Isomerism and the Physical Properties of Alkanes

Two or more compounds with the same molecular formula but different properties are called isomers. Isomers with different arrangements of bonded atoms are constitutional (or structural) isomers alkanes with the same number of C atoms but different skeletons are examples. The smallest alkane to exhibit constitutional isomerism has four C atoms two different compounds have the formula C4H10, as shown in Table 15.3 on the next page. The unbranched one is butane (common name, n-butane n- stands for normal, or having a straight chain), and the other is 2-methylpropane (common name, i obutane). Similarly, three compounds 

CHAPTER 15 Organic Compounds and the Atomic Properties of Carbon 

Because alkanes are nearly nonpolar, their physical properties are determined by dispersion forces. The four-C alkanes boil lower than the five-C compounds (Table 15.3). Moreover, within each group of isomers, the more spherical member (isobutane or neopentane) boils lower than the more elongated one (n-butane or n-pentane). As you saw in Chapter 12, this trend occurs because a spherical shape leads to less intermolecular contact, and thus lower total dispersion forces, than does an elongated shape. 

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Constitutional Isomerism and the Physical Properties of Alkanes Chiral Molecules and Optical Isomerism Alkenes Alkynes... 

Wiener H (1948a) Relation of the physical properties of the isomeric alkanes to molecular structure. Surface tension, specific dispersion, and critical solution temperature in aniline. J. Phys. Chem. 52 1082-1089. 

Branched-chain alkanes do not exhibit the same smooth gradation of physical properties as do the continuous-chain alkanes. Usually there is too great a variation in molecular structure for regularities to be apparent. Nevertheless, in any one set of isomeric hydrocarbons, volatility increases with increased branching. This can be seen from the data in Table 4-2, which lists the physical properties of the five hexane isomers. The most striking feature of the data is the 19° difference between the boiling points of hexane and 2,2-dimethylbutane. 

Abstract. Tungstated zirconia catalysts are stable and highly selective catalytic materials for the isomerization of alkanes when promoted by platinum and a transition metal oxide and when dihydrogen is present in the feed. Physical properties and the catalytic performance of these solids for the isomerization of n-pentane are discussed. 

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 ... 

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