Isomerism in hydrocarbons

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

In our studies of hydrocarbons in Sections 23-1 through 23-7, we saw some examples of constitutional isomerism in hydrocarbons. Recall, for instance, that there are three isomers of C5H12. 

KTB and KTA are superior to alkaU metal hydrides for deprotonation reactions because of the good solubiUties, and because no hydrogen is produced or oil residue left upon reaction. Furthermore, reactions of KTA and KTB can be performed in hydrocarbon solvents as sometimes requited for mild and nonpolar reaction conditions. Potassium alkoxides are used in large quantities for addition, esterification, transesterification, isomerization, and alkoxylation reactions. 

Isomerization is promoted by either acids or bases. Higher alkylbenzenes are isomerized in the presence of AlCb/HCl or BF3/HF olefins with most mineral acids, acid salts and silica-alumina saturated hydrocarbons with AlCb or AlBr,3 promoted by 0.1 percent of olefins. 

The isomerization of hydrocarbons in the petroleum industiy and the production of dodecyl benzene for detergents accounts for a further 10% each of the AICI3 used. 

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

The (=Si03)3TiH surface complex was active in the skeletal isomerization of alkanes at temperatures as low as 50°C and this remarkable activity is very promising because this catalyst has the possibility to isomerize light hydrocarbons under very mild conditions.261... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

In hydrocarbon reforming processes the vapour of an alkane is passed over a supported metal catalyst such as platinum on silica or alumina. Dehydrocyclization, isomerization and cracking reactions all take place to... 

One of the ways to generate the tetramethylenethane-type diradical 350, an important reference compound in connection with non-Kekule hydrocarbons [147], consists in the thermal isomerization of hydrocarbon 34 at -100 °C [43, 148], Under the reaction conditions, the six-membered ring of 350 ruptures to yield [4]dendralene (3,4-bismethylene-1,5 -hexadiene) (351). 

One might expect that a large number of isomers of polyboranes should exist as the number of boron atoms increases, similar to hydrocarbon chemistry. This, however, is not the case in borane chemistry. There are indeed only a few examples of proven isomerism, for instance for the B2oHi82 anions (see Section 5.8). However, this point has been addressed in many theoretical papers, and energy differences between various isomers of a polyborane are, in general, larger than in hydrocarbons. 

This situation continued much too long. In 1900, Moissan published a fifteen-page memoir on ethyl fluoride in which no real formula even appears. In 1890, Berthelot was still writing water as HO but finally published a note in 1891 using the atomic notation [H20], However, he continued to write benzene as C2H2 (C2H2) (C2H2) as late as 1898 and there is not the least use of structural formulas in his discussions of isomerism in a 1901 text on hydrocarbons. 18... 

The MM method proved to be remarkably useful for elucidating the complex mechanisms of the famous syntheses of adamantane and diamantane as well as other diamondoid molecules from isomeric polycyclic hydrocarbons under Lewis acid catalysis in Schleyer s laboratory (128,258,264-266),... 

Olefin isomerization reactions range from some of the most facile using acid catalysts to moderately difficult and, as components of more complex reaction schemes such as catalytic cracking, may be among the most common reactions in hydrocarbon processing. As stand-alone reactions, they are primarily used to shift the equilibrium between terminal and internal olefins or the degree of branching of the olefin. While olefin isomerization was considered for the production of MTBE, today stand-alone olefin isomerization processes are only considered for a few special situations within a petrochemical complex. 

Gajewski JJ (1981) In Hydrocarbon Thermal Isomerizations, Academic Press, New York, p 149 for the mechanism of this reaction see also ref [104]... 

The central bond does exhibit slower rotation, at. least in the lowest member where it. can be followed, as no coalescent temperature has been observed. This is at least partly caused by steric hindrance of the t-butyl group. The rate of cis/trans isomerization was found to increase greatly when the t-butyl was replaced by n-but.yl or another isoprene unit in the isoprene based derivative of lithium in hydrocarbon solvents the half life decreased to seconds at room temperature. The rate of cis/trans... 

The binuclear hydride salts are air sensitive, soluble in THF, acetone, CH3CN, MeOH, and diethyl ether, and insoluble in hydrocarbon solvents. They can be stored cold under an inert gas or in vacuo for several months. The salts tend to slowly decompose into W(CO)6 and [PPN] [FeH(CO)4], Carbon monoxide rapidly (within minutes) degrades the dimer into the same products.5 The salts react rapidly with CH3COOD or stronger deuterated acids to form the H-D exchanged products, [FeCrD(CO)9]. The hydrides also act as catalysts in olefin isomerization.10... 

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