Aromatic rings

A ring that does have a loop of π electrons is said to be aromatic. Thus, the rings in 2, 4, 5, and 7 are aromatic. A compound whose molecule contains one or more aromatic rings is called an aromatic compound. Thus, 2, 4, 5, and 7 are aromatic compounds. There are chemical and spectroscopic methods that can be used to experimentally determine whether a ring is aromatic or not. All aromatic rings share two structural features:

electrons is equal to 4n + 2, where n is zero or a positive whole number.

As is evident from the above examples, determining whether a ring obeys the Hückel’s rule or not using the Lewis diagram is straightforward.  However, determining whether a ring is planar or not requires careful consideration of many factors. A case in point is [10]annulene (10).

If the ring in 10, which obeys the Hückel’s Rule, were planar, the hydrogen atoms on C-2 and C-7, which are inside the ring, would have to occupy the same space, which would be physically impossible.  To accommodate the hydrogen atoms on C-2 and C-7 inside the ring, the ring is forced to deviate from planarity. Thus, 10 is not aromatic. If [10]annulene were to assume a configuration in which all double bonds were cis double bonds (11), the ring would be planar and, therefore, be aromatic.

Experimentally, however, [10]annulene exists exclusively as 10, indicating that the stability gained by having a loop of π electrons in 11 is outweighed by considerable angle strain