Isomerization positional isomers

Substitution of two or more hydrogens in benzene results in positional isomerism. Positional isomers have the same formula, but different physical and chemical properties. In disubstituted benzene, three positional isomers are possible, as shown above. If the substituents are the same or different, one is always listed in the 1-position. If the substituents are different, they are listed alphabetically. 

Two compounds can have different structures because of a differing arrangement of the some groups in the positional isomerism (e.g. midine vs. pseudouridine). Compounds with the same molecular formula but with different functional groups are structural isomers (e.g. D-glucose vs. D-fructose) in structural isomerism. Positional isomers and structural isomers have different chemical and physical properties because of the different arrangement of the atoms. These two types of isomers (i.e. positional isomers and structural isomers), which differ in the manner in which atoms are connected or bonded together, are also called constitutional isomers. 

Polyhydric phenols with more than two hydroxy groups (ie, the three positional isomers of benzenetriol, the three isomeric benzenetetrols, benzenepentol [4270-96-6] and benzenehexol [608-80-0]) are discussed in this article. The benzenediols are catechol, resorcinol, and hydroquinone (see Hydroquinone, RESORCINOL, AND CATECHOL). 

Diesters of Tetracarboxylic Acids with Diamines. Dianhydrides react with alcohols at a moderate temperature, producing the tetracarboxyhc acid diesters. Ben2ophenonetetracarboxyhc acid diester (BTTA diester) is obtained from B I DA as a mixture of three positional isomers. Partiy because it is an isomeric mixture, the diester of BTTA is soluble in methanol at high concentrations. A state-of-the-art thermoset polyimide,... 

The HLB numbers decrease with increasing chain length, e.g., from 13.25 for sodium decane 1-sulfonate to 9.45 for the C18 homolog [72]. Typical HLB numbers for positional isomers range from 12.3 for sodium dodecane 1-sulfonate to 13.2 for the more hydrophilic 6 isomer [73]. The HLB numbers of alkanesulfonates are less influenced by the isomeric position of the functional group and by substituents than the cM values [68]. HLB numbers can be correlated with partition coefficients for the distribution of a surfactant between the aqueous and oily phases, which emphasizes that the partition coefficient is dependent on the carbon number [68]. 

H-NMR spectroscopy can be used to determine alkenesulfonates in mixtures [115]. Under normal conditions, 1-alkenesulfonate shows a signal separated from the other positional isomers [122]. Moreover, the utilization of a lanthanide shift reagent makes possible even the separation of the signals of isomeric alkenesulfonic acids and hydroxyalkanesulfonic acids as their methyl esters [124]. 13C-NMR spectroscopy, which is not as quantitative, simply gives the cis/trans ratio of the main positional isomer. 

Commercial LASs are complex mixtures of four individual LASs (C10-C13) with 20 possible positional isomers. Isomeric separation can be achieved by solvophobic association with SDS or host-guest interaction with cyclodextrins. Complete resolution of 19 isomers was achieved using 10 mM phosphate buffer (pH 6.8) containing 40 mM SDS and 30% acetonitrile [4]. LAS isomers in technical products were separated using a-cyclodextrin, but complete resolution of all isomers was not achieved [5]. 

Xylanolytic systems of yeasts and fungi can also be induced by positional isomers of xylobiose. Induction with Xylp-p-(l- 2)-Xyl/7 is analogous to the sophorose induction of cellulase in filamentous fungi (76). Xylp-p-(l- 2)-Xyl/7 and Xylp-p-(l- 3)-Xyl/7 induced xylanase in C. albidus (77), Trichosporon cutaneum (73), A. pullulans (78) and A. terreus (Hrmova, M. et al., Slovak Academy of Sciences, Bratislava, submitted for publication, 1990). In C. albidus the positional isomers behave differently than does xylobiose (77), in that the response of the cells to them was slower but the enzyme yields were higher than in the presence of xylobiose. This in-icated that isomeric xylobioses were not direct inducers. In agreement with this idea, both Xylp-p-(l- 2)-Xylp and Xylp-p-(l- 3)-Xylp were transformed to Xyl/7-p-(l- 4)-Xyl/7, the natural inducer (79). 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

For the study of positional isomers in disubstituted bullvalenes, an isomeric mixture of dibromobullvalenes 3a-c is treated with an excess of lithium fluoride in the presence of silver (I) fluoride in pyridine at 115 C in the dark.5 This combination of metal fluorides enabled... 

A second form of isomerism occurs with mixed metal polyanions, such as [PW8V404o]7, in which the relative positions of the W and V atoms may vary.56 That such positional isomers do exist has been amply demonstrated by NMR and ESR spectroscopy.57 The numbers of possible isomers increase with the lowering of skeletal symmetry, e.g. five for ar-[PWioV204o]5-, fourteen for /3-[SiMo2Wi0O4ol4. ... 

In the Phillips neohexene process147 2,4,4-trimethyl-2-pentene (8) is converted by cleavage with ethylene to neohexene (9) used in the production of a perfume musk. The starting material is commercial diisobutylene. Since it is a mixture of positional isomers (2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-l-pentene) and the latter (7) participates in degenerative metathesis, effective utilization of the process requires the isomerization of 7 into 8. A bifunctional catalyst system consisting of an isomerization catalyst (MgO) and a heterogeneous metathesis catalyst is employed 131... 

Hay and Morrison (1970) identified the monoenoic positional and geometric isomers in milk fat and determined the amounts of each total acid class and percentage of trans isomers. The geometric and positional isomers of the monoenes are primarily the result of biohydrogenation of polyunsaturated fatty acids in the rumen. Stearate is also produced, and cis-9-18 l accounts for most of the monoenes. The several positional isomers in trans 16 1 and 18 1 are due to the positional isomerization of double bonds which accompanies elaidinization. 

Onions contain an amino acid that is a positional isomer of alliin. When acted upon by alliinase it produces 2-propene sulfenic acid, which isomerizes to the lachrima-tory factor that brings tears to the eyes of onion cutters.c This, too, decomposes to form many other compounds.3... 

When there are two or more substituents on a benzene ring, structural isomerization can result. For instance, there are three possible isomers of xylene (dimethylbenzene). The isomers are commonly designated as ortho, meta, and para. A letter (o, m, or p) designates the isomeric position. 

When w.//-perfluoroalk-l-enes longer than butene [CHF2(CF2)nCF = CF2, n = 2-7] are isomerized under conditions of thermodynamic control, however, mixtures of all possible positional isomers are obtained. Isomerization of these aik-1 -enes with antimony(V) fluoride under conditions of kinetic control yields the corresponding alk-2-enes as an EjZ mixture.20... 

Two molecules share an isomeric relationship if they have the same molecular formula. All molecules with the same molecular formula constitute a set of structural isomers and are to some degree similar. However, they may have different chemical constitutions, as indicated in Figure 1.2.1 for 1-butanol and five structural isomers. Any two of these molecules placed in the same row make a pair of constitutional isomers. For the purpose of property estimation, it is helpful to further classify the constitutional isomers according to type and position of the functional groups and branching of the isomers. In the dicussion that follows, we focus on two different types of isomeric sets positional isomers and branched isomers. 

CH2] and 1,3-pentadiene (CH2=CH—CH=CH-CH3), two different positional isomers can be formed instead. Isoprene can give two isomeric -allylic groups ... 

We may divide the large number of known cases (and the even greater number of hypothetical cases) of inorganic isomerism into two very broad classes (1) position isomers having the same sets of atoms but different sets of bonds (as is the ease with ethyl alcohol and dimethyl ether) and (2) stereoisomers having the same atoms, the same sets of. bonds, but differing somehow in the orientation of these bonds. Werner and many workers after him recognized a number of subclasses of position isomerisms. 

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