Atoms chemically equivalent

As an example, consider the molecule 1,3,5-trichlorobenzene. In the free molecule we term the three Cl atoms chemically equivalent. Another example is 2,4,6-trichloro-l-nitrobenzene in the free molecule the Cl atoms in positions 2 and 6 are chemically equivalent and the Cl atoms in position 4 is chemically inequivalent with respect to the other two Cl atoms. 

Kroto et al. have followed this announcement with a partial chemical separation of the soluble all-carbon molecules generated by the same procedure. They used mass spectrometric evidence to conclude that other air-stable C molecules are present (n = 62, 64, 66, 68, 70). They reported that chromatographic separation yields Qo and C70 in a 3 1 ratio, in contrast to the 2-10% of C70 estimated in ref 1. The reported C NMR spectrum of the Qo fraction, in particular, evidently confirms the existence of a species with all 60 carbon atoms chemically equivalent (proposed structures as shown in Chart 1),... 

In the genuine low-temperature chemical conversion, which implies the incoherent tunneling regime, the time dependence of the reactant and product concentrations is detected in one way or another. From these kinetic data the rate constant is inferred. An example of such a case is the important in biology tautomerization of free-base porphyrines (H2P) and phtalocyanins (H2PC), involving transfer of two hydrogen atoms between equivalent positions in the square formed by four N atoms inside a planar 16-member heterocycle (fig. 42). 

Chemical equivalence atomic nuclei in the same chemical environment are chemically equivalent and thus show the same ehemieal shift. The 2,2 - and 3,3 -protons of a 1,4-disubstituted benzene ring, for example, are ehemically equivalent because of molecular symmetry. 

Inspection of the citrate structure shows a total of four chemically equivalent hydrogens, but only one of these—the pro-/J H atom of the pro-i arm of citrate—is abstracted by aeonitase, which is quite stereospecific. Formation of the double bond of aconitate following proton abstraction requires departure of hydroxide ion from the C-3 position. Hydroxide is a relatively poor leaving group, and its departure is facilitated in the aeonitase reaction by coordination with an iron atom in an iron-sulfur cluster. 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

The equivalent weight of an ion (or an element) is the ratio of its formula weight to its valence. According to an alternative definition that is also suitable for compounds, an equivalent weight represents the amount of a substance which will react with one atomic weight of hydrogen or its chemical equivalent. 

Structure searching is the chemical equivalent of graph isomorphism, that is, the matching of one graph against another to determine whether they are identical. This can be carried out very rapidly if a unique structure representation is available, because a character-by-character match will then suffice to compare two structures for identity. However, connection tables are not necessarily unique, because very many different tables can be created for the same molecule depending upon the way in which the atoms in the molecule are numbered. Specifically, for a molecule containing N atoms, there are N ... 

This geometry is undoubtedly imposed by the skeletal stabilizing unit. Other eight membered phosphorus(III)-nitrogen rings, (MePNMe)i, (9) and (PrNO G NP) (11) are crown shaped containing all chemically equivalent phosphorus atoms. In XI, because the phosphorus atoms are of two sets and one set is in a highly protected environment, the possibility of selective, "cavitand," coordination at phosphorus sites exists. 

During the course of the nineteenth century, organic chemists developed a complex and integrated theory of chemical identity and constitution that in eluded theories of chemical atoms and equivalents, radicals, substitution, types, and valence, eventually unified in the so-called structure theory of constitutional formulas. But by midcentury, there was some discontent among chemists about the limitations of the overall theoretical framework within which they were working. 

In addition to atomism, the principal chemical theories of the nineteenth century included electrochemical dualism, the radical theory, the type theory, and the structure theory, the latter strongly identified with what chemists called the "law of linking" of carbon atoms. The valence theory evolved as a way of tying together the notions of chemical equivalence and chemical structure, and it carried along the old problem that some chemical elements (e.g., nitrogen) exhibit different combining values with another element in different circumstances. 

Lespieau s view was that organic chemistry must make use of physical methods and of physical chemistry and that the experimental work of Graham, Williamson, Wurtz, and, more recently, Raoult had confirmed the superiority of the atomic hypothesis over chemical equivalents. As for the meaning of the constitutional (structural, developed) formula,... 

Rocke, Alan J. "Atoms and Equivalents The Early Development of the Chemical Atomic Theory." HSPS 9 (1978) 225263. 

The rule on beta decay is new. It is that, when an atom emits a beta particle, which was known to be an electron, the resulting element is chemically equivalent to an element one column to the right from the parent in the periodic table. Consistent with rule 1, it is assumed that there is no change in atomic weight from this decay. 

Act = 226.5 the naming of the parents follows the nomenclature employed by Fajans in which the element uranium is referred to as UrI. A place in the periodic table is then defined by a specific row and a specific column. From the data, one then readily obtains the row and column of each of the elemental daughters. Since the characterization of decay products emphasized the fact that many of these products are chemically indistinguishable from one another, it is of course expected that many of the places in the last two rows of the periodic table are occupied by more than one elementary material. What is surprising is that these chemically equivalent materials have different atomic weights, some differing by as much as eight units. 

N-methylation of aniline is an interesting reaction from the products selectivity point of view, especially N-methylaniline (NMA), due to the presence of chemically equivalent protons attached to the nitrogen atom. The high nucleophilicity of the amine nitrogen results in the formation of mixture of N-methyl and N, N-dimethylated products. Ko et al [90]... 

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