Benzene Structure

10 Key excerpts on "Benzene Structure"

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  Petrochemistry

  • (Author)
  • 2014(Publication Date)
  • Library Press

    (Publisher)

  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. They showed that the electrons in benzene are almost certainly localized, and the aromatic properties of benzene originate from spin coupling rather than electron delocalization. This view has been supported in the next-year Nature issue, but it has been slow to permeate the general chemistry community. As is common in organic chemistry, the carbon atoms in the diagram above have been left unlabeled. Realizing each carbon has 2p electrons, each carbon donates an electron into the delocalized ring above and below the benzene ring. It is the side-on overlap of p-orbitals that produces the pi clouds. WT ____________________ WORLD TECHNOLOGIES ____________________ Benzene occurs sufficiently often as a component of organic molecules that there is a Unicode symbol in the Miscellaneous Technical block with the code U+232C ( ⌬ ) to represent it with three double bonds, and U+23E3 ( ) for a delocalized version. Substituted benzene derivatives Many important chemicals are derived from benzene by replacing one or more of its hydrogen atoms with another functional group. Examples of simple benzene derivatives are phenol, toluene, and aniline, abbreviated PhOH, PhMe, and PhNH 2 , respectively. Linking benzene rings gives biphenyl, C 6 H 5 –C 6 H 5 . Further loss of hydrogen gives fused aromatic hydrocarbons, such as naphthalene and anthracene. The limit of the fusion process is the hydrogen-free allotrope of carbon, graphite. In heterocycles, carbon atoms in the benzene ring are replaced with other elements. The most important derivatives are the rings containing nitrogen. Replacing one CH with N gives the compound pyridine, C 5 H 5 N.

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  Quantum Chemistry

  A Unified Approach

  • David B Cook(Author)
  • 2012(Publication Date)
  • ICP

    (Publisher)

  These chemical properties of benzene present a serious challenge to the theory of molecular electronic structure we have developed. It is clear that something new is happening in benzene which cannot be accounted for by the environment-insensitive substructures considered so far. This is not an isolated phenomenon confined to benzene. It is found in a wide class of compounds which all share a common property: Molecules which contain a continuous set of atoms bonded in a linear fashion, or in a ring in which the atoms of this 190 Delocalised Electronic Substructures: Aromaticity set are joined by alternating single and double bonds, show properties which are not explicable by using the properties of singly-and doubly-bonded atoms. They have, to a greater or lesser extent, some of the properties which are epitomised by the benzene molecule. Some typical examples are: naphthalene, azulene, butadiene, pyridine, the allyl radical. 1 Perhaps the easiest way to begin to understand the structures of these molecules is to look briefly at the earliest attempt to explain their prop-erties using the ‘conventional’ substructures which we have met already. The second and third of the items in the list at the start of Section 11.1 were the most puzzling: the fact that benzene is a regular hexagon and the existence of only one 1,2 disubstituted benzene. The explanation which appealed to chemists was that the single and double bonds in benzene were swapping places very rapidly, so rapidly that it was impossible to measure the lengths of the individual single and double bonds. One simply gets an average value for both. The same explanation would also work for the second puzzle. If the single and double bonds were swapping places very rapidly, then separation of the two isomers would be impossible.

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  Understanding Advanced Organic and Analytical Chemistry

  The Learner's ApproachRevised Edition

  • Kim Seng Chan, Jeanne Tan;;;(Authors)
  • 2016(Publication Date)
  • WS EDUCATION

    (Publisher)

  CHAPTER 6

  Arenes

  6.1 Introduction

  Arenes are a family of aromatic compounds known for their characteristic feature — the benzene ring. Benzene, C6 H6 , is made up of six sp2 hybridized carbon atoms, and its molecular structure is a resonance hybrid described by “an average” of two equivalent resonance forms (see Fig. 6.1 ). The six electrons are delocalized throughout the six-membered ring structure. As a result, the whole benzene molecule is planar in shape, with the 12 atoms (six carbon and six hydrogen) lying on the same plane.

  Q:

  Since arenes are aromatic compounds, do they all have nice fragrance?

  A:

  As a matter of fact, not all arenes have a nice fragrance, although the term “aromatic” literally means nice fragrance. In fact, the term “aromatic” refers to compounds that have a resonance stabilized pi network of electrons, consisting of a system of conjugated pi bonds. Based on molecular orbital theory (refer to Chapter 15 ), the German physicist Erich Hükel considered a molecule to be aromatic if the molecule possessed (4n + 2) number of conjugated pi electrons. Note that this rule is good enough to help us predict whether a molecule is aromatic, though there are some molecules that are aromatic in nature but do not follow the Hükel’s (4n + 2) rule. This would mean that such molecules are still resonance stabilized but do not have (4n + 2) number of conjugated pi electrons.

  Fig. 6.1.

  Q:

  What is resonance?

  A:

  Resonance refers to the delocalization of pi electrons as a result of the side-on overlapping of a few p orbitals that are parallel to each other. Note that the way the electrons are delocalized here is different from that in metal because of the need for p orbitals to be involved, whereas in metal it is not necessary.

  Q:

  Is there a limit to the number of atoms that can be involved in the delocalization?

  A:

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  Solomons' Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  14.1 THE DISCOVERY OF BENZENE The following are a few examples of aromatic compounds, including benzene itself. In these formulas we foreshadow our discussion of the special properties of the benzene ring by using a circle in a hexagon to depict the six π electrons and six-membered ring of these compounds, whereas up to now we have shown benzene rings only as indicated in the left-hand formula for benzene below. The study of the class of compounds that organic chemists call aromatic compounds (Section 2.1D) began with the discovery in 1825 of a new hydrocarbon by the English chemist Michael Faraday (Royal Institution). Faraday called this new hydrocarbon “bicar- buret of hydrogen”; we now call it benzene. Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil. In 1834 the German chemist Eilhardt Mitscherlich (University of Berlin) synthesized benzene by heating benzoic acid with calcium oxide. Using vapor density measurements, Mitscherlich further showed that benzene has the molecular formula C 6 H 6 : C 6 H 5 CO 2 H + CaO  → heat C 6 H 6 + CaCO 3 Benzoic acid Benzene The molecular formula itself was surprising. Benzene has only as many hydrogen atoms as it has carbon atoms. Most compounds that were known then had a far greater propor- tion of hydrogen atoms, usually twice as many. Benzene, having the formula of C 6 H 6 , should be a highly unsaturated compound because it has an index of hydrogen deficiency equal to 4. Eventually, chemists began to recognize that benzene was a member of a new class of organic compounds with unusual and interesting properties. As we shall see in Section 14.3, benzene does not show the behavior expected of a highly unsaturated compound. During the latter part of the nineteenth century the Kekulé–Couper–Butlerov theory of valence was systematically applied to all known organic compounds.

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  A Century of Science 1851-1951

  • Herbert Dingle(Author)
  • 2014(Publication Date)
  • Routledge

    (Publisher)

  unsaturated , since they easily add other atoms to form single bonds out of the double and triple bonds, such as in which the carbon atom has its usual valency of four, each carbon linking two hydrogens and one chlorine, and using its remaining bond to link another atom of carbon. The capacity of carbon atoms to link together into chains was also recognized by Kekulé, and it explains why so many carbon compounds are known.

  The Structure of Benzene

  The hydrocarbon benzene, C6 H6 , was discovered by Faraday in 1825. By 1850 it was known that many carbon compounds are derivatives of benzene. Phenol, or carbolic acid, for example, has the formula C6 H6 O, and by taking away the oxygen, benzene is formed from it. These substances often have peculiar and sometimes pleasant odours, and are called aromatic compounds, another large group of carbon compounds being called aliphatic compounds. Whereas the structural formulae of aliphatic compounds can all be represented by the kind of valency bond arrangement shown above, with chains of carbon atoms, this did not prove possible for the benzene derivatives. If we write the formula of benzene with alternate single and double carbon linkages, two valency bonds at the end of the chain are left over, in 1865 solved this problem by assuming that the chain forms a closed ring, the two free bonds at the ends linking together, and so the famous benzene ring came into being:

  Since there are so many benzene derivatives, chemists represent the benzene ring by a simple hexagon, the atoms or radicals which substitute atoms of hydrogen in the ring being shown. Phenol, for example, has a hydrogen atom substituted by a hydroxyl radical, −O−H, and its formula is written as ‘I’ below; aniline has H replaced by the amino-group −NH2

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  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  14.6 MODERN THEORIES OF THE STRUCTURE OF BENZENE 625 14.6 MODERN THEORIES OF THE STRUCTURE OF BENZENE It was not until the development of quantum mechanics in the 1920s that the unusual behavior and stability of benzene began to be understood. Quantum mechanics, as we have seen, produced two ways of viewing bonds in molecules: resonance theory and molecular orbital theory. We now look at both of these as they apply to benzene. 14.6A The Resonance Explanation of the Structure of Benzene A basic postulate of resonance theory (Sections 1.8 and 13.4) is that whenever two or more Lewis structures can be written for a molecule that differ only in the positions of their electrons, none of the structures will be in complete accord with the compound’s chemical and physical properties. If we recognize this, we can now understand the true nature of the two Kekulé structures (I and II) for benzene. • Kekulé structures I and II below differ only in the positions of their electrons; they do not represent two separate molecules in equilibrium as Kekulé had proposed. Instead, structures I and II are the closest we can get to a structure for benzene within the limitations of its molecular formula, the classic rules of valence, and the fact that the six hydrogen atoms are chemically equivalent. The problem with the Kekulé structures is that they are Lewis structures, and Lewis structures portray electrons in localized distri- butions. (With benzene, as we shall see, the electrons are delocalized.) Resonance theory, fortunately, does not stop with telling us when to expect this kind of trouble; it also gives us a way out. • According to resonance theory, we consider Kekulé structures I and II below as resonance contributors to the real structure of benzene, and we relate them to each other with one double-headed, double-barbed arrow (not two separate arrows, which we reserve for equilibria). Resonance contributors, we emphasize again, are not in equilibrium.

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  Organic Chemistry, Student Study Guide and Solutions Manual

  • David R. Klein(Author)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  (a) The term meta refers to a 1,3-disubstituted benzene ring. (b) A Frost circle is used to draw an energy diagram showing the relative energy levels of the MOs associated with a ring comprised of a continuous conjugated  system, such as benzene. (c) All six carbon atoms in benzene are sp 2 hybridized, with trigonal planar geometry. (d) A chair conformation is one of the conformations that a cyclohexane ring can adopt. Benzene is flat and does not adopt the conformations that are accessible to cyclohexane. (e) The term ortho refers to a 1,2-disubstituted benzene ring. (f) All six carbon atoms in cyclohexane are sp 3 hybridized, with tetrahedral geometry. (g) Benzene is resonance stabilized (while cyclohexane possesses no  electrons, and therefore has no resonance structures). (h) Benzene has  electrons, while cyclohexane does not. (i) The term para refers to a 1,4-disubstituted benzene ring. (j) Cyclohexane undergoes a conformational change called ring flipping. In contrast, benzene is planar and does not undergo ring flipping. (k) A boat conformation is one of the conformations that a cyclohexane ring can adopt. Benzene is flat and does not adopt the conformations that are accessible to cyclohexane. 17.32. (a) Each of the rings is comprised of a continuous system of overlapping p orbitals with a total of six  electrons. As such, each ring is aromatic. The compound would be aromatic if either ring alone were aromatic. This compound is most certainly aromatic. Notice that, in determining aromaticity, each ring is considered individually. (b) As described in Section 17.5, the hydrogen atoms positioned inside the ring experience a steric interaction that forces this compound, called [10]annulene, out of planarity. Since the molecule cannot adopt a planar conformation, the p orbtials cannot continuously overlap with each other to form one system, and as a result, [10]annulene does not meet the criteria for aromaticity.

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  Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Charles D. Winters Butylated hydroxytoluene (BHT) is often used as an antioxidant in baked goods to “retard spoilage.” Summary of Key Questions 303 SUMMARY OF KEY QUESTIONS 9.1 What Is the Structure of Benzene? • Benzene is a molecule with a high degree of unsaturation possessing the molecular formula C 6 H 6 . Each carbon has a single unhybridized 2p orbital that contains one electron. These six 2p orbitals lie perpendicular to the plane of the ring and overlap to form a continuous pi cloud encompass- ing all six carbons. • Benzene and its alkyl derivatives are classified as aromatic hydrocarbons, or arenes. 9.2 What Is Aromaticity? • According to the Hückel criteria for aromaticity, a cyclic compound is aromatic if it (1) has one 2p orbital on each atom of the ring, (2) is planar so that overlap of all p orbit- als of the ring is continuous or nearly so, and (3) has 2, 6, 10, 14, and so on, pi electrons in the overlapping system of p orbitals (i.e., it has 4 2 n electrons). • A heterocyclic aromatic compound contains one or more atoms other than carbon in an aromatic ring. 9.3 How Are Benzene Compounds Named, and What Are Their Physical Properties? • Aromatic compounds are named by the IUPAC system. The common names toluene, xylene, styrene, phenol, aniline, benzaldehyde, and benzoic acid are retained. • The C 6 H 5 group is named phenyl, and the C 6 H 5 CH 2 group is named benzyl. • To locate two substituents on a benzene ring, either num- ber the atoms of the ring or use the locators ortho (o), meta (m), and para (p). • Polynuclear aromatic hydrocarbons contain two or more fused benzene rings. 9.4 What Is the Benzylic Position, and How Does It Contribute to Benzene Reactivity? • The benzylic position is the carbon of an alkyl substituent immediately bonded to the benzene ring. • The benzylic position of a benzene ring can be oxidized by chromic acid without affecting any of the benzene ring atoms.

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  Organic Chemistry

  • 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.

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  Brown's Introduction to Organic Chemistry

  • William H. Brown, Thomas Poon(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  • A heterocyclic aromatic compound contains one or more atoms other than carbon in an aromatic ring. 9.3 How Are Benzene Compounds Named, and What Are Their Physical Properties? • Aromatic compounds are named by the IUPAC system. The common names toluene, xylene, styrene, phenol, aniline, benzaldehyde, and benzoic acid are retained. • The C 6 H 5 group is named phenyl, and the C 6 H 5 CH 2 group is named benzyl. • To locate two substituents on a benzene ring, either num- ber the atoms of the ring or use the locators ortho (o), meta (m), and para (p). • Polynuclear aromatic hydrocarbons contain two or more fused benzene rings. 9.4 What Is the Benzylic Position, and How Does It Contribute to Benzene Reactivity? • The benzylic position is the carbon of an alkyl substituent immediately bonded to the benzene ring. • The benzylic position of a benzene ring can be oxidized by chromic acid without affecting any of the benzene ring atoms. 9.5 What Is Electrophilic Aromatic Substitution? • A characteristic reaction of aromatic compounds is electro- philic aromatic substitution, which involves the substitu- tion of one of the ring hydrogens of benzene for an electrophilic reagent. • The five types of electrophilic aromatic substitution dis- cussed here are nitration, halogenation, sulfonation, Friedel–Crafts alkylation, and Friedel–Crafts acylation. 9.6 What Is the Mechanism of Electrophilic Aromatic Substitution? • The mechanism of electrophilic aromatic substitution can be broken down into three common steps: (1) generation of the electrophile, (2) attack of the electrophile on the aromatic ring to give a resonance‐stabilized cation intermediate, and (3) proton transfer to a base to regenerate the aromatic ring. • The five electrophilic aromatic substitution reactions stud- ied here differ in their mechanism of formation of the elec- trophile (Step 1) and the specific base used to effect the proton transfer to regenerate the aromatic ring (Step 3).

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