Is benzene an aromatic compound? What about cyclohexane, cyclohexene and 1,4-cyclohexadiene?

Yes, benzene is an aromatic compound. Aromatic compounds are a class of organic compounds that contain a specific ring structure called an aromatic ring. These rings are characterized by having alternating single and double bonds and being unusually stable compared to typical carbon-carbon bonds.

Benzene has a six-membered ring with three alternating double bonds and is often depicted as a regular hexagon with a circle inside it. The delocalization of electrons in the ring contributes to its stability and aromaticity.

As for the other compounds you mentioned:

Cyclohexane: Cyclohexane is not an aromatic compound. It is a cycloalkane, which means it is a saturated hydrocarbon with single bonds only. It has a six-membered ring of carbon atoms with each carbon bonded to two hydrogen atoms.

Cyclohexene: Cyclohexene is not an aromatic compound either. It is an unsaturated hydrocarbon known as a cycloalkene. It has a six-membered ring with one double bond.

1,4-cyclohexadiene: 1,4-cyclohexadiene is also not an aromatic compound. It is an unsaturated hydrocarbon and a cycloalkadiene, which means it has two double bonds in the six-membered ring, located at the 1st and 4th positions. Unlike benzene, the electrons in this molecule are not delocalized to form an aromatic system, and it lacks the characteristic stability of aromatic compounds.

In summary, only benzene is an aromatic compound among the ones you mentioned. The presence of a continuous, alternating double bond system within the ring is what distinguishes aromatic compounds from others.