Nitrogen preferential forms

The phototrophs that assimilate fixed nitrogen preferentially utilize ammonium over nitrate or nitrite. (Some phototrophs assimilate DON, such as urea, to meet their nitrogen needs, and, hence, are not strict autotrophs.) Ammonia is fevored over the more oxidized forms of DIN as less redox energy is required to incorporate its nitrogen into biomolecules, primarily amino acids and the nucleotide bases. 

In the thioaldehyde of pyrrolo[2,l-h]thiazole (73JCS(P1)657) (150), the C=S group is directed preferentially toward nitrogen (Z-form), as occurs in the aldehyde of indolizine 10, while bulky R substituents change this... 

Decompression sickness refers to other physiological effects that can occur during ascent to the water s surface—the decompression phase. If the decompression rate is too fast, the absorbed nitrogen will preferentially form bubbles. This process has variable results, depending on the size and location of the bubbles in the body. If they develop in the muscles or spine, extreme pain and muscle contraction can result—a consequence that has earned the term the bends. Bubbles can also block the bloodstream or cause pressure in the brain. 

The Schmidt transformation of ketones to IH-tetrazoles (predominantly isomer A) with sodium azide-titanium tetrachloride was rmorted by Suzuki el at [93S1218]. Mechanistically, a geminal diazide undergoes loss of nitrogen to form an a-azidonitrene which undergoes a [l,2]-substituent shift to lead to preferential formation of isomer A. However, in a few cases [R = QH5 and = 4-Me(X 6H4. = CgHs and R = CH3]... 

Evidence for the intermediate formation of nitrone species during the carbonylation of nitroarenes in e s-cyclooctene as solvent catalysed by Ru3(CO)i2 have been obtained [14], Moreover, zerovalent palladium species with nitrogen donor ligands have been shown to be active catalysts in the reductive carbonylation of organic nitro derivatives [41]. The hypothesis that an intermediate having the olefinic double bond coordinated to the metal is formed during the catalytic cycle is supported by the steric effect that has been observed in the case of p,p -dimethyl-o-nitrostyrene (7f) as substrate. Moreover, such an intermediate could explain why a pentaatomic indole nucleus is preferentially formed, even when a conjugated double bond is present in the olefinic chain ... 

Oxygen, nitrogen and air, like argon and krypton (1), are found to preferentially form gas hydrates of structure II, rather than structure I as previously expected for gas hydrates of small guest molecules. Lattice parameters from X-ray diffraction are given in the Table. 

Ozone, known for its beneficial role as a protective screen against ultraviolet radiation in the stratosphere, is a major pollutant at low altitudes (from 0 to 2000 m) affecting plants, animals and human beings. Ozone can be formed by a succession of photochemical reactions that preferentially involve hydrocarbons and nitrogen oxides emitted by the different combustion systems such as engines and furnaces. 

Canonical forms of type (442b) facilitate proton loss from the amino groups the anions formed react easily with electrophilic reagents, usually preferentially at the exocyclic nitrogen atom e.g. 443 -> 444) (79HC(34-2)9). 

The mesomeric anion (157 Scheme 10) reacts readily with electrophilic reagents such as alkyl or acyl halides at N-2, C-4 and the exocyclic oxygen atom. The percentages of the different products formed are controlled by the HSAB principle. The acylium ion (hard) attacks preferentially at oxygen (hard), whilst the softer alkylating agents attack the nitrogen atom. 

The mono- and some di-aminoacridines, 6- and 7-aminoquin-oline, and 3-aminopyridine form monocations in which protonation occurs preferentially at the ring nitrogen atom. However, ultraviolet spectral data show that 1,9-diaminoacridine (237, R = NH2) undergoes proton addition on the amino groups in preference to the cyclic nitrogen atom, whereas l-amino-9-methylacridine (237, R = Me) is monoprotonated principally at the amino group in ethanol... 

With alkali cyanides, a reaction via a SN2-mechanism takes place the alkyl halide is attacked by cyanide with the more nucleophilic carbon center rather than the nitrogen center, and the alkylnitrile is formed. In contrast, with silver cyanide the reaction proceeds by a SnI-mechanism, and an isonitrile is formed, since the carbenium intermediate reacts preferentially with the more electronegative center of the cyanide—i.e. the nitrogen (Kornblum s rule, HSAB concept). ... 

Thermal rearrangement of 2-vinylaziridine 203, with an electron-withdrawing substituent on the nitrogen atom, at reflux in decalin predominantly yielded 2-pyr-roline 204 along with some 3-pyrroline derivatives 205 (Scheme 2.50) [79]. A similar reaction was also observed with l-alkyl-2-phenyl-3-vinylaziridines 206 [80]. From these observations, the nature of the products formed depends on the natures of the substituents on the aziridine ring if the ring carbon carries a phenyl substituent, the thermolysis in most cases preferentially yields 2-pyrrolines. 

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