Carbocation

8 Key excerpts on "Carbocation"

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  eBook - PDF

  Reaction Mechanisms in Organic Chemistry

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

    (Publisher)

  Someone who knows and understands the chemistry of intermediates knows chemical reactions and reaction mechanisms. If the positive (+) charge is on a carbon atom in a heterolytic bond cleavage of a molecule A–B, the formal charge of that carbon atom is +1, and such compounds are called Carbocations. For many years, these compounds were called methyl, ethyl carbonium ions [1]. CH 3 H 3 C CH 2 H 3 C CH CH 3 H 3 C C CH 3 CH 3 Methyl Carbocation Ethyl Carbocation i-Propyl Carbocation t-Butyl Carbocation The Carbocations can be classified into two groups: classical Carbocations and nonclassical Carbocations. Classical car- bocations contain a carbon atom having a sextet of electrons with three σ bonds. Inductive or mesomeric effects sta- bilize the positive charge. However, nonclassical Carbocations have a three-center, two-electron structure and they are penta-coordinated. An example of a nonclassical Carbocation is the 2-norbornyl Carbocation. The CH 2 group is bonded to three carbon atoms (C1, C2, and C3) and two hydrogen atoms. Furthermore, the CH 2 group is bonded to the carbon atoms C1 and C2 through two electrons. Therefore, it is called a three-center, two-electron structure. H 3 C CH 2 CH 2 CH 2 Classical Carbocations Nonclassical norbornyl cation 1 2 3 7.1 Structure and Stability of Carbocations A Carbocation is a molecule in which a carbon atom has a positive charge and is bonded to three substituents. Carbocations have a trigonal planar structure because of their sp 2 hybrid orbitals. The vacant p orbital is perpendicular to the plane formed by the substituents and indicates its electron-deficient nature [2]. C H H H Carbocation Both NMR spectroscopic [3] and crystallographic studies [4] performed on the t-butyl Carbocation show that it has a planar structure and the angle between the methyl groups (C–C + –C) is 120 ∘ .

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  Introductory Organic Chemistry and Hydrocarbons

  A Physical Chemistry Approach

  • Caio Lima Firme(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  Chapter Twelve

  Carbocations

  DEFINITION AND CLASSIFICATION

  Carbocations are alkyl ions containing one trivalent, positively charged carbon atom and they are the intermediate of electrophilic addition to alkenes. Carbocations are electrophiles whose electrophilic center is the trivalent carbon atom. However, the positive charge is delocalized among neighbor hydrogen and carbon atoms by inductive effect. The trivalent carbon atom is sp2 and it has a trigonal planar geometry, i.e., the nucleophile might attack the Carbocation upwards or downwards.

  Carbocations can be defined as classical ion (or carbenium ion) and non-classical ion (or carbonium ion). In the former, there is a formal charge localized in just one carbon (although, actually the positive charge is delocalized among the neighbor hydrogen and carbon atoms by means of inductive effect). In the latter, the positive charge is delocalized to other two carbon atoms by a through-space assistance of a π-bond or σ-bond, forming a formal 3c-2e or 4c-2e multicenter bonding where some of them have homoaromatic properties.

  Carbocations can also be classified according to the number of alkyl groups bonded to the trivalent carbon: (1) methyl cation where only hydrogen atoms are bonded to the trivalent carbon; (2) primary Carbocation where one alkyl group is bonded to the trivalent carbon; (3) secondary Carbocation where two alkyl groups are bonded to the trivalent carbon; and (4) tertiary Carbocation where three alkyl groups are bonded to the trivalent carbon. All represented Carbocations in Fig. 12.1 (ethyl cation, 2-propyl cation and tert-butyl cation) are carbenium ions. Each alkyl group bonded to the trivalent carbon atom in all Carbocations in Fig. 12.1 is a methyl group.

  Alkyl group is a hydrocarbon fragment which is bonded to a hydrocarbon chain or an electrophilic/nucleophilic center. The alkyl group is an alkane with a missing hydrogen atom. It can be a methyl (CH3 -), ethyl (CH3 CH2 -), propyl (CH3 CH2 CH2 -), 2-propyl ((CH3 )2 CH-), butyl (CH3 CH2 CH2 CH2 -), tert-butyl ((CH3 )3 C-), and so on. The dash (-) represents the place where the alkyl group will be (or is) linked to the rest of the molecule. They are generally represented by R, R′, R″ or R1 , R2 , R3 . Each Carbocation can have, at most, three alkyl groups (see more information in chapter fourteen

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  Cationic Polymerizations

  Mechanisms, Synthesis & Applications

  • Krzysztof Matyjaszewski(Author)
  • 1996(Publication Date)
  • CRC Press

    (Publisher)

  2 Fundamentals of the Reactions of Carbocations with Nucleophiles HERBERT MAYR Institute for Organic Chemistry, Technical University of Darmstadt, Darmstadt, Germany I. INTRODUCTION Carbocations are molecules with a formal positive charge at carbon and an even number of electrons. Although the first examples of such ions were reported in 1902, when Baeyer [1,2] recognized the salt-like charac­ ter of the compounds formed from triphenylmethanol and sulfuric acid [3], the concept of Carbocation chemistry essentially was developed by Meerwein and Ingold in the 1920s [4], Meerwein rationalized the rear­ rangement of camphene hydrochloride to isobomyl chloride by suggesting the ionization of the C—Cl bond and successive rearrangement of the cationic intermediate [5]. Other molecular rearrangements were inter­ preted analogously [ 6 ]. Kinetic investigations led Ingold to the conclusion that nucleophilic aliphatic substitutions follow two different mechanisms, one of which (the so-called SN1 mechanism) involves the intermediacy of Carbocations [7]. Winstein’s kinetic and stereochemical studies of solvo-lytic displacement reactions revealed the importance of ion pairing [ 8 ] and anchimeric assistance [9] in reactions with a stepwise mechanism. These investigations furthermore led to the formulation of nonclassical carbo­ cations, i.e., Carbocations which cannot be properly described by two-electron two-center bonds [ 1 0 ]. Until the early 1960s information about Carbocations has been ob­ tained almost exclusively from indirect evidence. A major change oc­ curred in 1962 when Olah reported the generation and direct observation of Carbocations as long-lived species in solvents of low nucleophilicity (superacidic conditions) [11,12]. Many types of Carbocations have since 51 52 Mayr then been investigated by a combination of these methods, including quan­ tum chemical calculations [13], and nowadays Carbocations belong to the best characterized reactive intermediates [14].

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  Organic Reaction Mechanisms 2018

  An Annual Survey Covering the Literature Dated January to December 2018

  • Mark G. Moloney(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  227 6 Carbocations V. M. Moreira Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, Glasgow, UK Laboratory of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Coimbra, Portugal Center for Neuroscience and Cell Biology, University of Coimbra, Portugal CHAPTER MENU Introduction, 227 Alkyl and Cycloalkyl Cations, 228 Vinyl, Allyl and Propargyl Cations, 229 Benzyl Cations, 230 Arenium Ions, 235 Aromatic Cations, 235 Oxonium, Sulfonium and Iminium Cations, 236 New Cations and Synthetic Methods, 237 Nonclassical Carbocations, 238 Carbocation rearrangements, 239 Carbocations in biosynthesis, 240 References, 243 Introduction Two major studies using computational techniques have addressed Carbocation formation and stability. 1,2 Canonical molecular orbital energy decomposition analysis (CMOEDA) has been presented as new paradigm for explaining the order of stability of Carbocations, aiming to overcome the difficulty of the quantification of inductive/field and delocalization effects. 1 The analysis examines the interaction energy ( Δ E tot eda ) between two fragments of a given molecule resulting from its division into radicals or ions, which is then quantified by means of different contributions, namely electrostatic (electron–electron, nucleus–nucleus, electron–nucleus), exchange repulsions (between like-spin electrons), polarization (orbital relaxation), dispersion (calculated from electron correlation) and preparation (energy needed to bring the fragments to the geometry they adopt on the final molecule). Thus, for the given species R 3 C + , the car-bocation is first brought from its equilibrium geometry to that which it will adopt in the final molecule, i.e., R 3 C + (rigid), and then interacts with H – to give the final product, R 3 CH. This last step is the one used to compute the interaction energy to be determined.

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  eBook - ePub

  Reaction Mechanisms in Organic Chemistry

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

    (Publisher)

  2 group is bonded to the carbon atoms C1 and C2 through two electrons. Therefore, it is called a three-center, two-electron structure.

  7.1 Structure and Stability of Carbocations

  A Carbocation is a molecule in which a carbon atom has a positive charge and is bonded to three substituents. Carbocations have a trigonal planar structure because of their sp2 hybrid orbitals. The vacant p orbital is perpendicular to the plane formed by the substituents and indicates its electron-deficient nature [2] .

  Both NMR spectroscopic [3] and crystallographic studies [4] performed on the t-butyl Carbocation show that it has a planar structure and the angle between the methyl groups (C–C+ –C) is 120°. The racemization of optically active alkyl halides during the solvolysis reaction indirectly also indicates that Carbocations have a planar structure.

  The stability of Carbocations varies according to the nature of substituents attached to the carbon atom carrying the positive (+) charge. If these groups are alkyl groups, both the number of alkyl groups and the Carbocation's stability will be higher. Among the alkyl-substituted Carbocations, tertiary Carbocations are the most stable.

  One method used to determine Carbocations' stability is to measure the energy required to form the Carbocation from the corresponding alkyl halide. However, the Carbocation's relative stability should also be determined in the gas phase rather than in solution because the solvation influences Carbocations' stabilities in solution.

  The hydride ion affinity (HIA ) shows the relative stability of Carbocations. The HIA is defined as the negative value of the reaction enthalpy (ΔH) of the reaction between a Carbocation and a hydride ion in the gas phase.

  For example, the gas-phase reaction of methyl Carbocation and a hydride ion has the formation enthalpy of ΔH = −313.4.0 kcal/mol. The HIA values of some selected Carbocations are given in Table 7.1

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  eBook - ePub

  Organic Photochemistry

  • V. Ramamurthy(Author)
  • 1997(Publication Date)
  • CRC Press

    (Publisher)

  5

  Photochemistry and Photophysics of Carbocations

  Mary K. Boyd

  Loyola University of Chicago, Chicago, Illinois

  I.

  INTRODUCTION

  Carbocations are important intermediates in an array of organic chemical reactions, resulting in extensive investigations of their ground state chemistry, including reactivity, structure, and product studies [1 ]. Early studies in Carbocation photochemistry focused primarily on photoproduct determination following irradiation of cations thermally generated in acidic media [2 ,3 ,4 ]. In a recent comprehensive review, Childs and Shaw noted two broad classes of Carbocation photoreactions [2 ]. In very strongly acidic media such as FSO3 H, the typical reaction is photoisomerization. In less strongly acidic media, the predominant reaction is electron transfer to give radical intermediates which undergo coupling reactions to form dimeric materials or substitution products.

  More recently, considerable attention has been centered on the characterization, properties, and mechanistic studies of excited state Carbocations, and this attention is the main subject of this chapter. A separate, yet intimately related aspect of Carbocation photochemistry involves the use of photochemical methods to generate the cations. The photogeneration of Carbocations and the study of their subsequent (mainly thermal) chemistry have been the subject of two recent extensive reviews [5 ,6

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  The Vocabulary and Concepts of Organic Chemistry

  • Milton Orchin, Allan R. Pinhas, R. Marshall Wilson, Roger S. Macomber(Authors)
  • 2005(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Example. In the most frequently encountered examples, the carbon atom is trivalent and thus surrounded by a sextet of valence electrons, as in the t-butyl cation (Fig. 13.27a). In many cases, the charge on carbon may be delocalized on an adjacent hetero atom, as in one of the resonance structures of protonated acetone (Fig. 13.27b). These examples demonstrate some ambiguity in the definition since, as in the acetone case, they may be called oxonium (Sect. 13.37) ions as well. It is probably best to refer to the resonance structure with the charge on carbon atom as the Carbocation form and the resonance structure with the charge on oxygen as the oxonium ion form, bearing in mind, of course, that resonance structures are not separate species. All carbenium ions, such as Fig. 13.27a (Sect. 13.33), and all carbonium ions (Sect. 13.31), such as Fig. 13.27c (Sect. 13.31), the protonated methane or methonium ion, are Carbocations. 13.28 CLASSICAL CATION A cation in which the charge is either localized or delocalized, but is not distributed by means of a closed (bridging) multicenter bond (Sect. 3.46). Example. Methyl cation (Fig. 13.28a) and 1-hydroxyethyl cation (Fig. 13.28b). All carbenium ions (Sect. 13.33) are classical cations. Species in which the charge is on carbon, such as methyl cation, are frequently called carbonium ions, but according to IUPAC guidelines, because they are electron-deficient, they should be called car- benium ions (see Sect. 13.33). NONCLASSICAL CATION 517 OH C H 3 C H 3 C (a) OH C H 3 C H 3 C (b) (c) CH 5 (CH 3 ) 3 C Figure 13.27. (a) t-Butyl cation; (b) resonance structures of protonated acetone; and (c) pro- tonated methane or methonium ion. First prepared by George Olah and co-workers. OH C H H 3 C OH C H H 3 C (a) (b) CH 3 Figure 13.28. (a) Methyl cation and (b) 1-hydroxyethyl cation (protonated acetaldehyde).

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  Survey of Progress in Chemistry

  Volume 4

  • Arthur F. Scott(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Carbanions DONALD J. CRAM Department of Chemistry, University of California at Los Angeles, Los Angeles, California I. Carbanion Generation and Capture 45 . Orders of Carbanion Stability 47 A. Thermodynamic Acidity of Carbon Acids 47 B. Kinetic Acidity 49 C. Medium Effects 50 . Mechanism of Carbanion Stabilization 50 A. Structure of Unstabilized Carbanions 50 B. s-Orbital Effects 51 C. Conjugative Effects 52 D. Aromatization Effects 53 E. Homoconjugative Effects 54 F. Inductive Effects 55 G. Negative Hyperconjugative Effects 56 H. rf-Orbital Effects 56 IV. Carbanion Stereochemistry 57 A. Planar Carbanions in Asymmetric Environments 57 B. Asymmetric Carbanions V. Carbanionic Isomerization Reactions 3 A. Prototropic Rearrangements 3 B. Ring-Chain Rearrangements 66 References 67 I. CARBANION GENERATION AND CAPTURE A substantial fraction of all organic reactions involve carbanions as intermediates, particularly those transformations used to elaborate carbon chains. Most of the base-catalyzed condensation, fragment-ation, alkylation, and rearrangement reactions, and many of the 45 46 DONALD J. CRAM reactions of organometallic compounds, have negatively charged carbon as intermediates. These anions are usually generated by breaking a covalent or partially covalent bond in such a way that the pair of electrons of the bond remains with carbon, and that the other fragment (the leaving group) either assumes a positive charge or becomes neutral, depending on the charge type of the original starting material. A variety of leaving groups (L) are known, which involve many of the common elements bound in the starting material to carbon. I I-— C—L * C + L I -C-lH — C-W — C-fc 1 . i , I , -ç-lo -c-|§i -C -I P Generalized leaving groups (L) Usually anionic reactions are completed by carbanion capture by an electrophile (E), such as certain metal cations, or positive hydrogen, carbon, nitrogen, oxygen, or halogen donors.

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