Category: Aromaticity

Certain structural units containing heteroatoms can be substituted into conjugated systems in such a way that the system remains conjugated and isoelectronic with the original hydrocarbon. The most common examples are -CH=N- and -N=N- double bonds and divalent sp2 -O-, -S- and -NR- units. Each of these structural fragments can replace a -CH=CH- unit in a conjugated…

There is evidence that aromatic segments can exist as part of larger conjugated units, resulting in an aromatic segment in conjugation with a ‘localized’ double bond. For example, in acenaphthylene, the double bond in the five-member ring is both structurally and chemically similar to a normal localized double bond. The resonance energy 0.57β is slightly…

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. It is of interest to be able to predict the stability of such fused-ring compounds. Because Huckel’s rule applies only to monocyclic systems, it cannot be applied to the fused-ring compounds, and there have been many efforts to develop relationships…

In 1959 Winstein introduced the term ‘homoaromatic’ to describe compounds that display aromaticity despite one or more saturated linkages interrupting the formal cyclic conjugation. Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The saturated unit is generally a CH2 group, but can be a…

There are also striking stability relationships due to aromaticity in charged ring systems. These energy levels are applicable to charged species as well as to the neutral annulenes. A number of cations and anions that have completely conjugated planar structures are shown below. The Huckel rule predicts aromatic stability for cyclopropenium ion (A), cyclobutenium dication…

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. Conjugated monocyclic polyenes (CnHn), in which n is greater than or equal to 10, are usually called annulenes. The number in brackets tells us how many carbon atoms there are in the ring. The synthesis of annulenes has been extended well beyond the first two members…

An aromatic compound is more stable than the analogous acyclic compound. The planar cyclic 4n systems are referred to as antiaromatic. An antiaromatic compound is less stable than the analogous acyclic compound. Aromaticity is characterized by stabilization, whereas antiaromaticity is characterized by destabilization. relative stabilities aromatic compound > acyclic compound > antiaromatic compound A compound is said to…

The meaning of the word aromaticity has evolved, as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more…

Organic compounds have been classified on the basis of their structure. Open-chain structures containing carbon–carbon single (C–C), double (C=C) and triple bonds are said to be aliphatic compounds. The term ‘aliphatic’ is derived from the Greek word for fat or oil. In alicyclic molecules, the carbon atoms form a cyclic structure. The generic name aromatic was given originally to…