Straight-chain Isomers

The earhest modification of the Oxo process (qv) employed cobalt hydrocarbonyl, HCo(CO)4, as catalyst. The reaction was carried out in the Hquid phase at 130—160°C and 10—20 MPa (1450—2900 psi) to give a ratio of n- to isobutyraldehyde of between 2 1 to 4 1. / -Butyraldehyde, the straight-chain isomer and the precursor of 2-ethylhexanol, was the more valuable product so that a high isomer ratio of n- to isobutyraldehyde was obviously advantageous. 

Branched-chain acids contain at least one branching alkyl group attached to the carbon chain, which causes the acid to have different physical, and in some cases different chemical, properties than their corresponding straight-chain isomers. For example, stearic acid has a melting point of about 69°C, whereas isostearic acid has a melting point of about 5°C. Some properties of commercial branched-chain acids are shown in Table 1 (1,2). 

Pco (atm) Carbonyl equiv./olefin charged (%) Straight-chain isomer (%)... 

The thermal isomerization of tert. -butyl-diisobutylborane to triisobutylborane and the stepwise isomerization of triisopropyl and tri-sec.-butyl boranes to the corresponding straight-chain isomers have been studied over a range of temperature, both neat and using either the end product trialkylborane or diglyme as solvent117 . In all cases the reactions were first order and showed no solvent effect. 

DeAngelo AB, Garrett CT, Manolukas LA, et al. 1986. Di-n-octyl phthalate (DOP), a relatively ineffective peroxisome inducing straight chain isomer of the environmental contaminant di(2-ethylhexyl)phthalate (DEHP), enhances the development of putative preneoplastic lesions in rat liver. Toxicology 41 279-288. 

Gordon, J.E. and SubbaRao, G.N., Fuse organic salts. 8. Properties of molten straight-chain isomers of tetra-n-pentylammonium salts, /. Am. Chem. Soc., 100, 7445,1978. 

Olefin Isomerization. One other type of hydrocarbon isomerization is on the threshold of commercialization—namely, that of olefins. Processes for olefin isomerization were first developed some 15 years ago (11, 14, 20) after it was recognized that highly branched olefins have higher octane numbers than do their straight-chain isomers, and that the octane numbers of olefins increase as the double bond moves toward the middle of the molecule. 

The new Brownsville, Tex., plant for the manufacture of synthetic liquid fuels from natural gas makes use of this reaction to increase the octane number of its product by as much as 20 units. Synthetic naphtha produced over iron catalyst is highly olefinic and contains substantial amounts of straight-chain isomers with terminal double bonds (8). The shifting of these double bonds toward the center of the molecule may be accomplished by vapor-phase treatment employing synthetic cracking catalyst in the fluid state, under mild catalytic cracking conditions. Oxygenated compounds also present are converted under the isomerization conditions to hydrocarbons and water. 

Our results show that there is a clear difference between the sorption of /ert-butyl alcohol by the microporous phenyl-modified silica compared with the sorption of w-butyl alcohol. Comparison of total pore volumes and monolayer capacities with those for other sorptives leads us to conclude that sorption of the straight-chained isomer is significantly more sterically hindered than its branched analogue. This may be explained by the difference in the shapes of the two molecules. The spherical ten-butyl alcohol molecule is able to easily penetrate into the micropores of the unswollen sample without blocking the pore-entrance, while the straight-chain w-butyl alcohol molecule may block the pores upon adsorption. 

PROBLEM 23.1 Draw the straight-chain isomer with the formula C7H 6. 

Start with the straight-chain isomer of C5HI2. This compound has three different types of hydrogens, so three isomers of C5HUC1 result by replacing these hydrogens with chlorine. 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

First, draw all straight-chain isomers. Then proceed to the simplest branched structure. 

A contracted form of norbomanyl, the radical derived from norbornane A contracted form of norcaranyl, the radical derived from norcarane H3C(CH2)3CH(NH2)C0- the acyl radical from norleucine used in naming peptides in this case the prefix nor means normal, i.e., the straight-chain isomer of leucine... 

Species-specific evolution profiles for the PE-HY sample are shown in Figure 2.5. Unlike the PE-HZSM-5 results, volatile mixtures were primarily composed of C4-C8 paraffin rather than olefin products. Evolution profiles for paraffins and olefins had similar shapes with maximum evolution rates occurring at 230-240°C. Like the paraffin evolution profiles for the PE-HZSM-5 sample, isobutane and isopentane were significant paraffin products and no straight-chain isomers were detected. Alkyl aromatic yields for the PE-HY sample were much lower than for the PE-HZSM-5 sample. 

Asymmetric Hydrocarboxylation. The title reagent was used in the first example of an asymmetric hydrocarboxylation (eq 1). With the a-methylstyrene, the straight chain isomer was formed. The regiospecificity was much less pronounced, however, for other alkenic substrates. The influence of some reaction variables on the reaction shown in eq 1 was studied. For example, the presence of a solvent such as THF or benzene, the alcohol source, the effect of CO pressure, the effect of substitution on the phenyl ring, the PdC /DIOP molar ratio, or the presence of PPha along with DIOP, were varied to improve the optical yield. ... 

With alkanes having more than four carbons, the names of the straight-chain isomers are systematic and derive from Greek roots pentane for five C atoms, hexane for six, and so on. There are... 

Early identifications of long-chain monocar-boxylic acids (fatty acids) are generally believed to have been terrestrial contaminants (e.g., Smith and Kaplan, 1970). However, short-chain mono-carboxylic acids (extractable compounds in Murchison (Figure 1 and Table 1 Yuen and Kvenvolden, 1973 Lawless and Yuen, 1979 Yuen et al., 1984). They show complete structural diversity with all C2-C5 isomers present and with equal concentrations of both branched- and straight-chain isomers there is a general decrease in the abundance of short-chain monocarboxylic acids with increasing carbon number. 

Within the same series, the branched chain isomer generally has greater lipid solubility, hypnotic activity, and shorter duration of action than the straight chain isomer. Stereoisomers have approximately equal potencies. 

We first write the carbon skeleton of the straight-chain isomer (no branching). Then we remove one carbon from the straight-chain structure and reposition it to form the skeleton for a branched isomer. We check to see whether branching is possible at other carbon atoms. Then... 

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