CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12Number of C atomsn 12 3 4 5
Number of H atoms 2 n 2 46 8 10 12
The formula of each alkane differs from the next by CH 2 , a methylene group.
A series of compounds in which each member differs from the next by a specific number
and kind of atoms is called a homologous series.The properties of members of such a
series are closely related. The boiling points of the lighter members of the straight-chain
saturated hydrocarbon series are shown in Figure 27-6. As the molecular weights of the
straight-chain hydrocarbons increase, their boiling points also increase regularly. Proper-
ties such as boiling point depend on the forces between molecules (Chapter 13). Carbon–
carbon and carbon–hydrogen bonds are essentially nonpolar and are arranged tetrahe-
drally around each C atom. As a result, saturated hydrocarbons are nonpolar molecules,
and the only significant intermolecular forces are dispersion forces (Section 13-2). These
forces, which are due to induced dipoles, become stronger as the sizes of the molecules
and the number of electrons in each molecule increase. Thus, trends such as those depicted
in Figure 27-6 are due to the increase in effectiveness of dispersion forces.
Some systematic method for naming compounds is necessary. The system in use today
is prescribed by the International Union of Pure and Applied Chemistry (IUPAC). The
names of the first 20 straight-chain alkanes are listed in Table 27-2. You should become
familiar with at least the first ten. The names of the alkanes starting with pentane have
prefixes (from Greek) that give the number of carbon atoms in the molecules. All alkane
names have the -aneending.
Constitutional isomers(structured isomers) have the same molecular formula but
differ in the order in which their atoms are attached.We have seen that there are two saturated C 4 H 10 hydrocarbons. For the C 5 hydro-
carbons, there are three possible arrangements of the atoms. These three different C 5 H 12
alkanes are examples of constitutional isomers.
Figure 27-5 Ball-and-stick models
of the two isomeric C 4 H 10
hydrocarbons, butane,
CH 3 CH 2 CH 2 CH 3 , and
methylpropane, CH 3 CHCH 3.
A
CH 327-1 Alkanes and Cycloalkanes 1045 100 20002003004001002 4 6 8 10 12 14 16 18 20GasesLiquidsSolidsNumber of carbon atoms in alkaneBoiling point,CFigure 27-6 A plot of normal
boiling point versus the number of
carbon atoms in straight-chain,
saturated hydrocarbons.