The Stability of Organic Compounds

In this chapter some classes of unstable compound and their decomposition reactions will be presented, with the aim of sparing the reader unpleasant surprises in the laboratory. 

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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. 

The stability of organic compounds on thin layer chromatograms exposed to air has been studied, with mass spectrometry used to characterize the products of degradation. Aromatic thiols undergo oxidation in air, and dimerize to the disulphide (24). Arylindandiones, medicinal compounds isolated from various plants, also undergo degradation in air and light (25). The rates of formation of the dimers can be followed with TLC, with characterization by mass spectrometry. 

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). 

Stannic chloride, SnCLp a fuming liquid, is used in the preparation of organic compounds and chemicals to weight silk and to stabilize perfumes and colors in soap. 

The stability of organic chalcogen compounds decreases mosdyin the order sulfur > selenium > tellurium. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

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