Liquid isomers

We are being somewhat disingenuous in that we are taking the difference here of the enthalpy of formation of the naturally liquid (Z)-isomer and of the liquid (/ )-isomer obtained by summing the value for the solid and the ca 27 kJmol-1 enthalpy of fusion (at 398 K) from Domalski. We know of no measurement of the enthalpy of fusion for (Z)-stilbene at any temperature from which to derive an enthalpy of formation for the solid. 

It may be seen that a very high separation factor of organic liquid isomers through polymer membranes has been obtained for PrOH isomers [84],... 

Only in a few cases are the acyclic dienones stable enough to afford isolable products. Thus on treating 2,4,6-trialkylpyrylium salts with aqueous alkali cyanides, a mixture of two diastereomeric 5-cyano-2,4-dienones resulted. When the alkyls are methyl groups, the two liquid isomers are 2-Z,4-Z, 2,4-dimethyl-6-oxo-2,4-hep-tadienonitrile (5, 95%) and the 2-/ ,4-Z isomer (6, 5%). On diluting their solutions in concentrated hydrochloric acid, the corresponding crystalline diastereomers (7 and 8) are formed [60, 61]. 

All the enthalpies of formation of the gaseous dichlorobenzenes were obtained from our archive, as were the values for the liquid o- and m-isomer. The value for the liquid -isomer was found by summing the value for the solid and the enthalpy of fusion from W. E. Acree, Jr., Thermochim. Acta, 189, 37 (1991). 

Because it contains two saturated six-member rings, 4,4 -dicyclohexyl methane diisocyanate can exist in three isomeric forms, in which the orientations of the NCO groups with respect to the rings and each other are different. The trans-trans isomer is a solid at room temperature so that over time this isomer can precipitate from the remaining liquid isomers. Samples of 4,4 -dicyclohexyl methane diisocyanate that have been stored at cool temperatures for a while often have a solid layer of trans-trans precipitate in the bottom. 

We have evaluated several molecular sieves for the separation of 2,6-DMN from 2,7-DMN in the presence of liquid isomers present in a 257°-265 °C heart-cut of an aromatic extract of catalytic gas oil. This heart-cut contains ... 

Intuitively, one would predict that separation of a binary mixture should be simpler than separation of 2 components present in a complex mixture. However, this was not the case in our work. As shown in Table II, a much higher separation factor is obtained with a mixed stream than with a binary eutectic. Despite the dilution of 2,7-DMN present in adsorbate by liquid isomers, the weight of 2,7-DMN freed per 100 grams of sieve drops only slightly. This enhanced separation is illustrated further by the equilibrium diagrams for the binary eutectic (Figure 1) and for the heart-cut mixture (Figure 2). 

The product is again a mixture of cis- and trfl s-isomers tliat can be separated into the pure liquid isomers by preparative-scale glc. 

Naphthalene, obtained from a petroleum reformate, was chromato-graphically pure after recrystallization. The two monomethylnaph-thalenes were obtained by distillation from the same reformate. The 1-methylnaphthalene was a heart cut of 99-f% purity from a precise fractional distillation. 2-Methylnaphthalene from the same distillation was recrystallized to high purity. 1,3-Dimethylnaphthalene, 1,7-dimethylnaphthalene, and 1,8-dimethylnaphthalene were synthetic samples that were kindly donated by Dr. G. D. Johnson of Kansas State University and Dr. L. Friedman of Case Institute. The remaining isomers were obtained from commercial sources. They were treated with Raney nickel in methanol under reflux to destroy catalyst poisons such as sulfur compounds. The liquid isomers were further purified by preparative-scale gas chromatography. Octalins were prepared by reducing pure naphthalene with lithium in ethylamine (29). 

Racemic 2,3-butanediol was converted to the racemic ditosy-late, which was then treated with catechol under the influence of NaOH in dimethylformamide (DMF) to yield the desired compounds, cis- and trans-2,3-dimethyl-l,4-benzodioxane [3] and [4]. The solid trans isomer [4] was obtained by repeated recrystallization from methanol. Column chromatography of the mother liquid using AloOj as adsorbent and petroleum ether and diethyl ether in a 1 1 ratio as eluent, gave pure liquid cis isomer [3]. By comparison with the PMR spectra (8) of pure trans- and cis-2,3-dimethyldi-oxane, the solid isomer was identified as trans, and the liquid isomer as cis. The compounds were further characterized by IR, NMR, CMR and elemental analyses. The CMR spectral data for 2 and 4 are summarized in Table I. 

Enthalpies of combustion for the different liquid isomers of hexane have been... 

A high-temperature-stable polyimide occurs from the reaction of pyromellitic anhydride with aromatic diamines such as p,p-diaminodi-phenyl ether. Pyromellitic acid is produced either by direct oxidation of 1,2,4,5-tetramethyl benzene (Durol) or from xylylene. The xylylene mixture is chloromethylated. Crystalline l,3-dimethyl-4,6-di(chloromethyl)-benzene (I) is separated from the two liquid isomers (II) and (III) and oxidized to pyromellitic acid with HNO3 ... 

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