Isomers and isomerization

DavRSl Davidson, R. A. Isomers and isomerization Elements of Redfield s combinatorial theory. J. Am. Chem. Soc. 103 (1981) 312-314. 

LI0E85 Lloyd, E. K. Redfield s papers and their relevance to counting isomers and isomerizations. Unpublished article 1985. To appear in Discrete Applied Mathematics. 

C5Me5) was stable up to room temperature (108,109). A surprising feature of the matrix photochemistry (6,7) was that only the trans isomer of [CpFe(CO)2]2 was photolyzed, and it may be that this was the consequence of concerted CO loss from the trans isomer and isomerization to form the triply bridged product (6). 

Isomers are compounds with the same molecular formula but different structural formulas. Some organic and biochemical compounds may exist in different isomeric forms, and these different isomers have different properties. The two most common types of isomers in organic systems are cis-trans isomers and isomerism due to the presence of a chiral Ccirbon. 

The various heterocyclic ligands just described are easily cleaved from the respective molybdenum centers either by reaction with sulfur or through ligand exchange with l,2-bis(diphenylphosphino)ethane (Scheme 17).44a The Diels-Alder cycloaddition is successfully expanded to transient 1-phospha-1,3-diene complexes such as 25 and suitable dienophiles. As illustrated in Scheme 12, the isomeric 1,2-dihydrophosphetes (e.g., 26) are their masked stable precursors. Reaction of 26 with dimethyl acetylenedicarboxy-late or with benzaldehyde furnishes the Diels-Alder adducts of transient 25, compound 71, as a mixture of two isomers, and isomerically pure 72.44b (See Scheme 18.)... 

Like alkenes (Section 6.1.1), chromophores containing the N=N (azo compounds) or C=N (imines, oximes, etc.) bonds can undergo E Z (or trans cis) photoisomerization (Scheme 6.156) and the resulting isomer concentration ratio in the photostationary state (PSS) reflects the absorption properties of the isomers and isomerization quantum yields (see Scheme 6.1 in Section 6.1.1). Since conventional (dark) synthesis generally provides access to more stable E-isomers, photochemistry is an exceptional tool for preparing sterically hindered Z-isomers.1061,1062 The photoisomerization reaction can be induced by a direct irradiation or by sensitization and it often competes with other phototransformations, such as photofragmentation or photorearrangement (Section 6.4.2). 

Relative stabilities of isomers and isomerization barriers with respect to formaldehyde from experiment and several levels of theory. All values are in kcal/mol. 

Table 1. H cliemical shifts (ppm) for the aW-iraiis isomer and isomerization shifts for cis isomers in benzene-d(, at 281K... 

A nuclide is an atomic species as determined by its atomic number (proton number) Z and mass number (nucleon number) A = Z+N, where N is the number of neutrons in its nucleus. Atomic species with the same nuclear composition but different nuclear energy states with measurable lifetime are considered independent nuclides in their own right. Nuclides can be classified in different ways. Nuclides having the same atomic number Z (but different mass number A) represent the same chemical element and are called the isotopes of that element. Nuclides with the same mass number A (but different atomic number Z) are called isobars. Nuclides of the same number of neutrons N (but different atomic number Z) are called isotones. Nuclides of the same nuclear composition but different nuclear states are referred to as (nuclear) isomers. The terms isotope, isotopic, isobar, isobaric, isotone, isotonic, isomer, and isomeric can also be applied to nuclei, but the terms nuclide and nuclidic can only be applied to atoms. 

The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

Only relatively few examples of interesting target molecules containing rings are known. These include caryophyllene (E.J. Corey, 1963 A, 1964) and cubane (J.C. Barborak, 1966). The photochemical [2 + 2]-cycloaddition applied by Corey yielded mainly the /ranr-fused isomer, but isomerization with base leads via enolate to formation of the more stable civ-fused ring system. 

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

In this section we shall consider three types of isomerism which are encountered in polymers. These are positional isomerism, stereo isomerism, and geometrical isomerism. We shall focus attention on synthetic polymers and shall, for the most part, be concerned with these types of isomerism occurring singly, rather than in combination. The synthetic and analytical aspects of stereo isomerism will be considered in Chap. 7. Our present concern is merely to introduce the possibilities of these isomers and some of the vocabulary associated with them. 

Extraction of C-8 Aromatics. The Japan Gas Chemical Co. developed an extraction process for the separation of -xylene [106-42-3] from its isomers using HF—BF as an extraction solvent and isomerization catalyst (235). The highly reactive solvent imposes its own restrictions but this approach is claimed to be economically superior to mote conventional separation processes (see Xylenes and ethylbenzene). 

Propylene glycol, dipropylene glycol, and tripropylene glycol all have several isomeric forms. Propylene glycol has one asymmetric carbon and thus there are two enantiomers (R)-I,2-propanediol and (3)-1,2-propanediol. 1,3-Propanediol is a stmctural isomer. Dipropylene glycol exists in three stmctural forms and since each stmctural isomer has two asymmetric carbons there are four possible stereochemical isomers per stmcture or a total of twelve isomers. These twelve consist of four enantiomer pairs and two meso- compounds. Tripropylene glycol has four stmctural isomers and each stmctural isomer has... 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Ethyltoluene is manufactured by aluminum chloride-cataly2ed alkylation similar to that used for ethylbenzene production. All three isomers are formed. A typical analysis of the reactor effluent is shown in Table 9. After the unconverted toluene and light by-products are removed, the mixture of ethyltoluene isomers and polyethyltoluenes is fractionated to recover the meta and para isomers (bp 161.3 and 162.0°C, respectively) as the overhead product, which typically contains 0.2% or less ortho isomer (bp 165.1°C). This isomer separation is difficult but essential because (9-ethyltoluene undergoes ring closure to form indan and indene in the subsequent dehydrogenation process. These compounds are even more difficult to remove from vinyltoluene, and their presence in the monomer results in inferior polymers. The o-ethyltoluene and polyethyltoluenes are recovered and recycled to the reactor for isomerization and transalkylation to produce more ethyltoluenes. Fina uses a zeoHte-catalyzed vapor-phase alkylation process to produce ethyltoluenes. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

Refining and Isomerization. Whatever chlorination process is used, the cmde product is separated by distillation. In successive steps, residual butadiene is stripped for recycle, impurities boiling between butadiene (—5° C) and 3,4-dichloto-l-butene [760-23-6] (123°C) are separated and discarded, the 3,4 isomer is produced, and 1,4 isomers (140—150°C) are separated from higher boiling by-products. Distillation is typically carried out continuously at reduced pressure in corrosion-resistant columns. Ferrous materials are avoided because of catalytic effects of dissolved metal as well as unacceptable corrosion rates. Nickel is satisfactory as long as the process streams are kept extremely dry. 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples ofp-nitrophenol. Usually, the ri-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pK values (see Chapter 1). The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers... 

Although -cyclohexene has been postulated as a reactive intermediate, it has not been observed directly. MO calculations at the 6-3IG level predict it to be 56 kcal/mol less stable than the Z-isomer and yield a value for the barrier to isomerization of about 15 kcal/mol. ... 

The 7a-bromo-5a-6-ketone (56) is conveniently prepared from a mixture of the 5a- and 5j5-6-ketones (55) under equilibrating conditions. It is formed from the 5a-isomer (55) via the 5a-bromo-compound, and from the 5j -isomer (55) via the 7a-bromo-5i -6-ketone (see section II-A). Dehydrobromination is effected in DMF, and chromatography of the crude product separates the title compound (58) from remaining starting material and isomeric A -6-ketone (57). ... 

Dehydrotestosterone acetate (174) in nonprotic solvents (dioxane, benzene) undergoes a rearrangement to the isomer (175). This product is photolabile and isomerizes readily to new cross-conjugated dienones. Thus, ultraviolet irradiation of (174), its 1-, 2- and 4-methyl homologs, and its lOa-stereoisomer (188) in dioxane solution causes, in each case, a series of rearrangements as summarized on page 331 for (174) and (188). ... 

Compound (223), the 6a,7a-methano homolog of dienone (174), is extraordinarily stable on irradiation in dioxane, while the 6, 7 -isomer (224) isomerizes readily to the bicyclohexenone (225). The apparent failure of (223) to react may be attributed to the high steric strain which would be introduced by the two adjacent three-membered rings on ring B upon 1,5-bridging. This could cause reversal of primary product formation [cf. (226)] rather than ensuing formation of the corresponding zwitterion and... 

Photochemical chlorination of pentane gave a mixture of three isomeric monochlorides. The principal monochloride constituted 46% of the total, and the remaining 54% was approximately a 1 1 mixture of the other two isomers. Write structural formulas for the three monochloride isomers and specify which one was formed in greatest amount. (Recall that a secondary hydrogen is abstracted three times faster by a chlorine atom than a primary hydrogen.)... 

Quinoline,like pyridine, and the ester give a 1 2 molar labile adduct (117) which is not basic and isomerizes on heating or with acids to a stable isomer (118) as in the pyridine series. It has been... 

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