Originally published in 1969, this book analyses the special properties of benzene and its derivatives, which give them their distinct aromatic character. The text first discusses the structure of benzene, including the light shed on it by physical methods and wave mechanics, and the structure of polycyclic benzenoid hydrocarbons and heterocyclic aromatic compounds. Aromaticity is defined as a function of the electronic structure of the molecule or ion in question and it is shown that this has important consequences in respect of bond lengths, resonance energies, electronic absorption spectra and induced ring currents. Final chapters discuss how far aromatic character and aromaticity are associated with non-benzenoid hydrocarbons and with more complex systems.
Aromaticity and Antiaromaticity A comprehensive review of the science of aromaticity, as well as its evolution, from benzene to atomic clusters In Aromaticity and Antiaromaticity: Concepts and Applications, a team of accomplished chemists delivers a comprehensive exploration of the evolution and critical aspects of aromaticity. The book examines the new global criteria used to evaluate aromaticity, including the Nucleus Independent Chemical Shift (NICS) index and the electronic indices based on electronic properties. Additional discussions of inorganic aromatic compounds developed in this century, which give rise to new concepts like multifold aromaticity, are included. Three-dimensional aromaticity found in fullerenes and nanotubes, Möbius aromaticity present in some annulenes, and excited state aromaticity are explored as well. This volume explores the geometrical, electronic, magnetic, and thermodynamic characteristics of aromatic and antiaromatic compounds and their reactivity properties. It also provides: A thorough historical overview of aromaticity, as well as simple electronic and structural models Comprehensive explorations of organic and inorganic aromatic compounds, concepts of stability and reactivity, and geometric, energetic, magnetic, and electronic criteria of descriptors of aromaticity Practical discussions of heteroaromaticity, as well as Möbius aromaticity and excited state aromaticity In-depth examinations of sigma, pi, delta, and phi aromaticity Perfect for graduate students, researchers, and academics interested in aromaticity, organometallic chemistry, and computational chemistry, Aromaticity and Antiaromaticity: Concepts and Applications will also earn a place in the libraries of professionals and researchers working in organic, inorganic, and physical chemistry.
Metal clusters, an intermediate state between molecules and the extended solid, show peculiar bonding and reactivity patterns. Their significance is critical to many areas, including air pollution, interstellar matter, clay minerals, photography, catalysis, quantum dots, and virus crystals. In Aromaticity and Metal Clusters, dozens of international experts explore not only the basic aspects of aromaticity, but also the structures, properties, reactivity, stability, and other consequences of the aromaticity of a variety of metal clusters. Although the concept of aromaticity has been known for nearly two centuries, there is no way to measure it experimentally and no theoretical formula to calculate it. In order to gain insight into its exact nature, the authors of this volume examine various indirect characteristics such as geometrical, electronic, magnetic, thermodynamic, and reactivity considerations. The book begins by discussing the evolution of aromaticity from benzene to atomic clusters. Next, more specialized chapters focus on areas of significant interest. Topics discussed include: Computational studies on molecules with unusual aromaticity Electronic shells and magnetism in small metal clusters A density functional investigation on the structures, energetics, and properties of sodium clusters through electrostatic guidelines and molecular tailoring The correlation between electron delocalization and ring currents in all metallic aromatic compounds Phenomenological shell model and aromaticity in metal clusters Rationalizing the aromaticity indexes used to describe the aromatic behavior of metal clusters 5f orbital successive aromatic and antiaromatic zones in triangular uranium cluster chemistry This collection of diverse contributions, composed of the work of scientists worldwide, is destined to not only answer puzzling questions about the nature of aromaticity, but also to provoke further inquiry in the minds of researchers.
Evaluating the aromaticity of a molecular system and the influence of this concept on its properties is a crucial step in the development of novel aromatic systems. Modern computational methods can provide researchers with a high level of insight into such aromaticity, but identifying the most appropriate method for assessing a specific system can prove difficult. Aromaticity: Modern Computational Methods and Applications reviews the latest state-of-the-art computational methods in this field and discusses their applicability for evaluating the aromaticity of a system. In addition to covering aromaticity for typical organic molecules, this volume also explores systems possessing transition metals in their structures, macrocycles and even transition structures. The influence of the aromaticity on the properties of these species (including the structure, magnetic properties and reactivity) is highlighted, along with potential applications in fields including materials science and medicinal chemistry. Finally, the controversial and fuzzy nature of aromaticity as a concept is discussed, providing the basis for an updated and more comprehensive definition of this concept. Drawing on the knowledge of an international team of experts, Aromaticity: Modern Computational Methods and Applications is a unique guide for anyone researching, studying or applying principles of aromaticity in their work, from computational and organic chemists to pharmaceutical and materials scientists. Reviews a range of computational methods to assess the aromatic nature of different compounds, helping readers select the most useful tool for the system they are studying Presents a complete guide to the key concepts and fundamental principles of aromaticity Provides guidance on identifying which variables should be modified to tune the properties of an aromatic system for different potential applications
Heterocyclic chemistry is the biggest branch of chemistry covering two-thirds of the chemical literature. Aromaticity in Heterocyclic Compounds covers hot topics of frontier research summarized by reputed scientists in the field.
Examines aromaticity through the historical and conceptual framework of simple molecular orbital theory. This approach is unique in that theoretical considerations are integrated with experimental observations. The criteria for aromatic, antiaromatic and nonaromatic compounds are discussed in with reference to benzene, cyclobutadiene and cyclooctatetraene. Also includes a chapter on the likely classes of aromatic compounds.
Aromaticity is among the most frequently used concepts in the chemical literature. Nevertheless, it has to be stressed that aromaticity is not an observable that could be directly obtained from molecular wave function. So, it remains as a controversial concept and its physical origin is still being debated. This has lead to multiple ways for assessing the aromatic or anti-aromatic character of molecules. Thus, several indices of aromaticity have been proposed throughout the literature. It is well-known that aromaticity is strongly related to the electron delocalisation of the electrons along all the centres of the ring. The introduction of the n-order density matrix, and development of n-centre delocalisation indices (n-DIs), has provided a quantitative tool to measure electron delocalisation within a certain ring. Therefore, n-DIs can be regarded as a promising way to determine the actual role played by electron delocalisation in aromaticity. n-DIs are found to be in line with most of the qualitative trends predicted by the harmonic oscillator model of aromaticity indices, HOMAs, aromaticity indices derived from the Chemical Graph Theory, properties of the ring critical points, and circuit resonance energies. It has to be noticed that the direct comparison of n-DIs computed for rings with a different number of centers does not provide a picture of relative aromatic stabilisation. In fact, aromatic stabilisation does not only depend on the extension of the electron delocalisation over the n centres, but also on the number of centres. n-DIs dependence on the number of centres and electrons in a ring prevents to use their direct comparison as a measure of aromatic stabilisation unless an appropriate scaling is applied. Effective Scaled Electron Delocalisation indices, ESED, have been developed for comparing the total aromaticity of polyaromatic hydrocarbons. n-DIs exhibit very different behaviour for aromatic and antiaromatic hydrocarbons. Thus, an aromatic compound is one whose multicenter electron delocalisation is larger than that of the corresponding linear structure, whereas an antiaromatic compound is one where the multi-centre electron delocalisation is smaller than that of the linear molecule. Those compounds that display very small differences between linear and cyclic structures may be called non-aromatic. n-DIs are also found as a promising tool to study the aromaticity of transition states in concerted processes. This book also reports on its application to the study of the aromaticity in all metal compounds.
This is the perfect complement to "Chemical Bonding - Across the Periodic Table" by the same editors, who are two of the top scientists working on this topic, each with extensive experience and important connections within the community. The resulting book is a unique overview of the different approaches used for describing a chemical bond, including molecular-orbital based, valence-bond based, ELF, AIM and density-functional based methods. It takes into account the many developments that have taken place in the field over the past few decades due to the rapid advances in quantum chemical models and faster computers.
An advanced-level textbook of organic chemistry for the graduate (B.Sc) and postgraduate (M.Sc) students of Indian and foreign universities. This book is a part of the four-volume series, entitled “A Textbook of Organic Chemistry – Volume I, II, III, IV”. CONTENTS: CHAPTER 1. Nature of Bonding in Organic molecules: Delocalized Chemical Bonding; Conjugation; Cross Conjugation; Resonance; Hyperconjugation; Tautomerism; Aromaticity in Benzenoid and Nonbenzenoid Compounds; Alternant and Non-Alternant Hydrocarbons; Huckel’s Rule: Energy Level of p-Molecular Orbitals; Annulenes; Antiaromaticity; Homo-Aromaticity; PMO Approach; Bonds Weaker than Covalent; Addition Compounds: Crown Ether Complexes and Cryptands, Inclusion Compounds, Cyclodextrins; Catenanes and Rotaxanes CHAPTER 2. Stereochemistry: Chirality; Elements of symmetry; Molecules with more than one chiral centre: diastereomerism; Determination of relative and absolute configuration (octant rule excluded) with special reference to lactic acid, alanine & mandelic acid; Methods of resolution; Optical purity; Prochirality; Enantiotopic and diastereotopic atoms, groups and faces; Asymmetric synthesis: cram’s rule and its modifications, prelog’s rule; Conformational analysis of cycloalkanes (upto six membered rings); Decalins; Conformations of sugars; Optical activity in absence of chiral carbon (biphenyls, allenes and spiranes); Chirality due to helical shape; Geometrical isomerism in alkenes and oximes; Methods of determining the configuration CHAPTER 3. Reaction Mechanism: Structure and Reactivity: Types of mechanisms; Types of reactions; Thermodynamic and kinetic requirements; Kinetic and thermodynamic control; Hammond’s postulate; Curtin-Hammett principle; Potential energy diagrams: Transition states and intermediates; Methods of determining mechanisms; Isotope effects; Hard and soft acids and bases; Generation, structure, stability and reactivity of carbocations, carbanions, free radicals, carbenes and nitrenes; Effect of structure on reactivity; The Hammett equation and linear free energy relationship; Substituent and reaction constants; Taft equation CHAPTER 4. Carbohydrates: Types of naturally occurring sugars; Deoxy sugars; Amino sugars; Branch chain sugars; General methods of determination of structure and ring size of sugars with particular reference to maltose, lactose, sucrose, starch and cellulose. CHAPTER 5. Natural and Synthetic Dyes: Various classes of synthetic dyes including heterocyclic dyes; Interaction between dyes and fibers; Structure elucidation of indigo and Alizarin CHAPTER 6. Aliphatic Nucleophilic Substtitution: The SN2, SN1, mixed SN1 and SN2, SNi , SN1’, SN2’, SNi’ and SET mechanisms; The neighbouring group mechanisms; neighbouring group participation by p and s bonds; anchimeric assistance; Classical and nonclassical carbocations; Phenonium ions; Common carbocation rearrangements; Applications of NMR spectroscopy in the detection of carbocations; Reactivity- effects of substrate structure, attacking nucleophile, leaving group and reaction medium; Ambident nucleophiles and regioselectivity; Phase transfer catalysis. CHAPTER 7. Aliphatic Electrophilic Substitution: Bimolecular mechanisms – SE2 and SEi; The SE1 mechanism; Electrophilic substitution accompained by double bond shifts; Effect of substrates, leaving group and the solvent polarity on the reactivity CHAPTER 8. Aromatic Electrophilic Substitution: The arenium ion: mechanism, orientation and reactivity, energy profile diagrams; The ortho/para ratio, ipso attack, orientation in other ring systems; Quantitative treatment of reactivity in substrates and electrophiles; Diazonium coupling; Vilsmeir reaction; Gattermann-Koch reaction CHAPTER 9. Aromatic Nucleophilic Substitution: The ArSN1, ArSN2, Benzyne and SRN1 mechanisms; Reactivity – effect of substrate structure, leaving group and attacking nucleophile; The von Richter, Sommelet-Hauser, and Smiles rearrangements CHAPTER 10. Elimination Reactions: The E2, E1 and E1cB mechanisms; Orientation of the double bond; Reactivity –effects of substrate structures, attacking base, the leaving group and the medium; Mechanism and orientation in pyrolytic elimination CHAPTER 11. Addition to Carbon-Carbon Multiple Bonds: Mechanistic and stereochemical aspects of addition reactions involving electrophiles, nucleophiles and free radicals; Regio–and chemoselectivity: orientation and reactivity; Addition to cyclopropane ring; Hydrogenation of double and triple bonds; Hydrogenation of aromatic rings; Hydroboration; Michael reaction; Sharpless asymmetric epoxidation. CHAPTER 12. Addition to Carbon-Hetero Multiple Bonds: Mechanism of metal hydride reduction of saturated and unsaturated carbonyl compounds, acids, esters and nitriles; Addition of Grignard reagents, organozinc and organolithium; Reagents to carbonyl and unsaturated carbonyl compounds; Wittig reaction; Mechanism of condensation reactions involving enolates – Aldol, Knoevenagel, Claisen, Mannich, Benzoin, Perkin and Stobbe reactions; Hydrolysis of esters and amides; Ammonolysis of esters.
