Reactive Intermediates

7 Key excerpts on "Reactive Intermediates"

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  eBook - ePub

  Reaction Mechanisms in Organic Synthesis

  • Rakesh Kumar Parashar(Author)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Chapter 2

  Reactive Intermediates

  Reactive Intermediates1−4 are believed to be transient intermediates in the majority of reactions. The main types of Reactive Intermediates of interest to organic chemists are carbocations, carbanions, radicals, radical ions, carbenes, nitrenes, arynes, nitrenium ions and diradicals.

  Reactive intermediate chemistry assists chemists in the design of new reactions for the efficient synthesis of pharmaceuticals, fine chemicals and agricultural products. Reactive Intermediates are usually short lived, very reactive and are rarely isolated under normal reaction conditions. However, their structures are established by indirect means either by chemical trapping, spectroscopically or sometimes by isolating them at very low temperature. The shapes of these intermediates become important while considering the stereochemistry of reactions in which they play a role.

  Carbocations are electrophiles and carbanions are nucleophiles. Reactions of these intermediates involving, at some stage, the bonding of a nucleophile to an electrophile are sometimes called ionic reactions.

  2.1 Carbocations

  Carbocation has a positively charged carbon atom which has only six electrons in its outer valence shell instead of the eight valence electrons (octet rule).

  2.1.1 Structure and stability of carbocations

  The heterolytic fission of a C–X bond in an organic molecule, in which X is more electronegative than carbon, generates the negatively charged anion (X− ) and positive charged species known as carbocations (called carbonium ions in the older literature).

  The carbon atom in a typical carbocation is sp 2 hybridized. The

  pz

  orbital is empty and is perpendicular to the plane of the other three bonds. Thus, carbocation adopts a trigonal planar shape.

  Because carbocation assumes a planar structure, its formation is inhibited in compounds which do not permit attainment of a planar geometry as in the case of bridge head compounds. Also, on the basis of quantum mechanical calculations for simple alkyl carbocations, it has been found that the planar (sp 2 ) configuration is more stable than the pyramidal (sp 3 ) configuration by about 84 kJ (20 kcal) mol−1 . Thus, the difficulty in the formation of carbocations increases as the attainment of planarity is inhibited. The planar configuration of simple carbocations has also been confirmed by NMR and IR spectra. Ned Arnett measured carbocation stabilities directly by measuring the enthalpy of reaction for the ionization of the RX process in hypermedia SbF5 /FSO3 H/SO2

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  eBook - ePub

  Reaction Mechanisms in Organic Chemistry

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

    (Publisher)

  In some cases, the existence of the intermediate can be observed by spectroscopic methods. We have shown that at least two transition complexes must form in reactions to obtain an intermediate. The energy levels of these transition complexes determine whether the intermediate product can be isolated or not. For example, as can be seen from the energy diagrams given in Figure 7.2, if the activation energy of the second transition complex is lower than that of the first (Δ E 2 < Δ E 1), this intermediate product cannot be isolated. However, if the second's activation energy is greater than the first's (Δ E 2 > Δ E 1), it may be possible to isolate the intermediate product. When Reactive Intermediates are not observable, their existence must be inferred through experiments by changing the reaction conditions such as concentration and temperature and studying the chemical kinetics. Once information is obtained about the intermediate, the reaction mechanism is much better understood. Figure 7.2 The energy diagrams of a two-step reaction. Intermediates are generally electron-defi cient compounds. When atoms have fewer than eight electrons in the valence shell, they tend to react with nucleophiles to increase the number of electrons in the valence shell to eight and form more stable compounds. Electron-deficient carbon intermediates can be neutral, such as carbene and radicals, or positively (+) charged, such as carbocations. On the other hand, intermediates can also be negatively charged (−) species such as carbanions that carry a negative (−) charge on an atom, following the octet rule. However, those compounds also belong to the class of Reactive Intermediates because of the negative charge. In addition to these, there are some Reactive Intermediates that are neutral and satisfy the octet rule. These compounds are generally highly strained molecules. For example, dihydroaromates, such as benzyne, fall into this group and are unstable compounds

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  eBook - PDF

  Reaction Mechanisms in Organic Chemistry

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

    (Publisher)

  381 7 Reactive Intermediates: Carbocations New products are formed as a result of chemical reactions. How does the conversion of starting compounds (reactants) to products occur? It is not always easy to answer this question. There are some reactions in which reactants are directly converted into products. On the other hand, most chemical reactions do not proceed in a single step. In a multistep reaction, the reactants are first converted into short-lived and highly unstable intermediates that quickly convert into products or new intermediates. Reactants Intermediates Products k 1 k –1 k 2 k –2 As shown in the above equation, there is an equilibrium between the intermediates formed and the reactants. The intermediates can be converted back into the reactants as well as into products. However, if the intermediates are con- verted into products, they do not turn back into the intermediates. In general, the conversion of intermediates to products is faster (k 2 > k 1 ) than their formation rate, and so it is often not possible to isolate intermediates and examine their structures. Figure 7.1 shows the energy diagrams of two different reactions. The first one shows the energy diagram of a reaction in which no intermediate is involved. The second one shows the formation of an intermediate. First, let us look at the energy distribution of a single-step system. As an example, we can take an S N 2 reaction proceeding in a single step. R X Nu R X Nu R Nu Reactant Product Transition state In an S N 2 reaction, the forming of a bond between the carbon atom and the nucleophile and the breaking of the bond between the carbon atom and the leaving group occur simultaneously. The system’s free energy increases, forming the transition state, which is the highest point on the reaction coordinate diagram (Figure 7.1). As can be seen from the energy diagram, the transition complex cannot be isolated; it turns directly into a product in a single-step reaction.

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  Reviews of Reactive Intermediate Chemistry

  • Matthew S. Platz, Robert A. Moss, Maitland Jones, Robert A. Moss, Matthew S. Platz, Maitland Jones, Jr., Robert A. Moss, Matthew S. Platz, Maitland Jones(Authors)
  • 2007(Publication Date)
  • Wiley-Interscience

    (Publisher)

  1968, 72, 1552. 169. K. Rademann, H.-W. Jochims, and H. Baumgärtel, J. Phys. Chem. 1985, 89, 3459. 170. D. W. Kohn, E. S. J. Robles, C. F . Logan, and P. Chen, J. Phys. Chem. 1993, 97 , 4936. 171. J. M. Ajello, W. T. Huntress, Jr., and P. Rayermann, J. Chem. Phys. 1976, 64, 4746. 172. B. A. Levi, R. W. Taft, and W. J. Hehre, J. Am. Chem. Soc. 1977 , 99, 8454. 173. M. Born, S. Ingemann, and N. M. M. Nibbering, Int. J. Mass Spectrom. 2000, 194, 103. 174. H. E. K. Matimba, A. M. Crabbendam, S. Ingemann, and N. M. M. Nibbering, J. Chem. Soc., Chem. Comm. 1991, 644. 175. M. Moini and G. Leroi, J. Phys. Chem. 1986, 90, 4002. 247 CHAPTER 6 Reactive Intermediates in Combustion JOHN K. MERLE National Institute of Standards and Technology, Gaithersburg, MD CHRISTOPHER M. HADAD Department of Chemistry, The Ohio State University, Columbus, Ohio 6.1. Introduction 247 6.2. Combustion Chemistry 249 6.2.1. Low-Temperature Mechanisms 251 6.2.2. High-Temperature Mechanisms 254 6.3. Generation of Pollutants from Combustion 257 6.3.1 PAH and Soot Formation 257 6.3.2. NO x and SO x 261 6.4. Atmospheric Chemistry 262 6.5. Detection of Gas-Phase Transient Intermediates 264 6.6. Conclusions and Outlook 267 Acknowledgments 267 Suggested Reading 268 References 268 6.1. INTRODUCTION The fields of combustion and atmospheric chemistry are intimately connected. Both of these fields are dominated by the reactivity of radical intermediates. The oxidation (combustion) of fossil fuels and their derivatives converts chemical energy into heat Reviews of Reactive Intermediate Chemistry. Edited by Matthew S. Platz, Robert A. Moss, Maitland Jones, Jr. Copyright © 2007 John Wiley & Sons, Inc. 248 Reactive Intermediates IN COMBUSTION energy that can be used, for example, to power electrical generators and automobiles. Under ideal conditions, the complete combustion of a hydrocarbon-based fuel will consume molecular oxygen (the oxidant) and produce carbon dioxide and water.

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  Keynotes in Organic Chemistry

  • Andrew F. Parsons(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  4 Reactivity and Mechanism

  Key point . Organic reactions can take place by radical or, more commonly, by ionic mechanisms. The particular pathway of a reaction is influenced by the stability of the intermediate radicals or ions, which can be determined from an understanding of electronic and steric effects. For ionic reactions, nucleophiles (electron-rich molecules) form bonds to electrophiles (electron-poor molecules) and this can be represented using curly arrows. The energy changes that occur during a reaction can be described by the equilibria (i.e. how much of the reaction occurs) and also by the rate (i.e. how fast the reaction occurs). The position of the equilibrium is determined by the size of the Gibbs free energy change while the rate of a reaction is determined by the activation energy .

  4.1 Reactive Intermediates: Ions versus Radicals

  There are two ways of breaking a covalent bond. The unsymmetrical cleavage is called heterolytic cleavage (or heterolysis) and this leads to the formation of ions (positively charged cations and negatively charged anions). The symmetrical cleavage is called homolytic cleavage (or homolysis) and this leads to the formation of radicals.

  For an example of homolysis, see halogenation of alkanes (Section 5.2.1)

  Curly arrows can be used to represent bond cleavage. A double-headed arrow represents the movement of two electrons (and is used in polar reaction mechanisms). A single-headed arrow (or fish-hook) is used to represent the movement of a single electron (and is used in radical reaction mechanisms). Curly arrows therefore always depict the movement of electrons.

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  Molecular Encapsulation

  Organic Reactions in Constrained Systems

  • Udo H. Brinker, Jean-Luc Mieusset, Udo H. Brinker, Jean-Luc Mieusset(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  10 Encapsulation of Reactive Intermediates Jean-Luc Mieusset and Udo H. Brinker Institut für Organische Chemie, Universität Wien, A–1090 Wien, Austria

  10.1 Introduction

  The inclusion of molecules is a very common technique in the pharmaceutical sciences to protect sensitive compounds and increase their usability and their storage time. For example, drugs were enclosed in host molecules like cyclodextrins. This topic has been previously reviewed.1

  The same approach can be applied to organic chemistry where very labile species can be encapsulated. In this way, species that usually would rapidly decompose or polymerize even at low temperatures can be stored at room temperature, especially when the guest has no chance to escape from its host. To achieve this goal, hemicarcerands have been mostly used, leading to the preparation of cages containing highly reactive species such as 1,3-cyclobutadiene, cycloheptatetraene, or benzyne. This subject is presented in detail in a specially dedicated chapter (Reactions inside Carcerands, Ralf Warmuth).

  Similarly, very useful results can be obtained if the labile species is not incarcerated but can easily be released from its cage. Obviously, this property is significant in order to have a chance to use the guest or its products. In this section, we will first present what can be done to increase the thermodynamic and kinetic stability of a labile species with the aim to increase its relative concentration during the reaction or to make it more persistent. Second, we will show a few examples to demonstrate how to stabilize some specific noncovalent assemblies and to enforce a change in the conformation of the guest. Finally, the focus will be put on highly Reactive Intermediates (radicals, radical cations, carbenes, and nitrenes) showing how to protect them from the surroundings in order to prevent their trapping but also to modify their reactivity to be able to obtain products that would not be found otherwise. Taking examples from the supramolecular chemistry of carbenes and nitrenes, more details will be given about the methodology used in these studies.

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  Reactive Species Detection in Biology

  From Fluorescence to Electron Paramagnetic Resonance Spectroscopy

  • Frederick A. Villamena(Author)
  • 2016(Publication Date)
  • Elsevier

    (Publisher)

  The pairing of electrons between two atoms is the bedrock of molecular stability, but once such pairing breaks Reactive Intermediates can form as an ion pair (i.e., heterolytic cleavage): A − B → A + + B − If there is an equal partition of the electron pair between two atoms (i.e., homolytic cleavage), two neutral radical species are formed: A − B → A + B Aside from being broken into component species, molecules can undergo transformation by accepting or losing electrons through energy or electron-transfer processes to form the Reactive Intermediates of a radical anion and a radical cation: A − B → + e − A − B − A − B → − e − A − B + These ionic species (A + /B −) as well as neutral (A / B) or charged radicals (AB − / AB +). formed from stable molecules and are key contributors to biomolecular transformation; collectively, they are referred to as reactive species (RS) in biology. The driving force in most of these chemical transformations are biological oxidoreductants, mostly derived from oxygen, nitric oxide, sulfides, and halides. For examples, heterolytic cleavage of thiols (R-SHs) to form the nucleophile thiolate (RS −) often serves as a basis for thiol nucleophilicity; homolytic cleavage of hydrogen peroxide (H 2 O 2) forms the electrophilic (HO) and is the basis of reactive oxygen species (ROS) oxidative properties; the electron transfer via one-electron reduction of oxygen (O 2) to form superoxide (O 2 −) is the precursor of some of the RS known to exist in biological systems. Superoxide radical is transformed into a variety of RS that come as neutral or charged species, either as radicals (e.g., O 2 −, HO, HO 2, RO 2, RO, CO 3 −, RS, GSSG −, and NO 2) or as nonradicals (e.g., H 2 O 2, HOCl, O 3, O 2 1 Δ g, ROOH, ONOO − and ONOOCO 2 −) as shown in Fig. 2.1. They are also classified as reactive oxygen-, nitrogen-, sulfur-, or halogen-species. 1, 2 The formation of O 2 − and its products signals the first sign of oxidative burst in a biological system

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