Replacement by oxygen

Difluoropyridines. 2,4-Difluoropyridine can be prepared (26% yield) from 2,4-dichloropyridine and potassium fluoride in sulfolane and ethylene glycol initiator (403). The 4-fluorine is preferentially replaced by oxygen nucleophiles to give 2-fluoro-4-hydroxypyridine derivatives for herbicidal apphcations (404). 

Oxadethiacephalosporins. The first compound in which the sulfur of a cephalosporin was replaced by oxygen was 1-oxa-dethiacephalothin [54214-83-4] (64), C gH N20yS, (199) which is approximately twice as active as cephalothin (27) taking into account that the synthetic material was... 

The doped Zr02 stmctures are used as electrochemical sensors, as, for example, when used to detect oxygen in automotive exhaust (see Exhaust CONTROL, automotive). The sensor voltage is governed by the Nemst equation (eq. 17) where the activities are replaced by oxygen partial pressures and the air inside the chamber is used as reference. 

The replacement of rhodium from a wide range of rhodacycles to form condensed furans, thiophenes, selenophenes, tellurophenes and pyrroles has been widely explored and a range of examples is shown in Scheme 97. The rhodacycles are readily generated from the appropriate dialkyne and tris(triphenylphosphine)rhodium chloride. Replacement of the rhodium by sulfur, selenium or tellurium is effected by direct treatment with the element, replacement by oxygen using m-chloroperbenzoic acid and by nitrogen using nitrosobenzene. 

Ruonne atoms in aromatic nitrogen heterocycles are readily replaced by oxygen nucleophiles [77] Bistnfluoromethyl hydroxylarmne anion is an mterest-ing nucleophile for the mtroduction of oxygen mto perfluoropyndine Rearrangement of the product occurs at 125 °C [18] (equation 12)... 

One of the two carbon atoms of the ethylene bridge of the antidepressants may be replaced by oxygen. Attachment of the side chain via the olefinic linkage found in amytriptyline affords antidepressants with a biologic profile similar to the carbocy-clic prototype. 

Ciamician and Silber have found that. light has a marked effect on methyl-heptenone. The ketone was kept in a glass flask, exposed to the light for five months, the flask being exhausted of air, which was replaced by oxygen. When the seal was broken, the contents of the flask were found to be at reduced pressure, and the oxygen was mainly converted into carbon dioxide. 

The higher fluorides are prepared using excess fluorine (Figure A). All these compounds are stable in dry air at room temperature. However, they react with water te form compounds in which one or more of the fluorine atoms has been replaced by oxygen. Thus xenon hexafluoride reacts rapidly with water to give the trioxide... 

Inspection of the table shows that the quotient a/Wj e is in fact nearly constant that I changes much less rapidly than W e] and that the critical depth has doubled when the highest oxide is reached. All three conditions are reflections of the (positive) absorption effect that occurs in this binary system when iron is replaced by oxygen, which has a lower mass absorption coefficient. 

If one-fourth of the fluorine ions are removed and the others are replaced by oxygen ions, calcium being replaced by (Mn, Fe), a structure is obtained which approximates that of bixbyite, which differs from it only in small displacements of the ions. This similarity is shown by the fact that the highly distorted octahedra have corners which are nearly at six of the eight corners of a cube, the six being chosen differently for the 8e and the 24 e octahedra, as is seen from Fig. 4 and 5. This analogy was, indeed, pointed out by Zachariasen for his incorrect structure. As a matter of fact the "ideal structure, with u = 0 and... 

In macrocycle [5b] the difference between log Ki and log K2 is 3.6, reflecting the strong Coulomb repulsion between two neighbouring positive charges. When the nitrogen atoms have a larger separation as in the isomer [5c] the difference is smaller (1.1), and if the sulfur atoms are replaced by oxygen atoms it is even lower (0.7 in [5d]). 

In a similar vein, the keto bridge in 5 can be replaced by oxygen with retention of activity. Reduction of acetophenone derivative 19 by means of sodium borohydride leads to the corresponding alcohol (20). Reaction with phosphorus tribromide with cyanide gives... 

The properties of alkyl acrylate homopolymers vary accordingly to the number of carbon atoms in the alkyl radical. As the number of carbon atoms increases the Tg decreases from room temperature to well below 0 °C until the number exceeds 10. However, as the carbons in the side chain increase oil resistance decreases. When carbon atoms in the alkyl radical are replaced by oxygen, oil resistance improves. 

Numerous heteroradialenes, in which the exo-methylene groups are replaced by oxygen, nitrogen or sulfur atoms, have also been synthesized and studied because of their interesting electrical and magnetic properties. Heteroradialenes are, however, not included in this review. 

The situation is different when the benzyl methylene of [23] is replaced by oxygen. Relief of one 1,5- and one 1,4-CH HC repulsion is clearly responsible for the significant oxygen-atom effect observed in this case (Table 12). 

When phenyl isothiocyanate is heated with yellow mercuric oxide the sulphur is replaced by oxygen and the corresponding cyanate, which can be recognised by its exceptionally pungent odour, is obtained ... 

This quindimine contains the quinonoid imino-group, which is very unstable towards acids and is eliminated hydrolytically in the form of ammonia, being replaced by oxygen ... 

When you look at that formula, you will see that, as the name implies ( tetra is Greek for 4), carbon tetrachloride is made up of one atom of carbon (C) and four atoms of chlorine (Cl). Now, it so happens that in the presence of ultraviolet light, two of the chlorine atoms can become replaced by oxygen from the atmosphere to make a compound that can be represented like this ... 

The latter compound results from loss of two hydrogen atoms by metabolic oxidation at each of two of the carbon atoms of hexane, and their replacement by oxygen atoms. Schaumburg and Spencer not only demonstrated this, but also showed that the identical neurotoxic events result from direct exposure to compound II and also another common solvent called methyl-n-butyl ketone (III). Chemical III is, like hexane, readily metabolized to the active toxicant, molecule II. Because both I and III yield the same metabolite (II), and because this metabolite is the source of toxicity, then exposure to both of these chemicals produces the identical type of neuropathy. 

Photolytic. The simulated sunlight (>, >230 nm) photolysis as a thin film on silica gel or sand yielded 6 (l,l-dimethylethyl)-3-(methylthio)-l,2,4-triazin-5-(4/ /)-one and two additional photoproducts. In both of the unnamed photoproducts, the methylthio group in the parent compound is replaced by oxygen and one of the compounds also underwent TV-deamination (Bartl and Korte, 1975). 

In 1972 Cava and Sprecker reported the formation of analogs of 149, in which one of the two sulfur atoms is replaced by oxygen or nitrogen (Scheme 9). Reactions of tetrabenzoylethane (146) with HCl or methylamine in acetic acid afforded 3,4-dibenzoyl-2,S-diphenylfuran (152) or 3,4-dibenzoyl-l-methyl-2,5-diphenyl pyrrole (153), respectively. 

If chlorine is replaced by bromine, the equilibrium will be displaced toward A O + Br2, because the bromide has a lower heat of formation than the chloride. The oxidation of a bromide by oxygen is therefore a more common event than that of the corresponding chloride. Oxidation is commoner, still, with iodides where, in general, iodine is very easily replaced by oxygen A1I3, for example, reacts so easily with air or oxygen that the vapour can explode during the reaction... 

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