Annulenes

• They are monocyclic conjugated systems, that is, they have alternate double bonds in a cyclic system.
• The ring size is indicated in the name in brackets, thus benzene is [6]-annulene.
• All annulenes are not aromatic. Planar cyclic systems that have (4n+2) Pi electrons involved in cyclic delocalisation ( Huckel Rule) are aromatic and similarly the 4n systems are antiaromatic.

Importance of Annulenes

• Most of these compounds are of considerable academic importance. Of course compounds like benzene and its derivatives are of industrial importance as well, which is common knowledge.
• Before 1960 only two compounds benzene and cyclooctatetraene were known two verify Huckel observations. Subsequently a large number were synthesized and the predictions of Huckel rule were confirmed.

Two electron systems

• Huckel rule predicts cyclopropenyl cation to be aromatic. It has two electrons (n=0) involved in cyclic delocalisation.
• Many stable substituted systems have been prepared. Among others the trichloro, diphenyl, and dipropyl derivatives are known.
• Tricyclopropyl cyclopropenyl cation is one among the more stable cations known.

Four electron systems

• A closed loop of 4n systems is anti aromatic, less stable than the corresponding open chain counterpart.
• They have been studied by trapping them in solid matrices at low temperatures.
• Substituted cyclobutadienes of varying degrees of stability have been prepared.
• These molecules are relatively stable because dimerisation is sterically hindered.

Six electron systems