Isomeric pentanes

There are three isomeric pentanes, ie, saturated aHphatic hydrocarbons of molecular formula C H 2- They are commonly called / -pentane [109-66-0] isopentane [78-78-4] (2-methylbutane), and neopentane [463-82-1] (2,2-dimethylpropane). 

As the number of carbon atoms increases, the number of possible isomers becomes larger. Whereas there are only two isomeric butanes, there are three isomeric pentanes. With hve carbons, in addition to the open-chain compounds shown, a stable ring compound known as cyclopentane is also possible. Five- and six-membered carbon rings are very stable because the bonds between carbon atoms in these size rings are close to the 109° angle preferred by carbon. Three- and four-membered hydrocarbon rings are also known, but they are less stable because of the required distortion of the bond angle. 

Pentafining A process for isomerizing pentane in a hydrogen atmosphere, using a platinum catalyst supported on silica-alumina. Developed by the Atlantic Richfield Company. 

Pentane Isomerization. Pentane isomerization, although carried out on a much smaller scale, increased the critical supply of aviation gasolines toward the end of the war. Two pentane processes—one developed by Shell and one by Standard (Indiana) —were commercialized before the end of the war. The principal differences between the butane and pentane processes are the use in pentane isomerization of somewhat milder conditions and the use of an inhibitor to suppress side reactions, principally disproportionation. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stocks is less important. Catalyst life is much... 

While the chain of carbon atoms may become more or less branched it always remains an open chain. Examples of straight and branched chains may be given by two of the isomeric pentanes. 

Petroleum ether mostly isomeric pentanes. Ligroin mostly isomeric hexanes. 

Fig. 5. Effect of hydrogen on pentane isomerization. Pentane feed containing 12% isopentane. Conditions temperature, 212°F. contact time, 3 hours AICI3, 11.5 wt.% and HCl, 3.2 wt.% based on pentane.

The most notable fact observed in this table is that the tertiary alcohol has the lowest boiling point. The student should compare the relations observed here with those that exist in the case of the isomeric pentanes. 

The isomeric pentanes form an interesting series. With 7i-pentane the range is 168°, with the branched 2-methyl butane it is 187°, but with the completely symmetrical tetramethyl methane it falls abruptly to 30°, not very much more than for methane itself. Clearly the symmetrical molecule must possess in the solid state much of the freedom... 

The first programs with the ability of generating information in chemistry were those which constructed all isomers of a given molecular formula. There are for instance 3 isomeric pentanes, 5 hexanes, 9 heptanesand 366319 decanes. A more sophisticated example is shown in Figure 1. The 20 valence isomers of cyclooctatetraene have been calculated with IGOR using only a few seconds CPU time. 

Three isomers are possible for the isomeric pentanes (C5Hj2). Draw condensed formulas for these isomers. 

PROBLEM 2.55 Rationalize the differences in the melting points for the isomeric pentanes shown below. 

Measurements and analysis of /hc couplings performed for isomeric pentane and isotopically labelled alkane complexes allowed Lawes et al. to get an insight into binding of these alkanes to transition metals. 

Fig. 4. Density normalized mobility of thermal electrons as functions of fluid density in the coexistence vapor and liquid of the isomeric pentanes. , n-pentane A, iso pentane o, neo-pentane. Data from Gyorgy and Freeman, 1979. 

In superacid media (HF/SbFs) at 273 K, alkanes may be oxidized to carbenium ions and H2. A two-step mechanism is proposed in which the formation of the protonated intermediate from various isomeric pentanes is rate determining, whereas for methylcyclopentane its decomposition is rate determining. 

Can the three isomeric pentanes be distinguished unambiguously from their broad-band proton-decoupled NMR spectra alone Can the five isomeric hexanes be distinguished in this way ... 

We can see the obvious conclusion that more carbon means more heat, but the differences between the isomeric butanes and the isomeric pentanes are instructive. In each case, the more branched the isomer, the less energy we get on burning it. Since the products are identical each time, this means that the branched isomers are lower in energy than the linear ones. This is quite general for alkanes and indeed quite a number of other compound classes. 

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