Isomer, structural

Strategy Start with ethylene and see how many isomers, structural or geometric, you can make by substituting two chlorine atoms for hydrogen. 

Chloro-3-(5-chloropyrazol-3-yl)quinoxaline (137, R = H) gave mainly 2-(2-acetyl-5-chloropyrazol-3-yl)-3-chloroquinoxaline (137, R = Ac) [neat Ac20, reflux, 7 min, then 20°C, 30 min 86% (after separation from a little of the 1-acetyl isomer) structure confirmed by X-ray analysis] or mainly 2-(l-acetyl-5-chloropyrazol-3-yl)-3-chloroquinoxaline [neat Ac20, 80°C, 2 min ( ), then 20°C, 12 h 70% (after separation from the 2-acetyl isomer)] an extraordinary pair of regioselective reactions.495... 

We have concentrated on the proton-proton, homonuclear NOE experiments in this section but the potential use of analogous heteronuclear experiments should not be overlooked, if you have the appropriate hardware available to you. The 19F- 1H NOE experiment, for example, can be very useful in certain situations as demonstrated in the following example. You have one of two possible positional isomers (Structure 8.6). 

Structures I and II are identical. Hence, the facial (fac) geometrical isomer has no optical isomer. Structures III and IV are also identical. So, the meridianal (mer) geometrical isomer also has no optical isomer. The total number of isomers is 2 for [Cr(NH3)3Cl3]. 

Structures I and II are mirror images that are not superimposable hence the cis geometrical isomer consists of a pair of optical isomers. Structures III and IV are identical the trans geometrical isomer has no optical isomer. Therefore, [Pt(en)2Cl2]Cl2 has a total of 3 isomers. 

The photosensitivity tests were carried out only for the polyamides containing different isomers of thymine photodimers to determine the effect of isomer structure on photosensitivity. 

Fig. 2.6.14. MS-MS fragmentation patterns of some CAEC (NI mode) (a) CA9PE2C, (b) CA5PE2C, and (c) CA5PE1C. The exact branching of the alkyl chain is unknown the alkyl isomer structures shown are chosen arbitrarily. Reprinted with permission from Ref. [33] 2001 American Chemical Society.

An abreviated report from the analysis of the mixture of Aroclor standards just described is shown in Table I. The report indicates the individual isomer structure and degree of chlorination, ortho-, ortho- substitution, isomer concentration, total concentration. 

Table I. Isomer Structure Assignment, Response Factors, and Concentration in 1 1 1 1 Mixture of Aroclor 1242 1248 1254 and 1260.

Analogously, l,2-dicyano-l,2-diphenylethylene, which is free from steric strains, and its strained isomer l,l-dicyano-2,2-diphenylethylene are reduced at practically the same potentials (Ioffe et al. 1971, Todres and Bespalov 1972). In DMF, with the support of Et4NI, the reversible two-step one-electron reductions are characterized by the following potentials (mercury pool as a reference electrode) -0.48 and -0.98 V for 1,2-dicyanoethylene and -0.50 and -1.07 V for 1,1-dicyano isomer. Thus, electrochemical reduction does not fix the difference in isomer structures. 

Farnesene 69 is a constituent of hops oil and many other oils. a-Farnesene 70 is the structural isomer. Structural representations of a-farnesene and -farne-sene are illustrated in Structure 4.17. 

Substances with the same elementary chemical composition but with the elements occupying different positions in space are called isomers. Structural isomers are compounds with the same numbers of chemical... 

For nonquantitative -C-NMR techniques, this could be the extent of the analysis possible. However, the combination of F-NMR with C-NMR allowed us to quantitatively calculate the isomer composition and to investigate solvent effects on isomer formation. Figure 17.5 illustrates these concepts. Two possible isomers (structures in Figure 17.5) can be formed from the reaction of 3-fluorophthalic anhydride with 4-fluoroaniline. Upon formation of the amic acid based on 3-fluorophthalic anhydride with 4-fluoroaniline, two isomers were found in both NMP and chloroform reactions as shown by the F-NMR spectra in Figure 17.5a and b, respectively. Two signals were observed for each type of fluorine atom, labeled as Fi and F2 for the anhydride and amine fluorine atoms respectively. Ortho and meta isomers were formed in a ratio of 4.75 1 in solution in NMP, while the same ratio was 1.04 1 in chloroform, where the product precipitated. The major isomer was the ortho in each case as determined by C-NMR of the chloroform prepared amic acid (Table 17.1). 

Molecules such as w-pentane, mo-pentane, and eo-pentane, which have the same molecular formula but different structures, are known as structural isomers. Structural isomers have different physical and chemical properties. For example, -pentane has a boiling point of 36°C, wo-pentanes boiling point is 30°C, and < o-pentanc s is 10°C. 

The 300 MHz H NMR and 20 MHz 13C NMR spectra of poly(4-methyl-l-pentene) have been found to be more complex than the corresponding spectra of poly(3-methyl-l-butene) due to the presence of an additional isomer structure in the polymer. Investigation of the 20 MHz 13C NMR spectrum of the polymer has indicated that placement of units in different triad sequences is die cause of multiple methyl proton resonances which have been observed in the H NMR spectra of poly(3-methyl-l-butene) and poly(4-methyl-l-pentene). The use of a computer program for simulating and plotting spectra has enabled measurements of polymer composition to be made of poly(4-methyl-l-pentene) s prepared under a variety of synthesis conditions. 

Vinylfurans are again predicted as planar, with one vinylic CH bond eclipsing the furan 2,3-bond. For the 2-isomer this gives structure (50)., and for the 3-isomer, structure (51). The energy difference is again small (2.2 kJ mol f) and so are the rotational barriers... 

Isomer Structure Bolling point, °F Melting point, °F Specific gravity, 60760°... 

Structure- Property Relationships for Isomers Structure-property relationships for isomers may indicate an increase or decrease in properties as a function of... 

Unlike oxazoles, benzoxazoles do not undergo Diels-Alder reactions with DMAD. Benzoxazole itself, without solvent, gives pyrido[2,l-6]-benzoxazole (15), the earlier477 5a//-isomer structure being discarded 478 because of the 13C NMR spectrum of the compound. 2-Methylbenzox-azole in acetonitrile gave on one occasion479 the azepine 16 and, on another,480 a compound that is almost certainly cyclobuta[4,5]pyrrolo-[2,1 -6]benzoxazole (17).337,481... 

In recent years h.p.l.c. has become a valuable chromatographic tool for analytical and preparative scale work. In this latter area the separation of isomers (structural, diastereoisomeric, and enantiomeric) has been possible by the selection of appropriate column packing material and solvent systems. However, the equipment, operating costs, and column packing materials are more expensive than those in t.l.c., g.l.c. and conventional liquid-solid column chromatography. 

The simplest form of isomer is a structural isomer. Structural isomers have the same molecular formula but different bond-to-bond connectivity. 

D is correct. As per the passage, only compounds C and D are stereoisomers (thus answers A and B are wrong). But all the compounds are dichlorobutane, so any two must be structural isomers. Structural isomerism is the same as constitutional isomerism. 

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