Aromatic, Antiaromatic, and Nonaromatic Compounds

Our working definition of aromatic compounds has included cyclic compounds containing conjugated double bonds with unusually large resonance energies. At this point we can be more specific about the properties that are required for a compound (or an ion) to be aromatic. 

Aromatic compounds are those that meet the following criteria  

The structure must be cyclic, containing some number of conjugated pi bonds. 

Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp hybridized or occasionally sp hybridized.) 

The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. In most cases, the structure must be planar (or nearly planar) for effective overlap to occur. 

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Aromatic, Antiaromatic, and Nonaromatic Compounds 722 16-6 Huckel s Rule 722... 

Q Determine whether Hiickel s rule applies to a given structure, and predict whether the compound will be aromatic, antiaromatic, or nonaromatic. 

Tlie tautomerism of heteroaromatic compounds is intimately related to the problem of aromaticity and antiaromaticity (87MI1,94MI1), this being especially true for the compounds of this chapter, which are borderline between these showing Fliickel behavior and polyenic compounds (nonaromatic). 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

Nevertheless, the NICS values appear to readily classify standard molecules into three discrete categories. Aromatic molecules possess NICS values that are negative. The values at the center of the six-member rings of benzene and naphthalene and anthracene are -9.7 and -9.9, respectively. Charged aromatic molecules also have negative NICS values the values for cyclopentadienyl anion and tropy-lium cation are -14.3 and -7.6 ppm, respectively. Nonaromatic compounds like cyclohexane and adamantane have NICS values near zero. Lastly, antiaromatic molecules such as cyclopentadiene and planar Z>4 cyclooctatetrane have NICS values that are positive, 27.6 and 30.1 ppm, respectively. 

Zhou, Parr, and Garst showed that absolute hardness correlates well with theoretical measures of aromaticity but that the value of tj by itself does not allow the categorization of aromaticity. Zhou and Parr later defined the relative hardness of a species as the difference between its hardness and the hardness of an acyclic reference compound. Based on these correlations, the authors proposed that a compound is aromatic if its Hiickel absolute hardness (determined from the Hiickel HOMO-LUMO gap) is less than -O.ip, antiaromatic if the value of tj is less than -0.15j8, and nonaromatic if rj is between those two values. The corresponding division based on relative hardness is 0. That is, a cyclic molecule that is harder than an acyclic analog is aromatic, while one that is not as hard as an acyclic analog is antiaromatic. 

This reference structure gives the annulene aromaticities in Figure 10. These are perhaps slightly worse than the results in Figure 9 since only cyclobutadiene and cyclooctatetraene are computed to be antiaromatic. The order of the 11 compounds in column three of Figure 6 is fairly good. Benzene is now at the top of the list, cyclobutadiene is antiaromatic, and butadiene is nonaromatic. However, three... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

Though it is usually not difficult to classify a given compound as aromatic, nonaromatic or antiaromatic from a qualitative point of view, much more complex problems arise in attempting to describe the aromaticity in quantitative terms. Until now, three main groups of quantitative criteria of aromaticity have been elaborated energetical, structural and magnetic. 

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