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Reactive Intermediates in Organic Chemistry Structure and Mechanism von Singh, Maya Shankar (eBook)

  • Erscheinungsdatum: 22.01.2014
  • Verlag: Wiley-VCH
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Reactive Intermediates in Organic Chemistry

This unique text gives a detailed introduction into the chemistry of the most important reactive intermediates in modern organic chemistry, such as carbocations, carbanions, radicals, carbenes, carbenoids, nitrenes, arynes, etc. Each chapter is clearly structured and provides information on the formation, identification, structure, stereochemistry as well as stability and reactivity of the intermediates. Additionally, a problem section with different levels of difficulty is included for self-testing. By providing a deeper understanding of the underlying concepts in organic chemistry, this is a must-have reference for PhD and master students in organic chemistry as well as a valuable source of information for chemists in academia and industry working in the field of organic synthesis.

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    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 295
    Erscheinungsdatum: 22.01.2014
    Sprache: Englisch
    ISBN: 9783527678273
    Verlag: Wiley-VCH
    Größe: 15539 kBytes
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Reactive Intermediates in Organic Chemistry

Chapter 2

Carbocations

2.1 Introduction

In many cases, reactions proceed via so-called intermediates, which have in general very short lifetimes. One type of reactive intermediates is the so-called " carbocations ." Charged atoms and groups of atoms are common in inorganic chemistry. All of us know about table salt, which consists of positively charged sodium ions (cations) and negatively charged chloride ions (anions). The opposite is true for the large number of organic compounds, especially hydrocarbons, which are composed of only two elements, carbon and hydrogen. Carbocations have been well established as intermediates in numerous synthetic transformations. In such cases these intermediates had to have an extremely short lifetime, a billionth of a second or less, and due to their high reactivity their concentrations had to be very low. Their existence has been indicated by measurements of reaction rates and observations of the spatial arrangement of the atoms in space. For such purposes, a variety of ingenious experiments have been carried out. However, nobody was able to see these carbocations, not even with the most powerful microscopes or by spectroscopic methods. These techniques can be regarded as extensions of human vision. Consequently, there was no evidence for the existence of carbocations, in other words whether they were a reality independent of human consciousness or were only created by human imagination to describe the experimental results. Because it was not possible to detect carbocations with spectroscopic methods, different scientists interpreted their experiments differently, and a scientific feud took place in organic chemistry during the 1960s and 1970s.

Through a series of brilliant experiments Professor George Olah solved the problem. He created methods to prepare long-lived carbocations in high concentrations, which made it possible to study their structure, stability, and reactions with spectroscopic methods. He achieved this by using special solvents, which did not react with the cations. He observed that in these solvents, at low temperatures, carbocations could be prepared with the aid of superacids (acids 1810 times stronger than concentrated sulfuric acid). Through Olah's pioneering work he and the scientists who followed in his footsteps could obtain detailed knowledge about the structure and reactivity of carbocations. Olah's discovery resulted in a complete revolution for scientific studies of carbocations, and his contributions occupy a prominent place in all modern textbooks of organic chemistry.

Olah found that there are two groups of carbocations, namely, trivalent ones called carbenium ions , in which the positive carbon atom is surrounded by three atoms, and those in which the positive carbon is surrounded by five atoms, called carbonium ions ( Figure 2.1 ). The disputed existence of these pentacoordinated carbocations was the reason for the scientific feud. By providing convincing proof that pentacoordinated carbocations exist, Olah demolished the dogma that carbon in organic compounds could at most be tetracoordinated, or bind a maximum of four atoms. This had been one of the cornerstones of structural organic chemistry since the days of Kekulé in the 1860s.

Figure 2.1 Carbenium and carbonium ions.

Olah found that the superacids were so strong that they could donate a proton to simple saturated hydrocarbons, and that these pentacoordinated carbonium ions could undergo further reactions. This fact has contributed to a better understanding of the most important reactions in petrochemistry. His discoveries have led to the development of methods for the isomerization of straight chain alkanes, which have low octane numbers when used in combustion engines, to produce branched alkanes with high octan

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