Structural Isomers of Hexane

Tie structural formula of an organic compound can sometimes be written in a variety of ways, but sometimes structural formulas that appear similar can represent different compounds. The structural formulas below are ten ways of representing compounds having the molecular formula C6H14. 

In the spaces provided, write the correct name for each of the structural formulas, labeled a-j, above. 

How many different compounds are represented by the structural formulas above What are their names  

Copyright Glencoe/McGraw-Hill, a division of the McGraw-Hill Companies, Inc. 

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Isobutane, isopentane, and neopentane are common names or trivial names, meaning historical names arising from common usage. Common names cannot easily describe the larger, more complicated molecules having many isomers, however. The number of isomers for any molecular formula grows rapidly as the number of carbon atoms increases. For example, there are 5 structural isomers of hexane, 18 isomers of octane, and 75 isomers of decane We need a system of nomenclature that enables us to name complicated molecules without having to memorize hundreds of these historical common names. 

Draw the five structural isomers of hexane (C6H14). 

Chi indices for the various isomers of hexane. (Figure adapted in part from Hall L H and L B Kier 1991. The ir Connectivity Chi Indexes and Kappa Shape Indexes in Structure-property Modeling. In Lipkowitz K B and id (Editors) Reviews in Computational Chemistiy Volume 2. New York, VCH Publishers, pp. 367-422.)... 

It is possible for long chains of carbons to loop around and form a closed ring structure. If you take the linear isomer of hexane in Figure 6-9... 

Differences in structure cause differences in intermolecular forces. Thus there are differences in boiling points and melting points between the isomers of any particular alkane. Table 1-6 shows the physical properties of the isomers of hexane. An increase in branching causes a decrease in intermolecular attraction, which results in a lower boiling point,... 

Following are the structures of four isomers of hexane. Which of the names given correctly identifies a fifth isomer ... 

Thus there are only five distinct structural isomers of C6H14 hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane. 

As the carbon number gets larger in the paraffin series the number of possible structural isomers also gets larger so that hexane (C5H14), for example, has five structural isomers and heptane (CyHi ) six. All 18 isomers of octane have been isolated or synthesized, as have the 35 isomers of nonane. Beyond this, however, little is established about the natural occurrence of the 75 possible structural isomers of decane (C10H22) or the over 4,000 possible isomers with pentadecane (C15H32). However, many of the possible paraffin isomers have been found and isolated from exhaustive separations of natural petroleum [4]. 

Higher-order chi indices involve summations over sequences of two, three, etc., bonds. To illustrate the difference between these indices for a series of related structures we show in Figure 12.25 the °x, arid indices for the isomers of hexane. 

Write the structural formulae of the five isomers of hexane and name them. 

Isooctane is 2,2,4-trimethylpentane. Draw structural formulas for and name a branched heptane, hexane, pentane, and butane that are structural isomers of isooctane. 

There is a problem with the AHf values obtained from the simple bond increment data in Table 1.5. The five isomers of hexane listed in Table 1.6 aU have five C-C bonds and fourteen C-H bonds. Using the bond increment values in Table 1.5, we would predict each to have the same heat of formation (—39.97 kcal/mol). As shown in Table 1.6, however, the experimental heats of formation become more negative as the branching increases. Specifically, the structure with a quaternary carbon atom is more stable than an isomeric structure with two tertiary carbon atoms, and the structure with two tertiary... 

The phosphorescence spectra of chlorinated 1,4-dibenzodioxins, in hexane solutions at 77 and 4.2 K, have a well-resolved vibronic structure with distinctions in quasi-linear phosphorescence spectra even for closely related isomers of polychlorinated dioxins <1998MI129>. 

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