4 Key excerpts on "Aromatic Chemistry"

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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

  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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  Image and Reality

  Kekulé, Kopp, and the Scientific Imagination

  • Alan J. Rocke(Author)
  • 2010(Publication Date)
  • University of Chicago Press

    (Publisher)

  The very next year Hofmann’s student William Perkin discovered mauve, the first coal tar dye. Aromatic compounds thereafter became the object of intense commercial interest, even though the structural details of their molecules were not yet known. Today, the overwhelming majority of the millions of known organic substances contain the benzene nucleus somewhere in their structure and therefore are aromatic in the chemical sense (organic compounds of all kinds comprise about 99 percent of known chemical compounds). Consequently, a satisfying scientific understanding of the constitution of benzene, and thus of all aromatic compounds, was historically of crucial importance to the development of chemistry, both pure and applied.

  Of course, this judgment is made only with hindsight, and before 1850 it was not obvious that aromatic substances were as important as that, nor even that the relevant substances should be considered as members of a common family. Others besides Liebig and Wöhler studied aromatic substances in the 1830s, including Berzelius, Mitscherlich, Dumas, and Laurent. Laurent called benzene “phène,” from the Greek for illumination, hence the modern name of the “phenyl” radical, C6 H5 (benzene less one hydrogen atom), and “phenol,” C6 H5 OH (phenyl hydroxide, or carbolic acid). It soon became clear that benzoic acid was phenyl carboxylic acid, C6 H5 CO2 H, and that its relatives in the benzoyl series all contained the phenyl radical. In the late 1840s Hofmann and his English students at the Royal College of Chemistry took the lead in these studies. Hofmann himself had carried out the first investigation of the components of coal tar, many of which were aromatic, and his student Charles Mansfield continued this work, focusing especially on benzene, toluene (which is methyl benzene), nitrobenzene, and aniline (aminobenzene).4

  It was in the late 1850s, simultaneous with the rise of the coal tar dye industry, that Aromatic Chemistry became a recognized subfield of organic chemistry. In the course of this chapter, we will see that the development of the structural theory of aromatic substances is an outstanding example of the heuristic importance, indeed indispensability, of visual symbols and mental images in the pursuit of chemical science.

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  Biochemistry

  An Organic Chemistry Approach

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

    (Publisher)

  absence of a Lewis acid, much like a simple alkene, to yield 9,10-dibromo-9,10-dihydrophenanthrene. This addition type reaction does not occur with benzene or naphthalene.

  9.11 Heteroaromatic Compounds: Nitrogen, Oxygen, or Sulfur

  Another class of aromatic compounds are important in chemistry and biology, and they are characterized by replacement of one or more ring carbons with a heteroatom. These compounds are collectively known as heterocycles or heterocyclic aromatic compounds , and they comprise a class of compounds so large that an entire course is easily built around their chemistry. The most common heterocycles include five- and six-membered monocyclic derivatives that contain nitrogen, oxygen, or sulfur. There are several important bicyclic derivatives that contain nitrogen.

  In a “thought experiment,” replace the CH moiety of cyclopentadiene with an N—H unit, and the result is the aromatic compound, pyrrole, a constituent of coal tar and is found in bone oil. Pyrrole is an aromatic compound because the nitrogen atom has an unshared electron pair in an orbital that is parallel with the four π-electrons in the C=C units for a six π-electron system in a π-framework (see Figure 9.17 ). Pyrrole is a planar molecule and the hydrogen atom is coplanar with the carbon atoms and nitrogen, as shown in Figure 9.17 . Due to the fact that the lone electron pair is part of the aromatic sextet, the electrons cannot be donated without disrupting the aromaticity and pyrrole is not very basic. There are other aromatic five-membered ring amines, but they have two nitrogen atoms in the ring. The two nitrogen atoms in imidazole have a 1,3-relationship (note that an older term for an imidazole is azole ). The two nitrogen atoms in pyrazole

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