Aromatic compounds Hiickels rule

The first question that needed answering was whether the unsaturated NHC are indeed aromatic. The Hiickel rule states that aromatic systems are monocyclic, homonuclear, planar and possess a delocalised 4n-n2 jr-electron system [96]. Unsaturated NHC fulfil all the criteria except homonuclearity, which would make them heteroaromatic compounds. However, there was doubt as to the delocalisation of the 6 7t-electron system. Althongh there are 6 n-electrons, their distribution is extremely unequal, the backbone carbon atoms C and C contribute one rr-electron each, the nitrogen atoms provide a lone pair each, but the carbene carbon atom C gives none. Can there be a true delocalisation in such circumstances ... 

Hiickel 4n -r 2 rule and, 523-524 imidazole and, 529 Indole and. 533 ions and,525-527 isoquinoline and, 533 naphthalene and, 532 polycyclic aromatic compounds and,531-532 purine and, 533... 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Problem 10.7 Cyclooctatetraene (CgH ), unlike benzene, is not aromatic it decolorizes both dil. aq. KMnO and Brj in CCI4. Its experimentally determined heat of combustion is -4581 kJ/mol. (a) Use the Hiickel rule to account for the differences in chemical properties of CgHg from those of benzene, (b) Use thermochemical data af Problem 10.4 to calculate the resonance energy, (c) Why is this compound not antiaromatic (d) Styrene, CgH5CH==CH2, with heat of combustion —4393 kJ/mol, is an isomer of cyclooctatetraene. Is styrene aromatic ... 

The solubility properties suggest that these compounds are salts. The stability of the organic ions formed indicates that they conform to the Hiickel rule and are aromatic. 

An aromatic compound has a molecular structure containing cyclic clouds of delocalized tt electrons above and below the plane of the molecule, and the TT clouds contain a total of 4n -f 2) tt electrons (where n is a whole number). This is known as Hiickel s rule (introduced first by Erich Hiickel in 1931). Eor example, benzene is an aromatic compound. 

If n = 1, we have 4 x 1+2 = 6, which means that any compound containing a total number of six tt electrons is an aromatic compound. In the above structure of benzene, there are three double bonds and six tt electrons, and it is a planar molecule. Thus, benzene follows Hiickel s rule, and is an aromatic compound. 

Section 11.23 Hiickel s rule can be extended to heterocyclic aromatic compounds. 

Because these compounds are monocyclic aromatic compounds, they must obey Hiickel s Rule. Hiickel s Rule requires An + 2n electrons, so the simplest aromatic compound should contain 6 n electrons (n = 1). Pyrrole, furan, and thiophene appear, however, to have only 4 n electrons (2 n bonds). In systems such as these, the extra electrons needed to produce an aromatic condition come from the unshared electron pairs in sp hybrid orbitals around the hetero atom. 

While benzene was the first aromatic system studied, the formulation of HUckel s rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Hiickel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

This is the Hiickel rule. It prescribes that the aromatic monocyclic compounds should possess 2, 6, 10. .. jt electrons. 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

It turns out that in addition to being cyclic, planar, and completely conjugated, a compound needs a particular number of n electrons to be aromatic. Erich Hiickel first recognized in 1931 that the following criterion, expressed in two parts and now known as Huckel s rule, had to be satisfied, as well ... 

To fill the MOs, the six electrons are added, two to an orbital, beginning with the lowest energy orbital. As a result, the six electrons completely fill the bonding MOs, leaving the antibonding MOs empty. This is what gives benzene and other aromatic compounds their special stability and this is why six 7t electrons satisfies Hiickel s 4n + 2 rule. 

Certain cyclic, completely conjugated, rr systems show unusual stability. These systems are said to be aromatic. Hiickel originally narrowly defined aromatic compounds as those completely conjugated, monocyclic carbon compounds that contain (4n + 2) tt electrons. In this designation, n can be 0, 1, 2, 3,..., so that systems that contain 2, 6, 10, 14, 18,...ir electrons are aromatic. This criterion is known as Hiickel s rule. 

Example 1.10. Some aromatic compounds that strictly obey Hiickel s rule. 

T ekQtrons. That is to say, for the particular degree of stability that characterizes an aromatic compound, delocalization alone is not enough. There must be a partipular number of w electrons 2, or 6, or 10, etc. This requirement, called the 4/1 + 2 rule or Hiickel rule (after Erich Hiickel, of the Institut fiir theoretische Physik, Stuttgart), is based on quantum mechanics, and has to do with the filling up of the various orbitals that make up the tt cloud (Sec. 29.6). The Hiickel rule is strongly supported by the facts. 

Let us look at some of the evidence supporting the Hiickel rule. Benzene has six 7T electrons, the aromatic sextet six is, of course, a Hiickel number, corresponding to /f = 1. Besides benzene and its relatives (naphthalene, anthracene, phenanthrene. Chap. 30), we shall encounter a number of heterocyclic compounds (Chap. 31) that are clearly aromatic these aromatic heterocycles, we shall see, are just the ones that can provide an aromatic sextet. 

The Hiickel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended beyond simple mono-cyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two benzene-like rings fused together, anthracene, 1,2-benzpyrene, and coronene are all well-known compounds. Benzolalpyrene is particularly interesting because it is one of the cancer-causing substances that has been isolated from tobacco smoke,... 

The Hiickel rule is strictly applicable only to monocyclic compounds, but the general concept of aromaticity can be extended beyond simple monocyclic compounds to include polycyclic aromatic compounds. Naphthalene, with two... 

---