3 Key excerpts on "Benzyne"

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  eBook - ePub

  BIOS Instant Notes in Chemistry for Biologists

  • J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
  • 2020(Publication Date)
  • Taylor & Francis

    (Publisher)

  Section K - Aromatic Compounds Passage contains an image

  K1 Aromaticity

  DOI: 10.1201/9780203079522-43

  Key Notes

  Benzene	Benzene is an unsaturated molecule and, as such, would be expected to undergo reactions similar to those of other unsaturated hydrocarbons such as alkenes and alkynes. However, benzene is relatively inert, and when it does react favors substitution reactions over addition reactions. The unexpected chemical and physical properties of benzene may be explained by the concept of pi electron delocalization. Benzene is the classic example of an aromatic compound. The term aromatic is applied as benzene, and other ring systems that have similar delocalized pi systems, is fragrant.
  Molecular orbital description of benzene	Benzene is a planar molecule in which all of the bond angles about the carbon atoms are 120°. This bond angle is what would be expected for an sp2 hybridized carbon atom, and therefore means that at each of the six carbon atoms there is a singly occupied p-orbital. These p-atomic orbitals overlap to form six pi molecular orbitals. The molecular orbital picture of benzene helps explain the special stability of this molecule.
  Definition of aromaticity	In 1931 the physicist Erich Hückel carried out a series of calculations based on the molecular orbital picture of benzene, but extended this to cover all planar monocyclic compounds in which each atom had a p-orbital. The results of his work suggested that all such compounds containing (4n + 2) pi electrons should be stabilized through delocalization and therefore should also be termed aromatic.
  Related topics	(I3) Factors affecting reactivity	(K2) Natural aromatics

  Benzene

  The study of the class of compounds now referred to as aromatics began in 1825 with the isolation of a compound, now called benzene, by Michael Faraday. At this time the molecular formula of benzene, C6 H6 , was thought quite unusual due to the low ratio of hydrogen to carbon atoms. Within a very short time the unusual properties of benzene and related compounds began to emerge. During this period, for a compound to be classified as aromatic it simply needed to have a low carbon to hydrogen ratio and to be fragrant; most of the early aromatic compounds were obtained from balsams, resins or essential oils. It was sometime later before Kekulé and coworkers recognized that these compounds all contained a six-carbon unit that remained unchanged during a range of chemical transformations. Benzene was eventually recognized as being the parent for this new class of compound. In 1865 Kekulé proposed a structure for benzene; a six-membered ring with three alternating double bonds (Figure 1 ). However, if such a structure were correct then the addition of two bromine atoms to adjacent carbons would result in the formation of two isomers of 1,2-dibromobenzene (Figure 1 ). Only one compound has ever been found. To account for this apparent anomaly Kekulé suggested that these isomers were in a state of rapid equilibrium (Figure 1

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  eBook - ePub

  BIOS Instant Notes in Organic Chemistry

  • Graham Patrick(Author)
  • 2004(Publication Date)
  • Taylor & Francis

    (Publisher)

  SECTION I — AROMATIC CHEMISTRY

  I1 Aromaticity

  Key Notes

  Definition	Aromatic compounds such as benzene are more stable than suggested from their structure. They undergo reactions which retain the aromatic ring system, and behave differently from alkenes or polyenes.
  Hückel rule	Aromatic compounds are cyclic and planar with sp 2 hybridized atoms. They also obey the Hückel rule and have (4n + 2) π electrons where n = 1, 2, 3, ... Aromatic systems can be monocyclic or polycyclic, neutral, or charged.
  Related topic	(A4) sp 2 Hybridization

  Definition

  The term aromatic was originally applied to benzene-like structures because of the distinctive aroma of these compounds, but the term now means something different in modern chemistry. Aromatic compounds undergo distinctive reactions which set them apart from other functional groups. They are highly unsaturated compounds, but unlike alkenes and alkynes, they are relatively unreactive and will tend to undergo reactions which involve a retention of their unsaturation. We have already discussed the reasons for the stability of benzene in Section A4 . Benzene is a six-membered ring structure with three formal double bonds (Figure 1a ). However, the six π electrons involved are not localized between any two carbon atoms. Instead, they are delocalized around the ring which results in an increased stability. This is why benzene is often written with a circle in the center of the ring to signify the delocalization of the six π electrons (Figure 1b ). Reactions which disrupt this delocalization are not favored since it means a loss of stability, so benzene undergoes reactions where the aromatic ring system is retained. All six carbon atoms in benzene are sp 2 hybridized, and the molecule itself is cyclic and planar — the planarity being necessary if the 2p atomic orbitals on each carbon atom are to overlap and result in delocalization.

  Figure 1. Representations of benzene.

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  eBook - ePub

  Biochemistry

  An Organic Chemistry Approach

  • Michael B. Smith(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  n +2 series, it is aromatic and particularly stable.

  An aromatic hydrocarbon is resonance-stabilized and therefore particularly stable. The lack of reactivity is a result of the fact that electrons are not localized on a single atom, but rather delocalized on several atoms, so that structure tends to be more stable. The special stability of benzene is easily shown by a simple chemical reaction. The π-bond of an alkene reacts as a Brønsted–Lowry base with an acid HX (HCl, HBr, HI, etc.). The more tightly a π-bond holds its electrons, the more stable it will be and the less likely it is to donate electrons so that the C=C unit is considered to be a weaker base.

  A crude comparison of the relative reactivity of HBr with cyclohexene, cyclohexadiene, or benzene (remember that using one Kekulé structure implies both resonance forms). The reaction of cyclohexene with HBr is shown in Figure 9.2 . This reaction is rapid, and the product is a carbocation, which traps bromide to give bromocyclohexane. Cyclohexa-1,3-diene also rapidly reacts with HBr to yield an allylic carbocation (a C=C—C+ unit). The allylic carbocation is resonance-stabilized, easily formed, and it reacts with the bromide ion to yield an allylic bromide, 3-bromocyclohex-1-ene. While both cyclohexene and cyclohexa-1,3-diene react quickly and easily with HBr, benzene does not react with HBr, even with heating. Benzene is therefore a weaker base than an alkene.

  FIGURE 9.2 Comparative reactivity of HBr with cyclohexene, cyclohexadiene and benzene.

  9.2 Benzene Is a Carcinogen

  Benzene is one of the best studied of the known human carcinogens. It causes leukemia in humans and a variety of solid tumors in rats and mice.”1 “To be carcinogenic, benzene must first be metabolized in the liver, mainly via cytochrome P4502E1. The major product is phenol in Figure 9.3 , which is either conjugated (primarily to phenyl sulfate in humans) or further hydroxylated by P4502E1 to hydroquinone. Other major metabolites include catechol and trans–trans- muconic acid [(2E ,4E )-hexa-2,4-dienedioic acid)] The latter is presumed to be formed from the ring opening of benzene epoxide (7-oxabicyclo[4.1.0]hepta-2,4-diene or benzene oxepin), or perhaps benzene dihydrodiol (cyclohexa-3,5-diene-1,2-diol).2

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