Chemistry Reference
In-Depth Information
the ability to make a single bond. Carbon atoms have the ability to make up
to four single bonds, or two double bonds, or one triple and one single bond.
If we use up three bonding electrons for each carbon atom with a triple bond,
then each will have enough for one more single bond, as shown here.
CC
You will recall that each carbon atom can make one triple and one
single bond. Notice that four lines come out of each carbon atom. Each
carbon atom has used up three of its four bonding electrons, leaving just
one more line available for each.
Now, it should be obvious where the two hydrogen atoms go. Each of
the two hydrogen atoms can make a single bond, and there are two bond-
ing sites available. We simply add the hydrogen atoms to the spaces avail-
able, as shown in this summary:
A.
Molecular formula: “eth-” = 2 carbon atoms, or C
2
“-yne” means # of hydrogen atoms = 2n - 2 = (2 × 2) - 2
= 2 or H
2
so, the molecular formula for ethyne is C
2
H
2
B.
Structural formula for ethyne:
HCCH
Something strange seems to happen when we start constructing struc-
tural formulas for hydrocarbons with more than three carbon atoms. Sud-
denly, you will find that there seems to be more than one possible answer.
You can create more than one structural formula for a given molecular
formula. You can put all of the carbon atoms in a long chain, or you can have
one or more branches coming off the main chain. Two or more compounds
that have the same molecular formula but different structural formulas are
called isomers. Figure 4-4c on page 134 shows an example of isomerism.
For butane, as shown in Figure 4-4c, there are two possible isomers. As
you add more and more carbon atoms, there become more and more pos-
sible combinations. Molecular formulas with many carbon atoms will have
dozens of different possible structural formulas. For this reason, a more com-
plex naming system is required for more complex hydrocarbons. Depending
upon the course that you are taking, you might not need to know the more
complex naming system, but you will almost certainly be expected to under-
stand the concept of isomers and be able to identify examples of isomerism.
