Chemistry
Huckels Rule
Huckel's rule is a concept in organic chemistry that predicts whether a planar ring molecule will exhibit aromaticity. According to the rule, a molecule will be aromatic if it contains 4n + 2 π electrons, where n is a non-negative integer. Aromatic molecules are more stable and have unique properties due to their delocalized electron structure.
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3 Key excerpts on "Huckels Rule"
eBook - PDF- John Daintith(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
As this configuration was en-ergetically more stable than placing electrons in isolated double bonds, benzene’s stability fol-lowed directly from the model. Hückel went on to generalize his model to cover other cyclic molecules containing alter-nating double and single bonds. Aromatic mol-ecules were planar compounds which had precisely 4 n + 2 pi-electrons, where n = 0, 1, 2, 3 … . This is known as the Hückel rule . Benzene represents the case where n = 1; and n = 2 and n = 3 represent the 10 and 14 member aro-matic rings of naphthalene and anthracene. For n = 0, the predicted aromaticity of a 3 mem-ber ring was confirmed in 1962 with the dis-covery of the cyclopropenyl cation. Huffman, Donald Ray (1935– ) Ameri-can physicist Born at Fort Worth in Texas, Huffman was ed-ucated at Texas Agricultural and Mechanical College, at Rice University, Houston, and at the University of California, Riverside, where he completed his PhD in 1966. After spending a postdoctoral year at the University of Frank-furt, Huffman moved to the University of Ari-zona, Tucson, in 1967 and was later appointed professor of physics in 1975. He is currently emeritus professor at Arizona. In 1985 in the laboratory of Richard S MALLEY a new form of carbon had been discovered: C 60 , known as “buckminsterfullerene.” The C 60 was produced by vaporizing a graphite target with a pulsed laser beam. The sooty carbon produced in this manner certainly contained a detectable amount of C 60 , but all efforts to extract the sub-stance from the residue in amounts sufficient to carry out a detailed spectroscopic study failed. Huffman, in collaboration with Wolfgang Kratschmer of the Max Planck Institute for Physical Chemistry, Heidelberg, was involved in the discovery of the new forms of carbon known as fullerenes . For many years they had been interested in the nature of interstellar dust, which they believed to be mainly carbon.
eBook - PDF- Ilya Prigogine(Author)
- 2009(Publication Date)
- Wiley-Interscience(Publisher)
Pariser and Parr admitted a certain degree of arbi- trariness in the choice of their numerical values, but emphasized the possibility of correlating in a quantitative manner a good deal more experimental data than theoretical parameters. Subsequent work, on other aromatic molecules, has justified this scheme. The essential points of Pariser and Parr’s treatment are, there- fore, (1) the decision to work in terms of Huckel molecular orbitals and to restrict consideration to unexcited and singly MOLECULAR ORBITAL THEORY 355 excited configurations; (2) to adopt empirical rather than theore- tical values for the various integrals which are required in the quantitative calculations. Their theory does not make any refer- ence to the method of the self-consistent field. The next development in the theory of aromatic hydrocarbon spectra was a paper by Dewar and Longuet-Higgins.13 These authors were concerned particularly with the spectra of alternant hydrocarbons in which, at least according to the Huckel theory, the molecular orbital energies are grouped symmetrically around the zero of binding energy. They drew attention to the fact that electron repulsion might mix together particularly strongly a pair of excited configurations whose energies on the Huckel theory were equal. If the molecular orbitals are, in order of increasing energy, YlJ Y3j . . ' J Y m , Wm+l, * . .) Y z m then the orbitals yi and y2m+l-j will have equal and opposite binding energies. Hence the configurations lxmm+Z and 1 ~ ~ - ~ ~ + l would on the Huckel theory be degenerate and between these configurations there would be strong interaction. Dewar and Longuet-Higgins pointed out that this interaction was required by symmetry in the case of benzene and in cyclic polyenes of formula (CH)4p+z and that in these molecules the resulting upper states would be precisely those postulated by hloffitt in his LCAO perimeter theory.
eBook - PDF- David V. George(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
Hückel MO Theory 211 bonds. Examples are: butadiene, benzene, naphthalene, and the purine and pyrimidine bases of DNA. In fact, a very large number of important organic and biochemical molecules are of this type. Many of these mole-cules have been studied in great detail by experimentalists, resulting in a vast accumulation of experimental data. It has become very desirable, therefore, to try to understand the experimental results for these mole-cules in terms of quantum theory: Hiickel molecular-orbital theory has made a major contribution in this regard. The distinctive feature of conjugated molecules is that we can classify their valence electrons into two separate sets, -electrons and cr-elec-trons. The -electrons are assumed to be strongly localized in the indi-vidual bonds, the bond orbitals associated with them having the -type symmetry we discussed for diatomic molecules (cf. Section 10-7). These electrons are relatively unreactive. On the other hand, the -electrons are relatively much more reactive and therefore play a more important role in the chemical reactions of the molecules. 7r-electrons, unlike the cr-electrons, are assumed to be delocalized over the carbon framework of the molecule. Their name derives from the analogy between the mole-cular orbitals describing them and the 7r-type molecular orbitals we discussed for diatomic molecules. The classic example of a 7r-electron system is, of course, that which occurs in the benzene molecule; in this molecule we assume that six p orbitals, one from each of the carbon atoms, overlap to form the -orbital system —the six 7r-electrons are then delocalized over the whole molecule when we assign them to this system. It is -electron systems of this type that we are going to be concerned with here. We first assume that the wavefunction for the -electrons is indepen-dent of the cr-electron framework; i.e., the 7r-electron system acts independently.
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