Hexane structural isomers

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. 

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]. 

Draw the five structural isomers of hexane (C6H14). 

Draw all the structural isomers for CgHig that have the following root name (longest carbon chain). Name the structural isomers, a. hexane b. pentane... 

One of the most interesting problems in the catalytic cracking of paraffins is the behavior of structural isomers. The work of Good, Voge, and Greensfelder (6) is of particular interest in this respect since it deals with the catalytic cracking of the five isomeric hexanes. The results indicate that, with one exception, there is essentially no isomerization of... 

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. 

GLC results confirmed the presence of very slightly depressed levels of NPA after distillation in experiments 1-3 (Table 2). Analysis of experiment 4 by GLC was complicated, however, due to the hexanes introduced with TEA, which is used as a 25% solution in hexanes. The column used in the GLC analysis was the non-polar DC-200 (15%) on a Chromosorb P solid support. The more polar n-propyl alcohol elutes just prior to the elution of the non-polar (but lower boiling) hexane peak. The "hexane peak" is actually two peaks, probably corresponding to two structural isomers. The only problem with peak overlap occurs with the hexane peaks and an impurity intrinsic to Rohm and Haas MMA, possibly a reaction by product from the acetone-cyanohydrin process (ACH). Chromatogram (A) of Figure 3 shows the 1% NPA doped MMA prior to reaction with tri n-octyl aluminum (TOA). The peaks of interest are the air... 

Which of the four structures in Figure 14.34 is not an isomer of this hexane structure ... 

All except (b) are isomers of the given hexane structure. Structure (b) is not an isomer because it is identical to the given structure it has the six carbons in a chain. Twisting the chain to make it appear as shown in (b) does not make it an isomer. 

Butane and isobutane are structural isomers, molecules with the same molecular formula but different structures. Because of their different structures, they have different properties—indeed they are different compounds. Isomerism is ubiquitous in organic chemistry. Butane has 2 structural isomers. Pentane (C5H12) has 3, hexane (CgHi4) has 5, and decane (C10H22) has 75 ... 

Two or more compounds that have the same molecular formula but different molecular structures are called isomers. The two main classes of isomers are structural isomers and stereoisomers. Structural isomers differ in the order in which atoms are bonded to each other. For example, hexane, 2-methylpentane, and 3-methylpentane are structural isomers because they all have the molecular formula CgHj, but they have different carbon chains. Structural isomers have different physical and chemical properties. 

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.)... 

Normal-phase liquid chromatography is thus a steric-selective separation method. The molecular properties of steric isomers are not easily obtained and the molecular properties of optical isomers estimated by computational chemical calculation are the same. Therefore, the development of prediction methods for retention times in normal-phase liquid chromatography is difficult compared with reversed-phase liquid chromatography, where the hydrophobicity of the molecule is the predominant determinant of retention differences. When the molecular structure is known, the separation conditions in normal-phase LC can be estimated from Table 1.1, and from the solvent selectivity. A small-scale thin-layer liquid chromatographic separation is often a good tool to find a suitable eluent. When a silica gel column is used, the formation of a monolayer of water on the surface of the silica gel is an important technique. A water-saturated very non-polar solvent should be used as the base solvent, such as water-saturated w-hexane or isooctane. 

---