Now that you've seen how valence bond theory works, it's time to
learn the language that organic chemists use to describe the
structures of molecules in which atoms are connected by covalent
bonds. That language is pictorial, and, like other languages, it
contains a number of "dialects". In order to speak like a "native",
you have to understand their dialects. Those of primary concern are
Figure 1 presents three alternative pictures of methane, starting
with the depiction we saw during our discussion of valence bond
theory. Representation B in Figure 1 labels the nuclei of the atoms
with their atomic symbols. Drawing C shows just the atomic symbols
and the shared pairs of electrons that constitute the four C-H
covalent bonds. Drawing C is a Lewis electron dot structure for
Figure 2 animates the rules for drawing a Lewis electron dot
structure using C2H6as an example. Click within
the figure to view the animation.
The name of the compound on the left in Figure 4 is butane. It is a colorless liquid with a
boiling point of 0oC. It's what burns when you flick your
BIC. Butane is a linear molecule; you can trace a path from
the first to the fourth carbon atom without lifting your pencil from
The compound on the right in Figure 4 goes by different names. It
is commonly called isobutane. On formal
occassions it goes by its proper name 2-methylpropane. It is a colorless liquid with
a boiling point of -12oC. It has a branched
structure; you can't draw a line through all four carbon atoms
without lifting your pencil off the paper.
Exercise 4 How many
constitutional isomers are there for C4H8?
Drawing each bond in a molecule as two dots gets old very fast. To
save time chemists usually depict a bond as a line drawn from one
atomic symbol to another. Such representations are called Lewis
structures rather than Lewis electron dot structures. Figure 5 shows
Lewis structures for each of the molecules presented in Figures 1-4.
When a molecule contains atoms with one or more lone pairs of
electrons, the non-bonding pairs are normally shown as two dots.
Generally organic chemists depict molecular structures using a
convention known as bond line notation.
According to this approach, a bond between two atoms is represented
by a line. The ends of the line correspond to the positions of the
bonded atoms. Only atoms other than carbon and hydrogen are shown
explicitly. Thus the bond line notation for ethane is ___
. It is implied that there is a carbon atom at each end of the line.
Furthermore, since the valence of carbon is four, it follows there
are three hydrogen atoms connected to each carbon. Figure 6
illustrates the bond line notations for those compounds in Figure 5
that contain carbon-carbon bonds.
Exercise 5 Complete Table
1 by supplying the molecular formula for each of the bond line
Exercise 6 Complete Table
2 by supplying the molecular formula for each of the bond line
diagrams shown. N.B. The convention for writing molecular formulas
lists the atoms in the following order: C.. H.. N.. O.. other atoms
in alphabetical order, e.g.
Another convention that chemists use involves condensed formulas. Unlike bond line notation,
where carbon and hydrogen atoms are inferred while bonds are shown
explicitly, condensed formulas show all the atoms explicitly and the
bonds between them are inferred. Thus, the condensed formulas for
methane and ethane are CH4 and
CH3CH3, respectively. The condensed formula for
ethane makes it easy to forget that there is a bond between the two
carbons. While an alternative formulation,
H3CCH3, makes that bond more obvious, it is not
commonly used. Condensed formulas for linear structures are
straightforward. The formula for pentane, C5H12, a
colorless liquid with a boiling point of 36oC, is
Sometimes this is abbreviated
When writing the formula for a branched molecule, the convention
calls for enclosing the branching group(s) in parentheses. For
example, the condensed formula for isobutane (2-methylpropane) is
CH3CH(CH3)CH3. The carbon atom of
the methyl group enclosed in parentheses is attached to the carbon
atom bearing the single hydrogen. Alternatively, some chemists would
write this as (CH3)2CHCH3. The
condensed formula for neopentane (2,2-dimethylpropane), a colorless liquid that
boils at 10oC, is
equivalent formulations are
(CH3)4C. Compare the notations for pentane and
neopentane. Be certain you understand the differece between these two
You should practice drawing Lewis structures and bond line
notations of condensed formulas until your brain automatically fills
in the structural information hidden in such formulas.
Table 3 illustrates each or these bonding situations.
Finally, you will frequently encounter structural formulas that
contain a mixture of bond line notation and condensed formulas. For
example, a recent article in the Journal of Natural Products
reported the isolation of a compound known as glacin B from the
leaves of the pond-apple tree. Extracts of the leaves of this tree,
which is native to Florida, have been used in traditional medicines
as an insecticide and a parasiticide. The structure of glacin B is
shown in Figure 8. Note the various conventions that are used in this
depiction of the molecule.
Exercise 8 What is the
molecular formula of glacin B?