The Isomer Problem

The isomer problem, simply stated, is to find how many distinct fullerenes could be assembled from n carbon atoms. 

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A second consideration is apparent for tetrahydrofuranyl-18-crown-6 (Figure 3). In this case, the oxygen is jp -hybridized, so binding of an alkali metal cation by it or 18-crown-6 (see above) should not differ much on that account. The drawing of tetrahydrofuranyl-18-crown-6 does not, however, illustrate the isomer problem. The macroring is attached to tetrahydrofuran at the 2- and 5-positions but the stereochemistry could be cis or trans. These two possibilities are identical neither to each other nor to 18-crown-6. In many such cases, stereochemistry is not established and the details of differences, if any, in cation binding are uncertain. 

Ecological Aspects of the Catalyst Change. The decreasing demand for ortho-chlorotoluene has limited the possibilities for utilizing the excess material. The effect on cost that would result from incineration of orf/io-chlorotoluene had to be assessed. With this background, the development of new benzothiazepine cocatalysts relieved the isomer problem at just the right time. The isomer ratio has now been moved in the desired direction and adapted to the demand pattern. No incineration of an undesirable product is necessary. 

This section illustrates one procedure for working out an isomer problem, but any rational technique will work. The important thing is to devise some systematic way of searching. We happen to like the one described above in which one first finds all the structural isomers (in this case 1,1- and 1,2-dichlorocyclopropane), then searches for stereoisomerism of the cis/trans type, and finally examines each isomer for chirality. But it doesn t matter what procedure you use as long as you are systematic. What will not work is a nonrational scheme. So don t start a find the isomers problem by just writing out compounds without thinking first. No one can do it that way. 

Molecular formulas of organic compounds are customarily presented in the fashion C2H5Br02 The number of carbon and hydrogen atoms are presented first followed by the other atoms in alphabetical order Give the molecular formulas corresponding to each of the compounds in the preceding problem Are any of them isomers ... 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

Acetic anhydride and acetic acid increase the solubiUty of the two phases in each other, and they are employed for the commercial N-nitration of hexamethylenetetramine [100-97-0] (11) to form cyclotrimethylenetrinitramine [121-82-4] (RDX), (CH2)3(NN02)3. Renewed consideration has been given to replacing H2SO4 with an improved soHd catalyst to reduce the environmental problems of disposal or reconcentration of the waste acid and to increase production of desired nitrated isomers. For example, a catalyst with suitable pore size might increase the production of 4-MNT and reduce that of 3-MNT when toluene is nitrated. 

Chirahty at the phosphoms is an unavoidable problem in all phosphorothioate syntheses. The phosphoramidite method produces a mixture of both the and the diastereomers having a small excess of the isomer (53). Although some progress has been made in the chiral synthesis of dinucleoside phosphorothioates, low yields have limited the utility of these approaches. The chiral center may be eliminated by replacing the other, nonbridging oxygen with sulfur. Avoidance of the chirahty problem is one reason for the interest in phosphorodithioates. 

Preferably, high pressure Hquid chromatography (hplc) is used to separate the active pre- and cis-isomers of vitamin D from other isomers and allows their analysis by comparison with the chromatograph of a sample of pure reference i j -vitainin D, which is equiUbrated to a mixture of pre- and cis-isomers (82,84,85). This method is more sensitive and provides information on isomer distribution as well as the active pre- and cis-isomer content of a vitamin D sample. It is appHcable to most forms of vitamin D, including the more dilute formulations, ie, multivitamin preparations containing at least 1 lU/g (AOAC Methods 979.24 980.26 981.17 982.29 985.27) (82). The practical problem of isolation of the vitamin material from interfering and extraneous components is the limiting factor in the assay of low level formulations. 

The second problem concerns isomers of these various systems. As illustrated, 2d might be presumed to be planar. Unlike nitrogen, the inversion at phosphorus is very slow and there are, in fact, five possible isomers of it (three meso and two dl pairs). The interconversion of these isomers is reported in a subsequent paper by Davis, Hudson and Kyba along with x-ray crystallographic data on the structure of the so-called isomer B . 

AMI does not even get the correct sign for the energy difference. PM3 orders the isomers properly, but substantially underestimates the energy difference, and the Hartree-Fock value is only somewhat better. Electron correlation is needed to obtain even semi-quantitative results for this problem. ... 

These structural problems are also insoluble by physical methods alone. The infrared spectrum often gives an unambiguous decision about the structure in the solid state the characteristic bands of the carbonyl or the hydroxyl group decided whether the compound in question is a carbinolamine or an amino-aldehyde. However, tautomeric equilibria occur only in solution or in the liquid or gaseous states. Neither infrared nor ultraviolet spectroscopy are sufficiently sensitive to investigate equilibria in which the concentration of one of the isomers is very small but is still not negligible with respect to the chemical reaction. 

In the realm of chemical enumeration we note Polya s equation (4.4) which gives the generating function for stereoisomers of the alkyl radicals, or equivalently, alcohols — that is, equation (5.2) of this article. His equation (4.3) gives the corresponding result for the structural isomers of these compounds. His equations (4.2) and (4.5) correspond, respectively, to the cases of alcohols without any asymmetric carbon atoms and the number of embeddings in the plane of structural formulae for alcohols in general. The latter problem is not chemically very significant. 

Assume that you have carried out a radical chlorination reaction on (P)-2-chloro-pentane and have isolated (in low yield) 2,4-dichIoropentane. How many stereoisomers of the product are formed and in what ratio Are any of the isomers optically active (See Problem 10.24.)... 

Dimethyl butynedioate also undergoes a Diels-Alder reaction with (2 ,4Z)-hexadiene, but the stereochemistry of the product is different from that of the (2 ,4 ) isomer (Problem 14.55). Explain. 

Problem 20,16 How could you distinguish between the isomers cyclopentanecarboxylie acid and 4-hydroxycyclohexanone by and l3C NMR spectroscopy (See Problem 20.14.)... 

Although 2-substituted 2-cyclopentenones are in a base-catalyzed equilibrium with their 5-substituted 2-cyclopentenone isomers (Problem 22.38). the analogous isomerization is not observed for 2-subslitutcd 2-cyclohexenones. Explain. 

The structural homology between intermediate 4 and isostrych-nine I (3) is obvious intermediates 3 and 4 are simply allylic isomers and the synthetic problem is now reduced to isomerizing the latter substance into the former. Treatment of 4 with hydrogen bromide in acetic acid at 120°C results in the formation of a mixture of isomeric allylic bromides which is subsequently transformed into isostrychnine I (3) with boiling aqueous sulfuric acid. Following precedent established in 194810 and through the processes outlined in Scheme 8a, isostrychnine I (3) is converted smoothly to strychnine (1) upon treatment with potassium hydroxide in ethanol. Woodward s landmark total synthesis of strychnine (1) is now complete. 

Basic Explosives Manufacture. The major quantities and the toughest problems are here. They include a) acid waters, treated with lime or soda ash, chemical washes, spills, washdowns b) Red Water from TNT purification. A complex, brick-red soln of Na nitrate, Na sulfate, Na sulfite, Na nitrite, and about 17% organics which include sulfonated nitrotoluene isomers and complex, unidentified dye-bodies c) dissolved expls, eg, Pink Water which is approx lOOppm TNT in w d) suspended expl particles — dust and chips and e) sometimes solvents such as acet, benz, and dimethyl aniline... 

Unfortunately the double bond in (24) is not conjugated with the carbonyl group so that the 2+2 cyclo-addition does not give a good yield. It is also un-symmetrical so that a mixture of adducts is formed. Attempts to solve the first of these problems by using (25) or (26) made the second problem worse as the wrong adduct, e.g. (27) is the major isomer. 

For the chromatography of complex mixtures (separation of isomers or closely related compounds with similar retention), the main problem is to improve the... 

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