Cycloalkanes physical properties

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

Perfluorinated alkanes and cycloalkanes are prepared from the corresponding hydrocarbons, either by electrochemical fluorination or by cobalt trifluoride fluo-rination [3], Many perfluorinated solvents are available commercially covering a wide selection of boiling points and densities. Some examples of perfluorinated solvents are listed in Table 3.1 together with their key physical properties. 

Alkanes have similar chemical properties, hut their physical properties vary with molecular weight and the shape of the molecule. The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the weak dipole-dipole forces (see 2.5.1), which are easily overcome. As a result, compared with other functional groups, alkanes have low melting and boihng points, and low solubility in polar solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and dichloromethane. Most cycloalkanes also have low polarity. 

Because of the chemical inertness of the saturated aliphatic hydrocarbons and of the closely related cycloalkanes, no satisfactory crystalline derivatives can be prepared. A pure sample may be characterised by consideration of such physical properties such as the boiling point, the refractive index (and/or the density), and these physical constants are listed in Table 10.1. If required, confirmation of structure should be sought from a more detailed study of appropriate spectra, particularly 13C-n.m.r., and mass spectra. 

Aromatic hydrocarbons are nonpolar, and their physical properties resemble those of alkanes of similar molecular mass. However, as was the case with cycloalkanes, the symmetrical shapes of many aromatic hydrocarbons often result in higher melting points. For example, the melting and boiling points of benzene are nearly identical to those of cyclohexane. (Recall that cyclohexane melts at considerably higher temperatures than does hexane.) As expected, a mixture of benzene and water forms two layers, with benzene as the upper layer. 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Most cycloalkanes resemble the acyclic (noncyclic), open-chain alkanes in their physical properties and in their chemistry. They are nonpolar, relatively inert compounds with boiling points and melting points that depend on their molecular weights. The cycloalkanes are held in a more compact cyclic shape, so their physical properties are similar to those of the compact, branched alkanes. The physical properties of some common cycloalkanes are listed in Table 3-4. 

TABLE 3-4 Physical Properties of Some Simple Cycloalkanes ... 

Some physical properties of first five members of the cycloalkanes are given in table 8. 

Table 8 The first five cycloalkanes and some of their physical properties...

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