H{sub 2n + 2} (where n is an integer). In order
of increasing number of carbon atoms,
methane (CH
), ethane (C
H
), and
propane (C
H
) are the
first three members of the series.
Methane, ethane, and propane are the only alkanes uniquely defined
by their molecular formula. For C
H{sub 10} two different alkanes satisfy the rules of chemical
bonding (namely, that carbon has four bonds and hydrogen has
one in neutral molecules). One compound, called
n-butane, where
the prefix n- represents normal, has its four carbon
atoms bonded in a
continuous chain. The
other, called
isobutane, has a
branched chain.
Different compounds that have the same molecular formula are
termed isomers. Isomers that differ in the order in
which the atoms are connected are said to have different constitutions
and are referred to as
constitutional isomers.
(An older name is structural isomers.) n-Butane and isobutane
are constitutional isomers and are the only possible isomers
for the formula C
H{sub 10}. Because isomers are different compounds, they can
have different physical and chemical properties. n-Butane,
for example, has a higher boiling point (-0.5
C, or 31.1 F) than
isobutane (-11.7 C,
or 11 F).
No simple arithmetic relationship exists between the number
of carbon atoms in a formula and the number of isomers.
Graph theory has been
used to calculate the number of constitutionally isomeric alkanes
that are possible for values of n in C
H{sub 2n + 2} from 1 through 400. Representative values
are given in Table 21, where
it can be seen that the number of constitutional isomers increases
sharply as the number of carbon atoms increases. There is probably
no upper limit to the number of carbon atoms possible in hydrocarbons.
The alkane CH
(CH){sub
388}CH, in
which 390 carbon atoms are bonded in a continuous chain, has
been synthesized as an example of a so-called
superlong alkane. Several
thousand carbon atoms are joined together in molecules of hydrocarbon
polymers such as polyethylene, polypropylene, and polystyrene.
