274 ❯ STEP 4. Review the Knowledge You Need to Score High
Next, the longest chain should be numbered from one end to the other with the lowest
number(s) going to the branches. For the preceding example the numbering of the chain
(bold-face carbon atoms) would be:
1 2 3 4 5
6
7 8 9
Once these numbers have been assigned, do not alter them later.
All carbon atoms that are not part of the nine-atom main chain are branches. Branches
have -yl endings. It may help you to circle the carbon atoms belonging in the branches. In
the above example, there are three branches. Two consist of only one carbon and are called
methyl groups. The remaining branch has two carbons, so it is an ethyl group. The branches
are arranged alphabetically. If there is more than one of a particular type, use a prefix (di-, tri-,
tetra-, etc.). The two methyl groups are designated dimethyl. The position of each branch is
indicated with a number already determined for the main chain. Each branch must get its own
number, even if it is identical to one already used.
In the above example this gives: 5-ethyl-2,2-dimethylnonane
a. ethyl before methyl (alphabetical—prefixes are ignored)
b. two methyl groups = dimethyl
c. three branches = three numbers
Numbers are separated from other numbers by commas, and numbers are separated
from letters by a hyphen.
Another type of isomerism is optical isomerism. These molecules are capable of rotat-
ing light to either the left or right and are said to be optically active. The presence of an
asymmetric or chiral carbon (a carbon atom with four different groups attached to it) will
make a compound optically active.Common Functional Groups
If chemistry students had to learn the properties of each of the millions of organic com-
pounds, they would face an impossible task. Luckily, chemists find that having certain
arrangements of atoms in an organic molecule causes those molecules to react in a simi-
lar fashion. For example, methyl alcohol, CH 3 -OH, and ethyl alcohol, CH 3 -CH 2 -OH,
undergo the same types of reactions. The -OH group is the reactive part of these types of
molecule. These reactive groups are called functional groups. Instead of learning the proper-
ties of individual molecules, one can simply learn the properties of functional groups.
In our study of the simple hydrocarbons, there are only two functional groups. One
is a carbon-to-carbon double bond. Hydrocarbons that contain a carbon-to-carbon double
bond are called alkenes. Naming alkenes is very similar to naming alkanes. The major difference
is that the carbon base has an -ene ending instead of the -ane ending. The carbon backbone of
the base hydrocarbon is numbered so the position of the double bond has the lowest location
number.
The other hydrocarbon functional group is a carbon-to-carbon triple bond. Hydrocarbons
that contain a triple bond are called alkynes. Alkynes use the -yne ending on the base hydro-
carbon. The presence of a double or triple bond make these hydrocarbons unsaturated.
The introduction of other atoms (N, O, Cl, etc.) to organic compounds gives rise to
many other functional groups. The major functional groups are shown in Table 18.2, on
the next page.