Aromatic Ions

8 Key excerpts on "Aromatic Ions"

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  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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  Aromaticity and Metal Clusters

  • Pratim Kumar Chattaraj(Author)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  Although different definitions and indices of aromaticity have their own flaws, qualitatively there has never been a real disagreement on the fact that aromatic compounds are characterized by special stability and this additional stabi-lization is due to the cyclic electron delocalization [15]. In other words, aromaticity is essentially an excess property , that is, a deviation from an additive scheme and it is not a directly measurable experimental quantity. 4 Aromaticity and Metal Clusters In 2005, Schleyer and coworkers [16, p. 3844] proposed a qualitative definition of aromaticity as Aromaticity is a manifestation of electron delocalization in closed circuits, either in two or in three dimensions. This results in energy lowering, often quite substantial, and a variety of unusual chemical and physical properties. These include a tendency toward bond length equalization, unusual reactivity, and characteristic spectroscopic features. Since aromaticity is related to the induced ring currents, magnetic properties are particularly important for its detection and evaluation. Despite many controversial arguments regarding the definition and physical ori-gin of aromaticity [1,15–17], the concept of aromaticity has crossed the boundary of benzenoid hydrocarbons [with (4 n + 2) π -electrons] to include heterosystems [50] like pyridine, thiophine, cations such as tropylium [12] and cyclopropenium [13], anions like cyclopentadienyl [51], organometallic systems, namely ferrocene [52], purely carbon-free systems [53,54], namely P 5 − , [(P 5 ) 2 Ti] 2 − . The three-dimensional aromaticity of boron-based clusters [55] and of fullerenes [56], the homoaromaticity of cationic systems [57], aromaticity of triplet state annulenes [58], and pericyclic transition states [59] has enlarged the concept of aromaticity.

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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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  BIOS Instant Notes in Organic Chemistry

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

    (Publisher)

  2 hybridized.

  Bicyclic and polycyclic systems can also be aromatic (Figure 4 ).

  Figure 3. (a) Cyclopentadienyl anion; (b) cycloheptatrienyl cation. Figure 4. (a) Naphthalene; (b) anthracene; (c) benzo[a]pyrene.

  I2 Preparation and properties

  Key Notes

  Preparation	Simple aromatic structures such as benzene, toluene, or naphthalene are isolated from natural sources and converted to more complex aromatic structures.
  Properties	Many aromatic compounds have a characteristic aroma and burn with a smoky flame. They are nonpolar, hydrophobic molecules which dissolve in organic solvents rather than water. Aromatic molecules can interact by van der Waals interactions or with a cation through an induced dipole interaction. Aromatic compounds undergo reactions where the aromatic ring is retained. Electrophilic substitution is the most common type of reaction. However, reduction is also possible.
  Spectroscopic analysis	Aromatic compounds show characteristic absorptions in the IR spectrum due to ring vibrations. Signals due to Ar-H stretching and bending may also be observed. Signals for aromatic protons and carbons appear at characteristic positions in nmr spectra. Fragmentation ions can be observed in mass spectra which are characteristic of aromatic compounds.
  Related topics	(I3) Electrophilic substitutions of benzene (I7) Oxidation and reduction (P2) Visible and ultra violet spectroscopy (P3) Infra-red spectroscopy	(P4) Proton nuclear magnetic resonance spectroscopy (P5) 13 C nuclear magnetic resonance spectroscopy (P6) Mass spectrometry

  Preparation

  It is not practical to synthesize aromatic structures in the laboratory from scratch and most aromatic compounds are prepared from benzene or other simple aromatic compounds (e.g. toluene and naphthalene). These in turn are isolated from natural sources such as coal or petroleum.

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  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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  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  The aromatic structure is responsible for deuterium scrambling (Figure 6.8). H H H H H H H H H Figure 6.8 Side view of the cyclononatetraenyl cation. 6.2 Aromatic Ions Is it possible for an ion to be aromatic? Yes, Hückel’s rule also applies to charged molecules, if they meet certain criteria. They must have a planar structure and a cyclic delocalized π electron system containing (4n + 2) electrons, where n is any whole number. According to Hückel’s rule, compounds having 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3), 18 (n = 4), and so on, π electrons should be aromatic. A number of cations and anions with completely conjugated cyclic planar structures are shown below. Cyclopropenyl cation Cyclopropenyl anion Cyclobutene dication Cyclobutene dianion Cyclopentadienyl cation Cyclopentadienyl anion 6.2 Aromatic Ions 295 Tropylium cation Cycloheptatrienyl anion Cyclooctatrienyl dication Cyclooctatrienyl dianion Cyclononatetraenyl cation Cyclononatetraenyl anion Cyclopropene has a planar structure as three points define a plane. However, cyclopropene with two π electrons is not aromatic. It does not have an uninterrupted ring and so the electrons can delocalize. The hybridization of one of the ring atoms is sp 3 . For delocalization of the electrons, the next carbon must have an empty p orbital. Only sp 2 - and sp-hybridized carbons have p orbitals. Therefore, cyclopropene does not fulfill the Hückel criterion for aromaticity. O Breslow and coworker reacted 3-chlorocyclopropene by mixing it with antimony pentachloride, aluminum trichloride, or silver fluoroborate and obtained the cyclopropenyl cation [28]. The 1 H NMR spectrum of the cyclopropenyl cation shows a sharp singlet at 11.1 ppm, clearly indicating the presence of a strong diamagnetic ring current and the aromaticity of this compound. H Cl + H H H X – X = SbCl 6 = AlCl 4 = BF 4 Cyclopropene Chlorocyclopropene The electron configuration of the cyclopropenyl cation is shown in Figure 6.9.

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  Polyatomic Molecules

  Results of ab Initio Calculations

  • Robert S. Mulliken(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  C H A P T E R X I AROMATIC MOLECULES A. GENERAL CONSIDERATIONS Chapters I I -X have dealt with inorganic molecules and with aliphatic organic molecules, arranged largely in terms of symmetry and size. Aro-matic molecules are a rather distinctive class of mostly large molecules and ions that follow Hiickel's well-known rule of possessing An + 2 π electrons. Simplest is the cyclopropanyl cation Η with η = 0. More typical are benzene (C e H e ), the cyclopentadienyl anion (C5H5), the cycloheptanyl cation ( C 7 H 7 + ) , pyrrole ( C 5 H 5 N H ) , furan (C5H5O), and thiophene (C 5 H 5 S), all with η = 2. These are all monocy-clic. Then there are very numerous poly cyclic compounds containing one or more fused or unfused rings similar to those just mentioned, and/or containing heteroatoms or aliphatic or inorganic substituent atoms or 329 TABLE 1 Symmetry Operations and Characters for the D eh Group Ε 2C 6 (JC) 2 C e 2 = 2 C 3 C e 3 = Q ' 3 C t 3 Q 3cr v 3cr d 2 S e 2 S 3 S e 3 = S , = i Coordinates Α ι β 1 1 1 1 1 1 1 1 1 1 1 1 A lu -i -i -i -i Aj g 1 1 1 1 -1 -ι 1 -1 -1 1 1 1 A tu 1 1 1 1 1 1 -1 -1 -1 X B, e 1 -1 1 -1 1 -1 -1 -1 1 1 -1 1 B l u 1 -ι 1 -i -i 1 1 -i -i 1 -ι B,« -ι 1 -i -i 1 -i 1 -i 1 -i 1 Biu 1 _ 1 1 _ 1 1 1 -1 1 _ 1 1 _ 1 E „ 2 1 -1 -2 0 0 -2 0 0 -1 1 2 E l u 2 1 -1 -2 0 0 2 0 0 1 -1 -2 E „ 2 -1 -1 2 0 0 2 0 0 -1 -1 2 E l u 2 -1 -1 2 0 0 -2 0 0 1 1 -2 Β. S I X -M E M B E R E D R I N G A N D R E L A T E D M O L E C U L E S 331 B. SIX-MEMBERED RING COMPOUNDS AND RELATED MOLECULES 1. Benzene A very good SCF computation on ground-state benzene was made by Ermler and Kern [1] using a (9s 5p ld/4s lp) polarized Gaussian basis set contracted to (4s 2p ld/2s lp) with differently scaled s and ρ exponents. They found E h = -230.74938 and estimated the SCF limit as -230.82 ± 0.02. They give orbital energies and several computed one-electron prop-erties.

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  Nonbenzenoid Aromatics

  • James P. Snyder(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  4 Monocyclic and Polycyclic Aromatic Ions Containing Six or More π-Electrons P. J . GARRATT AND Μ . V . SARGENT I. Introduction 208 II. Monocyclic Systems with Six π -Electrons . . . . 2 1 0 A. Cyclobutadienyl Dianion 210 B. Cyclooctatetraenium Dication 212 III. Monocyclic Systems with Ten π -Electrons . . . . 2 1 3 A. Cyclooctatetraenyl Dianion and Related Systems . . 2 1 3 B. Cyclononatetraenyl Anion 227 C. Bridged Systems 230 IV. Monocyclic Systems with More than Ten π-Electrons · . 235 A. [16]Annulenyl Dianion 235 B. l-Methoxy-2,8,10-tridehydro[17]annulenyl Anion . . 236 C. l,3,7,9,13,15,19,21-Octadehydro[24]annulenyl Dianion . 237 V. Physical Properties of Monocyclic Ions 238 A. Electronic Spectra 239 B. Infrared and Raman Spectra 239 C. NMR Spectra . . 241 VI. Polycyclic Aromatic Ions 245 A. Systems with Four-Membered Rings 247 B. Systems with Five-Membered Rings . . . . 252 C. Systems with Six- or More Membered Rings . . . 257 VII. HomoAromatic Ions 259 A. Homocyclopropenium Cations 260 B. Homocyclopentadienyl Anions 262 C. Homotropylium Cations 264 D. Monohomocyclooctatetraenyl Anion and Dianion . . 269 E. 1-Methylsulfinylmethyl-l ,6-methanocyclodecatetraenyl Anion 270 VIII. Conclusions 271 208 P. J. GARRATT AND Μ. V. SARGENT I. Introduction The present chapter is concerned with the cyclic (4n + 2) 7r-electron systems having six or more ^-electrons and bearing one or more formal charges. The related two 7r-electron systems will be discussed in a future volume. The group of molecules under discussion consists of Aromatic Ions, and the first member of the group to be recognized was the cyclopentadienyl anion (1), prepared by Thiele in 1901 by treatment of cyclopentadiene with potassium. 1 The same worker also attempted to prepare the corresponding anion 2 from cyclohepta-triene, 10 and concluded from the failure of this reaction that cycloheptatriene was homoaromatic.

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