Alkanes isomeric structures

Alkenes also form a homologous series as the carbon number increases, the number of possible isomeric structures for each member increases more rapidly than in the case of the alkane series. 

The effective carbon number neff is helpful in characterizing surfactants with an inner functional group. Surfactants with isomeric structures can be compared by means of the hydrophobicity index / [69], which indicates the influence of the effective length of the alkane chain on cM ... 

The molecular formulas just shown for 10 alkane hydrocarbon molecules represent the proportions of carbon to hydrogen in each molecule. These formulas do not reveal much about their structures, but rather indicate the proportions of each element in their molecules. Each molecule may have several different structures while still having the same formula. Molecules with different structures but the same formulas are called isomers. For example, n-butane is formed in a straight chain, but in an isomer of butane, the CH branches off in the middle of the straight chain. Another example is ethane, whose isomeric structure can be depicted as H,C H,C-CH,. The name for the normal structure sometimes uses n in front of the name. 

Butane and all succeeding members of the alkanes exhibit structural isomerism. Recall from Section 20.4 that structural isomerism occurs when two molecules have the same atoms but different bonds. For example, butane can exist as a straight-chain molecule (normal butane, or -butane) or with a branched-chain structure (called isobutane), as shown in Fig. 22.4. Because of their different structures, these molecules exhibit different properties. For example, the boiling point of -butane is -0.5°C, whereas that of isobutane is — 12°C. 

But nearly every alkane can have a number of isomeric structures, and there must be an unambiguous name for each of these isomers. The butanes and pentanes are distinguished by the use of prefixes w-butane and isobutan penta, isopentane and neopentane. But there are 5 hexane T lTieptanes, and 75 decanes it would be diflicnli to devise, and even more difficult to remember, a different prefix for each of these isomers. It is obvious that some systematic method of naming is needed. 

Microporous and, more recently, mesoporous solids comprise a class of materials with great relevance to catalysis (cf. Chapters 2 and 4). Because of the well-defined porous systems active sites can now be built in with molecular precision. The most important catalysts derived from these materials are the acid zeolites. The acid site is defined by the crystalline structure and exhibits great chemical and steric selectivities for catalytic conversions, such as fluid catalytic cracking and alkane isomerization (cf. Chapter 2). In Section 9.5 we discuss the synthesis of zeolites and, briefly, of mesoporous solids. 

These results agree with those of various authors who have demonstrated the transformation of bicyclo[n. 1.0]alkane type structures under acid catalysis into compounds having a methyl group. However, it is not possible to visualize a prior isomerization into methylcycloalkenes followed by its polymerization. One must consider that polymerization occurs in a single step by transformation into the carbo cation. Even so, the presence of monomer units with the methyl group not in the side chain but carried by carbon atoms in the ring should not be excluded. If such units existed, they would be present in a very small proportion with respect to the main monomer unit. 

Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a Cg intermediate, which subsequently decomposes, preferentially into two iso-Cn structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". 

Figure 7.39. The structure sensitivity of light alkane isomerization and hydrogenolysis. Shown here are the reaction rates of isobutane catalyzed at 570 K and atmospheric pressure over four platinum surfaces shown in Figure 7.37. Isomerization is favored over Pt surfaces that have a square atomic arrangement. Hydrogenolysis rates are maximized when kink sites are present in high concentrations on the platinum surface [155].

Apart from the use of the carbon number index, the first use of gri h invariants for the correlation of the measured properties of molecules with their structural features was made in 1947. In that yeax, Wiener [121,122] introduced two parameters designed for this purpose. The first of these was termed the path number and was defined as the "sum of the distances between any two carbon atoms in the molecule, in terms of carbon-carbon bonds. A simple algorithm was given for the calculation of this number and it was shown that its value for normal alkanes assumes the form - n). The second parameter was called the polarity number and was defined as "the number of pairs of carbcm atthree carbon-carbon bonds it took the general value n-3 f< normal alkanes. Wiener proposed that the variation of any physical property for an isomeric structure as compared to a normal alkane would be ven by the linear expression ... 

As the number of carbon atoms in an alkane increases, the number of isomers increases dramatically. There are 5 isomers possible for six-carbon alkanes, 9 isomers possible for seven-carbon alkanes, and 75 isomers possible for ten-carbon alkanes. It is possible to draw more than 300,000 isomeric structures for C20H42 and more than... 

Butane and aU succeeding members of the alkanes exhibit structural isomerism. Recall from Section 21.4 that structural isomerism occurs when two molecules have the same atoms... 

Sen S, Cohen J M, McCoy J D and Curro J G 1994 The structure of a rotational isomeric state alkane melt near a hard wall J Chem. Phys. 101 9010... 

In Problem 2 5 you were asked to write structural formulas for the five isomeric alkanes of molecular formula C6H14 In the next section you will see how the lUPAC rules generate a unique name for each isomer... 

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