Region-selective oxygenation

It was established previously that the selectivity towards the production of acetic acid is increased when the electrolysis potential is set in the so-called oxygen region. This is understandable, because the reaction requires an extra oxygen atom which can be provided by an oxygenated Pt(OH, OOH, O) species. The adsorbed acetyl is the precursor of the acetic acid (steps 4 and 5) and the COads gives carbon dioxide (steps 5, 5" and 6 ). 

The examples of reaction rates of O2 in Figure 12.7 show that O2 reacts only with deprotonated species (e.g., phenolate anions) that is, the apparent rate constants decrease in the pH region below the pKa of the chemical. Singlet oxygen is selective it is an electrophile that reacts only with particular functional chemical structures such as are present in 1,3 dienes (see chemical structure of fiirfuiyl alcohol) or polycondensed aromatic hydrocarbons (with delocalized 7T electron bonds) or in sulfides or mercaptans (Hoigne, 1990). 

Introduction.—The oxidative dehydrogenation of alcohols to aldehydes and ketones over various catalysts, including copper and particularly silver, is a well-established industrial process. The conversion of methanol to formaldehyde over silver catalysts is the most common process, with reaction at 750—900 K under conditions of excess methanol and at high oxygen conversion selectivities are in the region 80—95%. Isopropanol and isobutanol are also oxidized commercially in a similar manner. By-products from these reactions include carbon dioxide, carbon monoxide, hydrogen, carboxylic acids, alkenes, and alkanes. 

Hosokawa et al. clearly spelled out the heteroatom coordination theory in explaining their region-selective formation of aldehydes.20 Terminal olefin 13 was converted to a mixture of aldehyde 14 and methyl ketone 15, with aldehyde 14 as the major product (14/15 = 70/30). The authors explained that Pd(II) coordinated with both oxygen of the amide as well as the olefin as depicted in intermediate 16, which blocked the normal Markovnikov hydration position and the addition of the peroxide took place at the terminal position as... 

This oxidation can be exploited to region-selectively oxygenate the a positions of the ring when the oxide is dissolved in refluxing acetic anhydride followed by hydrolysis with sodium hydroxide. This alcohol can then tautomerize to form the amide. 

Gettering is a black art. It consists in forcing selected impurities (typically, transition metals) to diffuse toward unimportant regions of tlie device. This is often done by creating precipitation sites and perfoniiing heat treatments. The precipitation sites range from small oxygen complexes to layers such as an A1 silicide. The foniiation of such a... 

Like the photosynthetic reaction center and bacteriorhodopsin, the bacterial ion channel also has tilted transmembrane helices, two in each of the subunits of the homotetrameric molecule that has fourfold symmetry. These transmembrane helices line the central and inner parts of the channel but do not contribute to the remarkable 10,000-fold selectivity for K+ ions over Na+ ions. This crucial property of the channel is achieved through the narrow selectivity filter that is formed by loop regions from thefour subunits and lined by main-chain carbonyl oxygen atoms, to which dehydrated K ions bind. 

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