20.
The
most common configurations for these four bonds around a
carbon atom are (a) four single bonds, (b) two single bonds
and one double bond, (c) one single bond and one triple bond,
or (d) two double bonds.
21.
These
arrangements are illustrated in Figure 10.2.
22.
Other
elements exhibit different bonding behavior in organic
compounds.
23.
A
hydrogen atom is always attached to another atom by a single
covalent bond.
24.
An
oxygen atom typically attaches either with two single bonds
(to two different atoms) or one double bond (to a single
atom).
25.
A
nitrogen atom commonly forms three single bonds (to three
different atoms), but also can form either a triple bond (to
one other atom), or a single and a double bond.
26.
Chemical
formulas such as C4H10 indicate the
kinds and numbers of atoms present in a molecule, but do not
show how the atoms are arranged or connected.
27.
To
get that higher level of detail, structural formulas are used
that show the atoms and their arrangement with respect to one
another in a molecule.
28.
Here
is the structural formula of normal butane, or n-butane (C4H10),
a hydrocarbon fuel used in cigarette lighters and camp stoves.@
29.
A
drawback to writing structural formulas, at least in a
textbook, is that they take up considerable space.
30.
To
convey the same information in a format that is easily typeset
into a single line, we use condensed structural formulas where
carbon-to-hydrogen bonds are not drawn out explicitly, but
simply understood to be single bonds.
31.
Here
are condensed structural formulas for n-butane.
32.
Note
that the carbons are bonded directly to other carbon atoms,
and that the hydrogen atoms do not intervene in the chain.
33.
Rather,
two or three hydrogens are attached to each carbon
atom, depending on its position in the molecule.
34.
The
same number and kinds of atoms can be arranged in different
ways, helping to explain why there are so many different
organic compounds.
35.
Isomers
are molecules with the same chemical formula (same number and
kinds of atoms), but with different structures and properties.
36.
You
already encountered isomers in the discussion of octane, C8H18,
in Chapter 4.
37.
Here
we illustrate isomers with C4H10.
38.
One
way to arrange these atoms is in a chain to form n-butane.
39.
Another
arrangement is possible in which the four carbon atoms are not
all in a line.
40.
This
other isomer is known as isobutane.
41.
The
linear n-butane is shown for comparison, now represented in a
more realistic zigzag form.@
42.
The
chemical formulas of these two isomers are the same; the way
the atoms are connected is different.
43.
Note
that the central carbon in isobutane has three carbons
connected to it and all of the other carbons have one other
carbon (and three hydrogens) connected to them.
44.
Rotating
this representation does not change how the atoms are
connected.
45.
Just
like its linear isomer, isobutane can be written using a
condensed structural formula.@
46.
Here,
the parentheses around the CH3 groups indicate that
they are attached to the carbon to their left.
47.
Note
that the CH3 attached to the central CH carbon atom
introduces a "branch" into the molecule.
48.
Figure
10.3 shows three depictions of n-butane and
isobutane.
49.
The
first column shows the simple structural formula and the
second a ball-and-stick model.
50.
In
the third column we see space-filling models that present a
more realistic view of the molecular shape.
51.
Only
two isomers of C4H10 exist.
52.
As
the number of atoms in a hydrocarbon increases, so does the
number of possible isomers.
53.
Thus,
C8H18 has 18 isomers and C10H22
has 75.@
54.
Given
a chemical formula, no simple calculation can be performed to
obtain the number of isomers.