Heptane isomers

Heptane isomers-cyclohexane Close-hoihng Anihne, phenol ... 

Natural gas liquids may contain significant amounts of cyclohexane, a precursor for nylon 6 (Chapter 10). Recovery of cyclohexane from NGL hy conventional distillation is difficult and not economical because heptane isomers are also present which hoil at temperatures nearly identical to that of cyclohexane. An extractive distillation process has been recently developed by Phillips Petroleum Co. to separate cyclohexane. ... 

The above transformations and reactions can be used to explain the formation, as an example, of different heptane isomers when alkylating isobutane with propylene. As the first step in the mechanism, a tertiary butyl carbonium ion derived from isobutane reacts with a propylene molecule to form a carbonium ion of seven carbon atoms as outlined in reaction 2. This ion may then react directly with a molecule of isobutane as in reaction 6 to form the expected heptane molecule and to convert the isobutane molecule to a tertiary butyl carbonium ion. This seven-carbon carbonium ion, however, may undergo isomerization by the mechanism outlined in reaction 3 or 4 before reacting with isobutane to form an isomeric heptane molecule. 

There are nine heptane isomers of formula C7H16. Write structural formulas for each. Name each by the IUPAC system. (In working a problem such as this, proceed systematically by constructing first the heptane, then all the possible hexanes, the pentanes, and so on. Should you inadvertently duplicate a structure, this will become apparent when you name it duplicate names usually are easier to spot than duplicate structures.)... 

C4 alkenes/C4 alkanes/ C4 dienes C5 alkenes/C5 alkanes/ C5 dienes Heptane isomers-cyclohexane Heptane isomers-toluene Close-boihng and minimum-boiling azeotropes Close-boihng and minimum-boiling azeotropes Close-boihng Eurfural, DMF, acetonitrile, n-methylpyrohdone Furfural, DMF, acetonitrile, n-methylpyrohdone Anihne, phenol ... 

The next example shows results for the cracking of heptane isomers over H-ZSM-5 (Table 7-4). 

The sum-of-numbers scheme leads to different results for the two heptane isomers. A test must now be made as to whether the sum of products of edge 6 values yields unique numbers for a larger group of alkanes, say, the nine ismers of heptane. Examination of the first column of Table 1 reveals two pairs of identical values. Thus, the summation... 

L. H. Hall and L. B. Kier TABLE 1. Index values for heptane isomers Chemical graph... 

This combination of operations for the two examples from Figure 2 leads to different numbers. We can test the uniqueness of values from these operations by calculating values for the heptane isomers shown in Table 1. All values are unique. It appears that we can derive numbers for each molecule based on the number of neighbors on each atom (3 values). These are unique for heptane isomers and may carry information about relative structure. It is this quality of uniqueness which suggests that these graph-derived numbers may be a basis for developing QSARs. 

The consequences of this additional operation can now be seen in column 3 of Table 1. The more elongated, less branched molecules like heptane, 2-methylhexane and 3-methylhexane have larger numbers. These molecules are larger, as is reflected by computed cavity surface areas . At the other extreme, more branched, smaller molecules, such as the last three heptane isomers, have lower numerical values. 

In what ways does the information encoded in x differ from xl To reveal this, we can examine these two indices for the heptane isomers in Table 4. The most striking observation is that the first-order index, x is computed from the same number of... 

Another reaction sequence (Fig. 22) will proceed in a similar fashion. It starts with a y-heptyl cation and leads to the formation of 3-MH and 2,4-DMP cations through the isomerization, with the energy barriers at 8.83 and 12.77 kcal/mol. These branched cations will quickly abstract a hydride to form stable alkanes. The similar magnitude of the energy barriers between the formation of the mono- and di-methyl branched isoheptanes may explain the observed ratios among the four heptane isomers. 

The formation of aromatics from alkanes with less than six carbon atoms in their main chain (eg, methylpentanes) is also possible but proceeds much less readily than direct Ce-dehydrocyclization (13,85). This mechanism should involve a sort of dehydrogenative bond shift isomerization. The optimum hydrogen pressure (Fig. 5a) for aromatization of heptane isomers with less than six carbon atoms... 

Heptane is available both as >96% n-heptane and >99.9% as saturated n-heptane isomers. The 30% premium paid for heptane over the cost of hexane is seldom warranted, except in particularly demanding separations. A major advantage heptane offers is a vapor pressure that is approximately 1/4 that of hexane (and 1/12 that of pentane). 

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