Aromatic Chemistry

8 Key excerpts on "Aromatic Chemistry"

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  eBook - ePub

  The Chemical Bond

  Chemical Bonding Across the Periodic Table

  • Gernot Frenking, Sason Shaik(Authors)
  • 2014(Publication Date)
  • Wiley-VCH

    (Publisher)

  14 Chemical Bonding in Inorganic Aromatic Compounds

  Ivan A. Popov and Alexander I. Boldyrev

  14.1 Introduction

  The concept of aromaticity was born in chemistry soon after Faraday discovered benzene. Chemists noticed that certain chemicals containing benzene ring are not particularly reactive in spite of having unsaturated carbon atoms. The term aromaticity was introduced in chemistry by Kekulé [1–3], who associated aromaticity with the presence of C6 units in aromatic compounds. Kekulé assumed that the similarity to benzene is essential for a compound to be aromatic. This concept undergoes a significant transformation nowadays. According to the IUPAC's definition, aromaticity is a concept of spatial and electronic structure of cyclic molecular systems displaying the effects of cyclic electron delocalization which provide for their enhanced thermodynamic stability (relative to acyclic structural analogues) and tendency to retain the structural type in the course of chemical transformations. A quantitative assessment of the degree of aromaticity is given by the value of the resonance energy. It may also be evaluated by the energies of relevant isodesmic and homodesmotic reactions. Along with energetic criteria of aromaticity, important and complementary are also a structural criterion (the lesser the alternation of bond lengths in the rings, the greater is the aromaticity of the molecule) and a magnetic criterion (existence of the diamagnetic ring current induced in a conjugated cyclic molecule by an external magnetic field and manifested by an exaltation and anisotropy of magnetic susceptibility). [4] Initially, aromaticity was associated with planar molecules and with delocalization of π-electrons. On the basis of quantum chemical analysis of molecular orbitals (MOs) the famous 4n + 2 π-electrons Hückel's [5, 6] rule was proposed for aromatic molecules. Breslow [7, 8] introduced in 1970 the concept of antiaromaticity, which can be understood as the destabilization of the cyclic systems possessing 4n

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  Organic Chemistry

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

    (Publisher)

  617 C H A P T E R 14 Aromatic Compounds In ordinary conversation, the word “aromatic” conjures pleasant associations—the odor of freshly prepared coffee, a warm cinnamon bun, a freshly cut pine tree. Similar associations occurred in the early history of organic chemistry when pleasantly aromatic compounds were isolated from natural oils produced by plants. Once the structures of these materials were elucidated, many were found to possess a unique, highly unsaturated, six-carbon structural unit also found in benzene. This special ring became known as the benzene ring. Aromatic compounds that contain a benzene ring are now part of a much larger family of compounds classified as aromatic, not because of their smell (since many of the molecules that contain them have no odor—for example, aspirin), but because they have special electronic features. IN THIS CHAPTER WE WILL CONSIDER: • the structural principles that underlie the use of the term “aromatic” • the initial challenge of determining the correct structure of benzene • a rule that helps to predict what kinds of molecules possess the special property of aromaticity • special groups of molecules that are also aromatic photo credits: (pine needles) Diana Taliun/Shutterstock (bottles with essential oils): Madlen/Shutterstock 618 CHAPTER 14 AROMATIC COMPOUNDS [ WHY DO THESE TOPICS MATTER? ] At the end of the chapter, we will explore the question of just how large the rings of these molecules can be and still be aromatic, noting that chemists have been able to make aromatic rings larger in size than those of molecules obtained from nature, but largely by using design clues derived from those natural molecules. See for additional examples, videos, and practice. 14.1 THE DISCOVERY OF BENZENE The following are a few examples of aromatic compounds, including benzene itself.

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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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  New Frontiers in Nanochemistry: Concepts, Theories, and Trends

  Volume 1: Structural Nanochemistry

  • Mihai V. Putz, Mihai V. Putz, Mihai Putz(Authors)
  • 2020(Publication Date)
  • Apple Academic Press

    (Publisher)

  CHAPTER 4

  Aromaticity

  MIHAI V. PUTZ1, 2 and MARINA A. TUDORAN2

  1 Laboratory of Structural and Computational Physical Chemistry for Nanosciences and QSAR, Biology-Chemistry Department, West University of Timişoara, Pestalozzi Street No. 44, Timişoara, RO-300115, Romania, Tel.: +40-256-592638, Fax: +40-256-592620, E-mail: [email protected] , [email protected]

  2 Laboratory of Renewable Energies-Photovoltaics, R&D National Institute for Electrochemistry and Condensed Matter, Dr. A. Paunescu Podeanu Str. No. 144, Timişoara, RO-300569, Romania

  4.1 DEFINITION

  According to the IUPAC definitions, the aromaticity concept is referring to a spatial and electronic structure of cyclic molecular systems, which displays the cyclic electron delocalization effects. An aromatic compound present enhanced thermodynamic stability, which is relative to acyclic structural analogs, and tends to retain the structural type during the chemical transformations.

  4.2 HISTORICAL ORIGIN(S)

  The word “aromatic” as a chemical term was used for the first time by August Wilhelm Hofmann in 1855 and was referring to compounds containing the phenyl radical (Hofmann, 1855). In 1865, August Kekulé proposed the cyclohexatriene structure for benzene, structure accepted by most chemists, due to the fact that it respects the isomeric relationships of Aromatic Chemistry known at that time, even if the unsaturated molecule of benzene was unreactive toward addition reactions. After J.J. Thomson, who discover the electron, placed three equivalent electrons between each carbon atom in benzene in 1921, and along with the introduction of the term aromatic sextet as a group of six electrons resisting disruption by Sir Robert Robinson in 1925 (Armit and Robinson, 1925), the stability of benzene was explained. Still, the quantum mechanical origins of aromaticity were modeled for the first time in 1931 by Hückel who separate the bonding electrons in sigma and pi electrons.

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  eBook - ePub

  Aromaticity and Antiaromaticity

  Concepts and Applications

  • Miquel Solà, Alexander I. Boldyrev, Michal K. Cyrañski, Tadeusz M. Krygowski, Gabriel Merino(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  3 Aromaticity from Organic to Inorganic Compounds

  “Human science fragments everything in order to understand it, kills everything in order to examine it.”

  Leo Tolstoy

  3.1 Introduction

  Initially the concept of aromaticity was introduced in organic chemistry. Thus, after Michael Faraday reported the isolation of benzene by distillation in 1825 [1] and noted that it was much less reactive than other unsaturated hydrocarbons, Kekulé introduced the term aromatic for a general classification of benzene derivatives [2] . Kekulé also observed a characteristic odor or fragrance of these substances and related them to relatively low chemical reactivity. Subsequent research in the area of aromaticity revealed that the low reactivity of “aromatic” compounds with unsaturated carbon–carbon bonds is not associated with the aroma. Instead, the exceptional stability and low reactivity was recognized to originate from peculiarities of the chemical bonding and electronic structure. Hückel [3] demonstrated that a closed‐shell monocyclic system should have 4n + 2 valence π‐electrons in order to be aromatic. The concept of antiaromaticity, relating to the corresponding destabilization seen in cyclic systems with 4n π‐electrons, was introduced by Breslow [4 , 5 ]. Dewar introduced σ‐aromaticity in order to explain the conjugation pattern in cyclopropane [6] . However, Schleyer and coworkers proved that cyclopropene is not a σ‐aromatic molecule [7] . The first doubly aromatic system, the 3,5‐dehydrophenyl cation, was identified by Chandrasekhar, Jemmis, and Schleyer as being doubly (σ‐ and π‐) aromatic [8] . All these are milestone works in advancing aromaticity, antiaromaticity, and double aromaticity concepts in organic chemistry. The concept of aromaticity has been extended into inorganic chemistry. It turns out that aromaticity in inorganic chemistry is more complex than in organic chemistry. Participation of d‐atomic orbitals (AOs) and f‐AOs in chemical bonding metal systems allows to introduce δ‐aromaticity/antiaromaticity and φ‐aromaticity/antiaromaticity [9] (see chapter 12 of Ref. [10] for further details). These new types of aromaticity lead to complicated combination of aromaticities and antiaromaticities. Finally, a term conflicting aromaticity introduced by Boldyrev and Wang [8] deals with the simultaneous presence of one of more types of aromaticity and one or more types of antiaromaticity simultaneously. There are a few reviews [9 –23

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  eBook - ePub

  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  5

  Aromatic Compounds

  5.1 Aromatic Compounds

  The term aromatic means “fragrant” (Ancient Greek, aroma). For this reason many fragrant substances were called “aromatic compounds.” Many of these compounds contain a benzene ring (Section 1.6 ) that is bonded to one or more substituents. Oil of sassafras, oil of wintergreen, and vanillin are well-known examples of fragrant, aromatic compounds.

  Today, the classification of aromatic compounds is no longer based on odor because many compounds containing a benzene ring are not fragrant. Many aromatic compounds are solids that have little or no odor. Solid aromatic compounds include the pain relievers, or analgesics, aspirin, ibuprofen, and acetaminophen, and the antibiotic chloramphenicol.

  The common feature of aromatic compounds is not their odor, but the benzene ring. This six-carbon unit is usually not affected by reactants that alter the rest of the structure. The distinguishing characteristic of aromatic compounds is the very low reactivity of the benzene ring.

  5.2 Aromaticity

  Benzene, C6 H6 , is highly unsaturated—it has six fewer hydrogen atoms than cyclohexane, C6 H12 – its cyclic saturated counterpart. Although benzene is represented by a hexagon that contains three double bonds, unlike alkenes it does not undergo addition reactions with reagents such as bromine, HBr, or water. The lack of reactivity of benzene contradicts what we know about unsaturated compounds. That is, benzene does not behave like the “triene” depicted by its Lewis structure. Benzene typically undergoes substitution reactions, a reaction not typical of alkenes. Benzene reacts with bromine, in the presence of iron(III) bromide as a catalyst, to give a single monosubstituted product, C6 H5 Br.

  This result indicates that all six hydrogen atoms of benzene are chemically equivalent. There are three possible isomeric dibromobenzenes, C6 H4 Br2

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  eBook - ePub

  Organic Chemistry II For Dummies

  • John T. Moore, Richard H. Langley(Authors)
  • 2023(Publication Date)
  • For Dummies

    (Publisher)

  Part 2

  Discovering Aromatic (And Not So Aromatic) Compounds

  IN THIS PART …

  • Examine aromatic systems, including the spectroscopy of aromatic compounds.
  • Organize some aromatic substitutions by electrophiles.
  • Get nucleophiles in on the aromatic substitution action.

  Passage contains an image Chapter 6

  Introducing Aromatics

  IN THIS CHAPTER

  Exploring benzene

  Coping with the aromatic family

  Getting acquainted with heterocyclic aromatic compounds

  Shedding light on the spectroscopy of aromatic compounds

  Organic chemistry has two main divisions. One division deals with aliphatic (fatty) compounds, the first compounds you encountered in Organic Chemistry I. Methane is a typical example of this type of compound. The second division includes the aromatic (fragrant) compounds, of which benzene is a typical example.

  Compounds in the two groups differ in a number of ways. The two differ chemically in that the aliphatic undergo free-radical substitution reactions and the aromatic undergo ionic substitution reactions. In this chapter, we examine the basics of both aromatic and heterocyclic aromatic compounds, concentrating on benzene and related compounds.

  Benzene: Where It All Starts

  Benzene is the fundamental aromatic compound. An understanding of the behavior of many other aromatic compounds is much easier if you first gain an understanding of benzene. For this reason, you may find it useful to examine a few key characteristics of benzene, which we discuss in the following sections.

  Figuring out benzene’s structure

  Benzene was first isolated in 1825 from coal tar. Later, chemists determined that it had the molecular formula C6 H6 . Further investigation of its chemical behavior showed that benzene was unlike other hydrocarbons in both structure and reactivity.

  Chemists proposed many structures for benzene. However, the facts didn’t support any of the possibilities until Kekulé proposed a ring structure in 1865. Some of the proposed structures, including Kekulé’s, are shown in Figure 6-1

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  Biochemistry

  An Organic Chemistry Approach

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

    (Publisher)

  9  Aromatic Compounds and Heterocyclic Compounds

  Benzene is a special type of hydrocarbon. Derivatives are known by replacing the hydrogen atoms of benzene with substituents and/or functional groups. There are hydrocarbons related to benzene that have two, three, or more rings fused together (polycyclic compounds). The unifying concept of all these molecules is that they are aromatic, which means that they are especially stable with respect to their bonding and structure.

  9.1 Benzene and Aromaticity

  Benzene is a hydrocarbon with the formula C6 H6 . It is the parent of a large class of compounds known as aromatic hydrocarbons. The structure and chemical reactivity of aromatic hydrocarbons are so unique that benzene derivatives are given their own nomenclature system. This discussion will begin with the unique structure of benzene.

  The structure of benzene is shown in Figure 9.1 . It is known that the C—C bond length in ethane is 1.53 Å (153 pm), and the C—C bond length is 1.536 Å (153.6 pm) in cyclohexane. The bond distance for the C=C bond in ethene is 1.34 Å (134 pm). These data indicate that a C=C unit has a shorter bond distance than a C—C unit. If benzene has a structure with both single and double bonds, it should have three longer C—C single bonds and three shorter C=C units. It would then be called cyclohexatriene. It has been experimentally determined that all six carbon–carbon bonds have a measured bond distance of 1.397 Å (139.7 pm), a value that lies in between those for the C–C bond in an alkane and the C=C unit of an alkene. This observation means that the C–C bonds in benzene are not single bonds, nor are they C=C double bonds where the π-electrons are localized between two carbons in a π-bond. This molecule is not cyclohexatriene, it is benzene . Each carbon in benzene is sp2 hybridized, however, which means each has a trigonal planar geometry. The planar geometry is seen more clearly in the molecular model of benzene in Figure 9.1

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