Alkanes and Alkane Isomers

Before beginning a systematic study of the different functional groups, let’s look
first at the simplest family of molecules—the alkanes—to develop some general
ideas that apply to all families. We saw  that the carbon–carbon
single bond in ethane results from σ (head-on) overlap of carbon sp3 hybrid
orbitals. If we imagine joining three, four, five, or even more carbon atoms by
C - C single bonds, we can generate the large family of molecules called alkanes.

Alkanes are often described as saturated hydrocarbons: hydrocarbons

because they contain only carbon and hydrogen; saturated because they
have only C - C and C - H single bonds and thus contain the maximum possible
number of hydrogens per carbon. They have the general formula
CnH2n12, where n is an integer. Alkanes are also occasionally called aliphatic
compounds, a name derived from the Greek aleiphas, meaning “fat.” We’ll
see many animal fats contain long carbon chains similar
to alkanes.

Think about the ways that carbon and hydrogen might combine to make

alkanes. With one carbon and four hydrogens, only one structure is possible:
methane, CH4. Similarly, there is only one combination of two carbons with six
hydrogens (ethane, CH3CH3) and only one combination of three carbons with
eight hydrogens (propane, CH3CH2CH3). When larger numbers of carbons and
hydrogens combine, however, more than one structure is possible. For example,
there are two substances with the formula C4H10: the four carbons can all
be in a row (butane), or they can branch (isobutane). Similarly, there are three
C5H12 molecules, and so on for larger alkanes.

Compounds like butane and pentane, whose carbons are all connected in a
row, are called straight-chain alkanes, or normal alkanes. Compounds like
2-methylpropane (isobutane), 2-methylbutane, and 2,2-dimethylpropane,
whose carbon chains branch, are called branched-chain alkanes.
Compounds like the two C4H10 molecules and the three C5H12 molecules,
which have the same formula but different structures, are called isomers, from
the Greek isos 1 meros, meaning “made of the same parts.” Isomers are compounds
that have the same numbers and kinds of atoms but differ in the way
the atoms are arranged. Compounds like butane and isobutane, whose atoms
are connected differently, are called constitutional isomers. We’ll see shortly
that other kinds of isomers are also possible, even among compounds whose
atoms are connected in the same order.
Constitutional isomerism is not limited to alkanes—it occurs widely throughout
organic chemistry. Constitutional isomers may have different carbon skeletons
(as in isobutane and butane), different functional groups (as in ethanol
and dimethyl ether), or different locations of a functional group along the
chain (as in isopropylamine and propylamine). Regardless of the reason for the
isomerism, constitutional isomers are always different compounds with different
properties but with the same formula.

A given alkane can be drawn in many ways. For example, the straight-chain,
four-carbon alkane called butane can be represented by any of the structures
shown in Figure 3.2. These structures don’t imply any particular threedimensional
geometry for butane; they indicate only the connections among
atoms. In practice, , chemists rarely draw all the bonds
in a molecule and usually refer to butane by the condensed structure,
CH3CH2CH2CH3 or CH3(CH2)2CH3. Still more simply, butane can be represented
as n-C4H10, where n denotes normal (straight-chain) butane.

Straight-chain alkanes are named according to the number of carbon atoms
they contain, as shown in Table 3.3. With the exception of the first four
compounds—methane, ethane, propane, and butane—whose names have historical
roots, the alkanes are named based on Greek numbers. The suffix -ane is
added to the end of each name to indicate that the molecule identified is an
alkane. Thus, pentane is the five-carbon alkane, hexane is the six-carbon alkane,
and so on. We’ll soon see that these alkane names form the basis for naming all
other organic compounds, so at least the first ten should be memorized.