Aromatic states

Sulphur. The general method is to heat the compound at 200-260° with the theoretical amount of sulphur required to bring it to the aromatic state ... 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

The dearomatization of benzene requires that a resonance energy estimated at 36 kcal mol-1 be overcome [14]. For this reason, when benzene attacks an electrophile, the end result is typically a substitution product rather than an addition product. The initial cyclohexadienyl cation is unstable to deprotonation by even extremely poor bases, and it therefore loses a proton and returns to an aromatic state. 

Which compounds are aromatic For any compound that is not aromatic, state why this is so. 

In the first stage, the aromatic molecule is attacked by the electrophile and a Ti-complex is formed, which retains the aromatic state. The Ti-complex is converted in a second reaction stage into the a-complex. The high stability, which the aromatic electron sextet gives to the hydrocarbons, is lost in this process. The a-com-plex is a reactive intermediate, which can be isolated through substitution with suitable electron donors. 

Hiickel s rale of aromaticity states that monocyclic conjugated hydrocarbons with 4m + 2 Jt-electrons are aromatic, whereas systems with 4m ti-electrons are antiaromatic. Consequently, the number of t-electrons is crucial to determine stability, structure, and reactivity of aromatic and antiaromatic systems. In 1954, Doering and... 

First, we observe that both electronic configurations correspond to aromatic states. The A[ state is fourfold (twice of o-type, one with two electrons and the other... 

Pyrene tethered by eight carbon atoms The threshold for the cyclization is a tether of eight carbon atoms 45, where no two-electron reduction of the pyrene occurs even, after prolonged contact with either Li or K metals. It was suggested that the strain in 45 is large enough not to permit either an anti-aromatic state, or the formation of an intramolecular cr-bond [124]. 

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