Chemistry
Kekule Structure of Benzene
The Kekule structure of benzene is a historical model that represents benzene as a cyclic molecule with alternating single and double bonds between carbon atoms. Proposed by German chemist August Kekule in 1865, this model was later revised to incorporate the concept of resonance, which describes the delocalization of electrons within the benzene ring.
Written by Perlego with AI-assistance
Related key terms
1 of 5
7 Key excerpts on "Kekule Structure of Benzene"
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
A close look at one example, naphthalene, will illustrate what we mean by this. According to resonance theory, a molecule of naphthalene can be considered to be a hybrid of three Kekulé structures. One of these Kekulé structures, the most important one, is shown in Figure 14.15. There are two carbon atoms in naphthalene (C4a and C8a) that are common to both rings. These two atoms are said to be at the points of ring fusion. They direct all of their bonds toward other carbon atoms and do not bear hydrogen atoms. 1 2 3 4 5 6 7 8 9 10 3 2 1 10 9 8 7 6 5 4 7 6 5 4 3 8 2 1 7 6 5 10 8 9 4 3 2 1 Naphthalene C 10 H 8 Anthracene C 14 H 10 Phenanthrene C 14 H 10 Pyrene C 16 H 10 Benzo[a]pyrene C 20 H 12 Dibenzo[a,l ]pyrene C 24 H 14 FIGURE 14.14 Benzenoid aromatic hydrocarbons. Some polycyclic aromatic hydrocarbons (PAHs), such as dibenzo[a,l]pyrene, are carcinogenic. (See “Important, but hidden, epoxides” at the end of Chapter 11.) 7 6 5 4 4a 8 1 2 3 8a C C C H C H C C H H H C H C H C C H or FIGURE 14.15 One Kekulé structure for naphthalene. How many 13 C NMR signals would you expect for acenaphthylene? Acenaphthylene Strategy and Answer Acenaphthylene has a plane of symmetry which makes the five carbon atoms on the left (a–e, at right) equivalent to those on the right. Carbon atoms f and g are unique. Consequently, acenaphthylene should give seven 13 C NMR signals. SOLVED PROBLEM 14.4 Acenaphthylene a a b b c c d d e e f g 14.11 Other Aromatic Compounds 651 Molecular orbital calculations for naphthalene begin with the model shown in Figure 14.16. The p orbitals overlap around the periphery of both rings and across the points of ring fusion. When molecular orbital calculations are carried out for naphthalene using the model shown in Figure 14.16, the results of the calculations correlate well with our experimental knowledge of naphthalene.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
The Kekulé structure pre- dicts that there should be two different 1,2-dibromobenzenes, but there are not. In one of these hypothetical compounds (below), the carbon atoms that bear the bromines would be separated by a single bond, and in the other they would be separated by a double bond. Br Br Br Br and These 1,2-dibromobenzenes do not exist as isomers. • Only one 1,2-dibromobenzene has ever been found. To accommodate this objection, Kekulé proposed that the two forms of benzene (and of benzene derivatives) are in a state of equilibrium and that this equilibrium is so rapidly established that it prevents isolation of the separate compounds. Thus, the two 1,2-dibromobenzenes would also be rapidly equilibrated, and this would explain why chemists had not been able to isolate the two forms: Br Br Br Br There is no such equilibrium between benzene ring bond isomers. • We now know that this proposal was also incorrect and that no such equilibrium exists. Nonetheless, the Kekulé formulation of benzene’s structure was an important step forward and, for very practical reasons, it is still used today. Now we understand its meaning differently. The tendency of benzene to react by substitution rather than addition gave rise to another concept of aromaticity. For a compound to be called aromatic meant, experimentally, that it gave substitution reactions rather than addition reactions even though it was highly unsaturated. Before 1900, chemists assumed that the ring of alternating single and double bonds was the structural feature that gave rise to the aromatic properties. Since benzene and benzene derivatives (i.e., compounds with six-membered rings) were the only aromatic compounds known, chemists naturally sought other examples. The compound cyclooctatetraene seemed to be a likely candidate: Cyclooctatetraene In 1911, Richard Willstätter succeeded in synthesizing cyclooctatetraene. Willstätter found, however, that it is not at all like benzene.
eBook - ePub- Nenad Trinajstic(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
Chapter 8 ENUMERATION OF KEKULÉ VALENCE STRUCTURES I. THE ROLE OF KEKULÉ VALENCE STRUCTURES IN CHEMISTRYKekulé valence structures have been used in organic chemistry1 , 2 , 3 , 4 since 1865, when Kekulé proposed a hexagonal structure (see Figure 1 ) for benzene.5 , 6 , 7 , 8 , 9 , 10 The continued interest in Kekulé structures11 is related to their use in simple classical pictures of delocalized chemical bonding;12 , 13 pictures that carry over to modem quantum-chemical theories of benzenoid hydrocarbons.14 , 15Kekulé valence structures are the basis of several variants of valence bond (VB) resonance-theoretical models16 , 17 , 18 such as the Pauling-Wheland model,12 , 13 , 19 the Simpson-Herndon model,20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 and the conjugated-circuit model28 (see the next chapter). These models have been shown to be useful for predicting the properties of conjugated molecules11 , 12 , 13 , 14 , 15 , 22 , 23 , 29 , 30 , 31 , 32 , 33 , 34 in spite of some rather strong criticisms.35In the computationally simplest form of the resonance theory19 the ground states of conjugated molecules are described in terms of wave functions built from the Kekulé structures of equal weights. Therefore, the essential step in the application of the resonance theory is the enumeration of Kekulé valence structures. If the number of Kekulé structures K is known, one can, for instance, estimate quite accurately the total π-electron energy, Eπ , of a benzenoid hydrocarbon with N = the number of carbon atoms and M = the number of carbon-carbon bonds, by means of the formula36
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
Others have speculated that Kekulé's story in 1890 was a re-parody of the monkey spoof, and was a mere invention rather than a recollection of an event in his life. Kekulé's 1890 speech in which these anecdotes appeared has been translated into English. If one takes the anecdote as the memory of a real event, circumstances mentioned in the story suggest that it must have happened early in 1862. The cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale in 1929. Structure Benzene represents a special problem in that, to account for all the bonds, there must be alternating double carbon bonds: ________________________ WORLD TECHNOLOGIES ________________________ The various representations of benzene Using X-ray diffraction, researchers discovered that all of the carbon-carbon bonds in benzene are of the same length of 140 picometres (pm). The C–C bond lengths are greater than a double bond (135pm) but shorter than a single bond (147pm). This intermediate distance is explained by electron delocalization: the electrons for C–C bonding are distributed equally between each of the six carbon atoms. The molecule is planar (ignoring quantum/thermal vibrations), although many calculations predict otherwise. One representation is that the structure exists as a superposition of so-called resonance structures, rather than either form individually. The delocalization of electrons is one explanation for the thermodynamic stability of benzene and related aromatic compounds. It is likely that it is this stability that contributes to the peculiar molecular and chemical properties known as aromaticity. To reflect the delocalized nature of the bonding, benzene is often depicted with a circle inside a hexagonal arrangement of carbon atoms: The delocalized picture of benzene has been contested by Cooper, Gerratt and Raimondi in their article published in 1986 in the journal Nature.
eBook - ePubImage and Reality
Kekulé, Kopp, and the Scientific Imagination
- Alan J. Rocke(Author)
- 2010(Publication Date)
- University of Chicago Press(Publisher)
These same considerations apply to the benzene theory. Baeyer proceeded to summarize recent research on the structure of benzene. The benzene formula proposed years earlier by James Dewar had quickly been ruled out as unable to explain the chemical data. Albert Ladenburg’s “prism” formula and Adolf Claus’s “diagonal” formula were tougher nuts to crack, but Baeyer summarized research results that argued against these alternatives, as well. There could no longer be any question that the benzene molecule is indeed a symmetrical hexagon formed of six CH groups. The only question that still remained was: what to do about the fourth valence of each carbon atom? Baeyer proposed that so-called centric benzene (in which the fourth valence of each carbon atom was somehow directed toward the center of the ring) and the Kekulé structure (in which the fourth valence of each carbon atom formed a double bond with one of its two neighbors) were in a sense both true; they were “limiting states” of benzene. So Baeyer could now answer his initial question. Kekulé’s benzene theory was, like all theories, a temporary though highly fruitful heuristic tool; but “for ordinary usage” it was also “the best expression of the facts.” 11 Baeyer concluded with some final thoughts about the relationship between Kekulé’s theory of chemical structure and his aromatic theory. The crucial breakthrough, he stated, was the former, which had been published seven years earlier (in 1858). Structure theory demonstrated that “the general laws of mechanics do not suffice to explain the essence of matter, that atoms possess specific properties, a knowledge of which must precede the application of mechanics
eBook - PDFFoundations of Chemistry
An Introductory Course for Science Students
- Philippa B. Cranwell, Elizabeth M. Page(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Benzene has a delocalised electron system above and below the ring. For a refresher on the shapes of molecules, please see Chapter 2. Foundations of Chemistry: An Introductory Course for Science Students , First Edition. Philippa B. Cranwell and Elizabeth M. Page. © 2021 John Wiley & Sons Ltd. Published 2021 by John Wiley & Sons Ltd. Companion website: www.wiley.com/go/Cranwell/Foundations In the skeletal and Kekulé representations of benzene (Figure 15.1a,b), it can be seen that there are alternating single and double carbon – carbon bonds. If this were true, then there would be two different bond lengths in benzene: that for a single carbon – carbon bond and that for a double carbon – carbon bond. Experimentally, this is not observed, and in benzene all of the car-bon – carbon bonds are the same length. This therefore leads to the representa-tion of benzene where the delocalised π system is represented as a circle in the middle of a cyclohexane (Figure 15.1c). This representation is often used at A-level, but during undergraduate studies structure (b) is usually used. You should become used to using both (b) and (c) interchangeably. Finally, Figure 15.1d shows the p orbitals overlapping above and below the benzene ring to give the conjugated π system. A total of six electrons are in the π system above and below the ring. In 1931, German chemist Erich Hückel proposed a theory that would allow chemists to determine whether a compound was aromatic by looking at the structure, and would also allow the properties of the compound to be hypothesised. This led to the four criteria for aromaticity, Hückel ’ s rules, which chemists still use today. Worked Example 15.1 Explain, using Hückel ’ s rules and a diagram, why benzene, C 6 H 6 , can be thought of as aromatic.
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.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
