physical properties of alkanes-concept of homology
The series of continuous-chain alkanes, CH,(CH,),-,CH, , shows a remarkably smooth gradation of physical properties (see Table 3.3 and Figure 3-2). As you go up the series, each additional CH, group contributes a fairly constant increment to the boiling point and density and, to a lesser extent, to the melting point. This makes it possible to estimate the properties of an unknown member of the series from those of its neighbors. For example, the boiling points of hexane and heptane are 69" and 98", respectively; a difference in structure of one CH, group therefore makes a difference in boiling point of 29". This places the boiling point of the next higher member, octane, at 98" + 29", or 127", which is close to the actual boiling point of 126".
Members of a group of compounds with similar chemical structures and graded physical properties and which differ from one another by the number of atoms in the structural backbone, such as the n-alkanes, are said to constitute a homologous series. The concept of homology, when used to forecast the properties of unknown members of the series, works most satisfactorily for the higher-molecular-weight members. For these members, the introduction of additional CH, groups makes a smaller relative change in the overall composition of the molecule. This is better seen from Figure 3.2,
which shows how the boiling points and melting points of the homologous series of normal alkanes change with the number of carbons, a. See also Figure 3-3.
Branched-chain alkanes do not exhibit the same smooth gradation of physical properties as the n-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 3.4, in which are listed the physical
properties of the five hexane isomers; the most striking feature is the 19" difference between the boiling points of hexane and neohexane.
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