Page 278 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 18] ORGANIC CHEMISTRY 267

These are two distinctly different compounds, with different chemical and physical properties; for example, they
have different boiling points.
Note that there are only two possible isomers having the molecular formula C 4 H 10 . Condensed formulas
written as
CH 2 CH 2 CH 3 CH 2
and
CH CH
CH 3 CH 3 2 3

are both butane. They have a continuous chain of four carbon atoms and are completely saturated with hydrogen
atoms; they have no carbon branches.

EXAMPLE 18.3. Write the line formulas (a) for butane and (b) for methylpropane, both C 4 H 10 .
Ans. (a) CH 3 CH 2 CH 2 CH 3
butane
(b) CH 3 CH(CH 3 )CH 3 or CH 3 CH(CH 3 ) 2 or CH(CH 3 ) 3 or (CH 3 ) 3 CH
methylpropane
Similarly, three isomers of pentane, C 5 H 12 , exist. As the number of carbon atoms in a molecule increases,
the number of possible isomers increases markedly. Theoretically, for the formula C 20 H 42 there are 366 319
possible isomers. Other types of isomerism are possible in molecules that contain atoms other than carbon and
hydrogen atoms and in molecules with double bonds or triple bonds in the chain of carbon atoms.
Compounds called cycloalkanes, having molecules with no double bonds but having a cyclic or ring structure,
are isomeric with alkenes whose molecules contain the same number of carbon atoms. For example, cyclopentane
and 2-pentene have the same molecular formula, C 5 H 10 , but have completely different structures:

CH 2 CH 2
CH 2 CH 2 CH 3 CH CH CH 2 CH 3
CH 2

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is
much more reactive. Similarly, ring structures each containing a double bond, called cycloalkenes, can be shown
to be isomeric with alkynes.
Literally thousands of isomers can exist. Even relatively simple molecules can have many isomers. Thus,
the phenomenon of isomerism accounts in part for the enormous number and variety of compounds of carbon.

18.6. RADICALS AND FUNCTIONAL GROUPS
The millions of organic compounds other than hydrocarbons can be regarded as derivatives of hydrocarbons,
where one (or more) of the hydrogen atoms on the parent molecule is replaced by another kind of atom or group
of atoms. For example, if one hydrogen atom in a molecule of methane, CH 4 , is replaced by an —OH group,
the resulting compound is methanol, also called methyl alcohol, CH 3 OH. (This replacement is often not easy to
perform in the laboratory and is meant in the context used here as a mental exercise.) In most cases, the compound
is named in a manner that designates the hydrocarbon parent from which it was derived. Thus, the word methyl
is derived from the word methane. The hydrocarbon part of the molecule is often called the radical. To name a
radical, change the ending of the parent hydrocarbon name from -ane to -yl. The names of some common radicals
are listed in Table 18-3. Note that the radical derived from benzene, C 6 H 5 —, is called the phenyl radical. Some
other radicals are also given names that are not derived from the names of the parent hydrocarbons, but these
other cases will not be discussed here. Since, in many reactions, the hydrocarbon part of the organic compound
is not changed and does not affect the nature of the reaction, it is useful to generalize many reactions by using
the symbol R— to denote any radical. Thus, the compounds CH 3 Cl, CH 3 CH 2 Cl, CH 3 CH 2 CH 2 Cl, and so forth,
all of which undergo similar chemical reactions, can be reprersented by the formula RCl. If the radical is derived