Isomerization phenomena

The reaction products were first hydrogenated to saturate small amounts of olefins, and then split up into fractions by distillation in vacuo. 

The application of several methods for structural analysis of mineral oils is, in general, limited to those fractions in which no structural elements are present in larger quantities than normally occur in mineral oil fractions. In highly aromatic concentrates, for instance, the normal analytical methods (n-d-M v-n-d) may give inaccurate results, because different types of aromatics may influence the physical constants of the oil differently. 

The development of newer techniques (chromatography, thermodiffusion) for the separation of the different groups of hydrocarbons from mineral oil fractions allows a better characterization of such type-concentrates with the aid of physical constants. Combination of physical methods of separation with the statistical analysis of the products obtained, may lead to a more detailed and more complete knowledge of the composition of oils. 

By a simple percolation through silicagel a sharp separation into non-aromatic and aromatic compounds is achieved. By the same percolation approximate separations into mono-, di- and tri-aromatics have been obtained39 40. The result is a semi-quan- 

BRANCHING ANALYSIS OF HYDROGENATED PLATFORMING PRODUCTS OBTAINED BY HYDROISOMERIZATION OF PARAFFIN WAX 

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Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

Also, the coordination polymerisation of 2,4-hexadiene and 1,4-deuterated 1,3-butadienes is of particular interest owing to isomerism phenomena shown by their polymers. 2,4-Hexadiene and other monomers of the CHR=CH CH=CHR type exist in three isomeric forms - (E,E), (Z,Z) and (E,Z) ... 

The pr e-Woodwardhn era largely concerned itself with the collection and classification of synthetic tools chemical reactions suited to broad application to the constitutional construction of molecular skeletons (including Kiliani s chain-extension of aldoses, reactions of the aldol type, and cycloadditions of the Diels-Alder type). The pre- Woodwardian era is dominated by two synthetic chemists Emil Fischer and Robert Robinson. Emil Fischer was emphasizing the importance of synthetic chemistry in biology as early as 1907 [30]. He was probably the first to make productive use of the three-dimensional structures of organic molecules, in the interpretation of isomerism phenomena in carbohydrates with the aid of the Van t Hoff and Le Bel tetrahedron model (cf. family tree of aldoses in Scheme 1-6), and in the explanation of the action of an enzyme on a substrate, which assumes that the complementarily fitting surfaces of the mutually dependent partners are noncovalently bound for a little while to one another (shape complementarity) [31],... 

Z. Slanina, Theoretic Aspects of the Isomerism Phenomenon in Chemistry (in Russian). Mir, Moscow, 1984. 

It is pertinent here to briefly review those aspects of the recent experimental investigations which bear directly on the possibility of a formal quantum-mechanical treatment of the isomerization phenomenon.175-188... 

One desirable property of a photoactivated system is the possibility to switch it on and off in a reversible manner. In this case, full control over the initiation and termination of the reaction may be achieved. Lemcoff et al. [44, 45] further expanded the S-chelated ruthenium benzylidene series by attaching aromatic groups to the sulfur atom. This change led to a pronounced photo-isomerization phenomenon (Figure 12.18). Complexes 46 were tested in PROMP, and 46b and 46e gave the best results. In a typical procedure, 46b and 46e were mixed with COD, COE, or endo, e o-NBE dimethylester monomers in dichloromethane... 

Anhydrous ammonia adds tofluorooleftns to produce nitriles Tins phenomenon IS used to characterize chemically the terminal difluoromethylene olefin that IS claimed to be m equilibrium with the internal isomer [4] (equation 2) Thus isomerization to the terminally unsaturated isomer prior to attack by ammonia yields the cyanoenamine... 

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

In 1882 Griess discovered that in aqueous acidic solution 4-diazobenzenesulfonate and 4-methylaniline react quantitatively to yield 4-toluenediazonium ion and 4-ami-nobenzenesulfonic acid. This phenomenon is called diazo migration or diazo exchange. We now know that it is a consequence of the tautomerism of the initially formed l-(4 -methylphenyl)-3-(4 -sulfophenyl)-triazene, as discussed above. Griess also found another reaction that could not be explained at his time, but which is based on the tautomerism of intermediate triazenes occasionally, the reaction of an arenediazonium salt with a primary aromatic amine in weakly acidic solution yields a mixture of two isomeric aminoazo compounds (Scheme 13-21). 

The oldest and best known structure is that proposed by Kekule (I). The objections to it are twofold. First, it suggests that ortho-disubstituted derivatives should exist in two isomeric forms—a phenomenon which has never been observed. Kekule avoided this difficulty, however, by assuming that the double bonds were in a state of constant oscillation such that any two adjacent carbon atoms were connected part of the time by a single bond and part of the time by a double bond. For example ... 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

The substituted Bfx and Fx exhibit the isomerization or, well known, ring-chain tautomerism equihbrium depicted in Scheme 3. This phenomenon... 

When 2-norbornyl type radicals are generated from exo/endo isomeric precursors differences in rate are generally observed. The higher rate of decomposition of the exo-isomer is usually explained on steric grounds12 18). This phenomenon is demon-strated by the following examples ... 

Literally thousands of isomers can exist. Even relatively simple molecules can have many isomers. Thus, the phenomenon of isomerism accounts in part for the enormous number and variety of compounds of carbon,... 

As a further illustration of the phenomenon of H-bond resonance coupling let us consider the intramolecular H-bond of (3-hydroxyacrolein (0=CHCH=CH0H), a prototypical enolone (2-en-3-ol-l-one, or enol isomer of (3-diketone).55 This molecule may be envisioned as existing in two distinct isomeric forms, according to the position of the proton in the O- H—O hydrogen bond ... 

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