Structure and Stability of Organic Compounds

In each of the chapters of this text, we will explore the use of different models to explain and predict the structures and reactions of organic compounds. For example, we will consider alternative explanations for the hybridization of orbitals, the c,7c description of the carbon-carbon double bond, the effect of branching on the stability of alkanes, the electronic nature of substitution reactions, the acid-base properties of organic compounds, and the nature of concerted reactions. The complementary models presented in these discussions will give new perspectives on the structures and reactions of organic compounds. 

As chemists learned more about the effects of structure on the stability of organic compounds, it became apparent that Dewar benzene is much less stable than benzene. Not only does it have a considerable amount of angle strain, but it also has none of the stabilization due to aromaticity that benzene has. Because of these factors, Dewar benzene is 71 kcal/mol (297 kJ/mol) less stable than benzene. Because the conversion of Dewar benzene to benzene is so exothermic and involves an apparently simple electron reorganization, many chemists believed that the isolation of this strained isomer would prove to be impossible. They thought that if it were prepared, it would rapidly convert to benzene. In support of tills view, numerous attempts to synthesize Dewar benzene met with failure. 

What then, can organic chemistry as a science draw out from quantum chemistry In the search for the answer it is useful to look at the already accumulated experience of the interactions in these closely related areas of chemical science. In the last decades there have evolved various methods for the non-empirical and semi-empirical calculations of structure and reactivity of organic molecules based on quantum mechanics. In numerous cases these calculations turned out to be of extreme usefulness in obtaining quantitative information such as the charge distribution in a molecule, the reaction indices of alternate reaction centers, the energy of stabilization for various structures, the plausible shape of potential energy surfaces for chemical transformations, etc. This list seems to include almost all parameters that are needed for the explanation and prediction of the reactivity of a compound, that is, for solving the main chemical task. Yet there are several intrinsic defaults that impose rather severe limitations on the scope of the reliability of this approach. 

Hyperconjugation by a C-Sn o bond (and indeed by most carbon-metal a bonds) is much more effective than C-H hyperconjugation, and it is an important factor in determining the structure and stability of not only radicals and cations, but also of compounds with filled n systems such as allyl-, benzyl-, and cyclopentadienyl-stannanes. The importance of vinyl-, allyl-, and aryl-stannanes in organic synthesis owes much to the stabilisation of radical and cation intermediates by a stannyl substituent, and under suitable conditions this can accelerate a reaction by a factor of more than 1014. 

The concept of an atom s oxidation state see Oxidation Number) can provide fundamental information about the structure and reactivity of the compound in which the atom is found. In fact, it can be argued that oxidation states provided the basis for Medeleev s initial organization of the periodic table. For the main group elements, the relative stability of lower oxidation states within a given group increases as the atomic number increases. This trend in the periodic table see Periodic Table Trends in the Properties of the Element is generally attributable to the presence of an inert s pair see Inert Pair Effeci) caused by relativistic effects see Relativistic Effects). 

Chapter 5 begins with a look at the structure, nomenclature, and stability of alkenes—compounds that contain carbon-carbon double bonds—and then introduces some fundamental principles that govern the reactions of organic compounds. You will revisit how to draw curved arrows to show how electrons move during the course of a reaction as new covalent bonds are formed and existing covalent bonds are broken. Chapter 5 also discusses the principles of thermodynamics and kinetics, which are central to an understanding of how and why organic reactions take place. 

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. Scheme 9.2 gives the structures, names, and stabilization energies of a variety of such hydrocarbons. Derivatives of these hydrocarbons having heteroatoms in place of one or more carbon atoms constitute another important class of organic compounds. 

Recent developments in drug discovery and drug development spurred the need for novel analytical techniques and methods. In the last decade, the biopharmaceutical industry set the pace for this demand. The nature of the industry required that novel techniques should be simple, easily applicable, and of high resolution and sensitivity. It was also required that the techniques give information about the composition, structure, purity, and stability of drug candidates. Biopharmaceuticals represent a wide variety of chemically different compounds, including small organic molecules, nucleic acids and their derivatives, and peptides and proteins. 

The development of catalysts for the oxidation of organic compounds by air under ambient conditions is of both academic and practical importance (1). Formaldehyde is an important intermediate in synthetic chemistry as well as one of the major pollutants in the human environment (2). While high temperature (> 120 °C) catalytic oxidations are well known (3), low temperature aerobic oxidations under mild conditions have yet to be reported. Polyoxometalates (POMs) are attractive oxidation catalysts because these extensively modifiable metal oxide-like structures have high thermal and hydrolytic stability, tunable acid and redox properties, solubility in various media, etc. (4). Moreover, they can be deposited on fabrics and porous materials to render these materials catalytically decontaminating (5). Here we report the aerobic oxidation of formaldehyde in water under mild conditions (20-40 °C, 1 atm of air or 02) in the presence of Ce-substituted POMs (Ce-POMs). 

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