarbons are known as aromatics. We need not concern ourselves
with exactly what makes a compound aromatic, but all such compounds have in common a cyclic,
planar structure and possess a higher degree of stability (a lower enthalpy of formation) than
expected. This extra stability comes from the effects of resonance. The prime example of an
aromatic compound is benzene, C 6 H 6 , which in a Lewis structure is represented as having
alternating double and single bonds that can switch their positions to give an equivalent resonance
structure:
It is important, however, to keep in mind what resonance structures such as these really mean:
Benzene does not exist as an equilibrium mixture of the two Lewis structures, nor does it flip back
and forth between the two structures as time passes. Instead, a benzene molecule is always in a
state that is intermediate between the two structures that cannot be accurately captured by a
normal Lewis structure: Every carbon-carbon bond in the molecule has characteristics intermediate
between those of a single and those of a double bond. If we had to try to depict this using a Lewis
structure, we would draw the following: