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DRAWING ORGANIC MOLECULES
Drawing organic molecules is no different than drawing inorganic molecules: you will still use the
rules of Lewis structures, calculate formal charges, etc. The major difference is that you will use some
shortcuts for drawing organic molecules that we do not (traditionally) use for inorganic molecules.
Before we start, let’s review the rules of valence for neutral atoms that are especially important in
organic chemistry;
o Neutral Carbon has 4 valence e– available to make 4 covalent bonds (valence = 4);
o Neutral Oxygen has 2 valence e– available to make 2 covalent bonds + 2 lone pairs
(valence = 2);
o Neutral Nitrogen has 3 valence e– available to make 3 covalent bonds + 1 lone pairs
(valence = 3);
o Neutral Halogen has 1 valence e– available to make 1 covalent bonds + 3 lone pairs
(valence = 1);
o Hydrogen has a valence of 1 (one bond);
So in the molecules methane (CH4), ethane (C2H4) and acetylene;
H
H
H
C
C
H
H
H
C
H
H
C
H
C
H
the carbons are all neutral, as each has a TOTAL of four bonds. Likewise, in methanol (CH3OH),
methyl amine (CH3NH2), acetonitrile (CH3CN) and formaldehyde (CH2O):
H
H
H
C
H
H
O
H
H
C
H
H
N
H
O
C
H
C
N
C
H
H
H
all the atoms are neutral as well (Note: you would have arrived at the same conclusions had you
calculated the formal charge for each atom:
Formal Charge = # valence electrons that atom has – bonds that atom has – unshared electrons that atom has
Using the idea of valence is a quick way to determine whether you need to calculate the formal charge
or not). Notice that the lone pairs on N and O are not shown. It is understood that they are present
(remember that in drawing molecular structures, the lone pairs are not usually shown). With this
information in mind we can move on to a discussion of the drawing of organic molecules.
There are three basic ways to draw organic molecules: Extended, or Lewis, structure, Condensed
structure and Line-Angle structure. We will examine each of these separately.
EXTENDED STRUCTURE
This is nothing new; in this format EVERY bond to carbon, hydrogen, oxygen, etc. is explicitly
shown. It is not necessary to show the geometry at each atom, although you can if you wish. For the
molecule butane (C4H10) this can be demonstrated by the structures below:
H
H
H
C
H
C
C
C
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
B
A
The first one (A) shows all atoms and bonds AND the geometry at each atom while the second (B)
shows just the connections. Both are acceptable; structure B is usually the one shown (reasons for this
will become more evident when you get into Chem 140A!).
Extended structure is fine but requires a lot of writing. Showing all the atoms and the bonds…takes
too much time! What about a molecule that has 40 carbons and 80 hydrogens…that’s a lot of ink (or
pencil). Is there a quicker way? Yes!
CONDENSED STRUCTURE
In condensed format we will show what atoms are attached to what atoms, but we will not explicitly
show each bond. For example, the carbon on the end of butane (C4H10) has 3 hydrogens and a carbon
attached to it; that carbon has two hydrogens and another carbon attached to it, and so on. As a result
it would be easier (and faster) to do the following;
H
H
H
H
H
C
C
C
C
H
H
H
H
same as
CH3CH2CH2CH3
H
Condensed structure is used almost to the exclusion of extended structure (in fact, rarely is the
extended Lewis structure ever used unless there is a special reason to do so). It is very easy to use and
there are very few pitfalls, but there are some rules to remember:
1. The hydrogens attached to a particular carbon are always written directly next to the carbon to
which they are attached (either to the right (usually) or left (less often) of the C);
2. Halogens are treated like hydrogens except they are written after the H’s;
3. If there are groupings of atoms other than hydrogen attached to a carbon they can be shown
using parentheses (and subscript numbers if more than one grouping of the same are present)
4. The hydrogens or other atoms attached to any element other than a carbon are always written
directly to the right of the atom to which they are attached;
5. Any valences left open are used for multiple bond formation.
Lets look at each of these.
For #1: Look at the example given above for butane. Note that it is RARE that H’s are written to the
left of a C and this practice is usually ONLY done for the carbon on the FAR left.
For #2: Look at the example below:
H
H
Cl
H
H
C
C
C
C
H
H
H
H
Cl
same as
CH3CHClCH2CH2Cl
For #3: See below; note that it is understood that the groups inside the parentheses are attached to the
carbon immediately to the left:
H
H
H
H
OH
H
H
C
C
C
C
H
H
OH
H
H
C
H
H
H
C
C
H
H
C
H
H
C
C
H
H
Cl
H
H
same as
CH3CH(OH)CH(OH)CH3
same as
CH3C(CH3)2CH2CH3
OR
(CH3)3CCH2CH3
Notice that for the second structure there are TWO equivalent ways of representing the structure
using condensed structure.
For #4: See the example below, as well as #3 above:
H
H
H
C
C
N
H
H
H
H
same as
CH3CH2CH2NH2
CH3CH2CH4N
NOT
NOR
CH3CH2CH2H2N
For #5: This is a little harder to see. Let’s use three examples to demonstrate this: formaldehyde,
acealdehyde, acetic acid and acetone, respectively.
O
O
C
C
H
C
C
CH3
H
O
O
H
CH3
OH
CH3
CH3
CH2O
CH3CHO
CH3COOH OR CH3CO2H
CH3COCH3
formaldehyde
acetaldehyde
acetic acid
acetone
Notice that H’s attached TO a C are shown just as before. Let’s start with formaldehyde; notice that
the C has three atoms shown attached to it (two H’s and one O). Since carbon normally has a valence
of four but only three are shown in formaldehyde, the fourth must be due to a multiple bond to the
oxygen. Let’s skip to acetone: if we deal with the C’s and H’s first we get:
O
CO
H
C
H
H
C
H
C
CH3
H
H
CH3
acetone
The wavy lines indicate that those carbons are bonded to some other atom. All we have left is one
C and one O. The ONLY way to give C four valences and O two is to complete the structure the way
we did on the right (in the old days, molecules of this type would be written CH3C(O)CH3, where the
(O) indicated a double bond to the C. This representation has all but been abandoned). Carboxylic
acids (like acetic acid) are figured out the same way: after taking care of all C’s and H’s, then all H’s
on other atoms we get:
O
CO
H
H
C
H
C
O
H
CH3
OH
acetic acid
We are once again left with the CO unit, just like in acetone. Condensed structures are nice: fast,
convenient, relatively easy, but is there an even faster way? Yes!
LINE-ANGLE STRUCTURE
Line-angle represents the ultimate in simplicity. In line-angle format we do NOT have to
EXPLICITLY show ANY C’s or H’s attached to C’s; in fact the only atoms that MUST be shown are
any non-carbons AND the H’s that may be attached to non-carbons. In line-angle format we will use
an angled line to represent a carbon-carbon bond but will not explicitly show the C’s; since we know
that C has a valence of four it will be understood that all valences not shown are filled by H’s. Thus,
any time a line ends or two or more lines come together a C must be at that end or intersection. When
C is bonded to an element other than C or H, that will also be represented by an angled line but the
non-carbon will be explicitly shown. Therefore, the end of a line must represent a C; one valence
(bond) is shown therefore it must represent a CH3. When two lines meet we have a C at the
intersection of those two lines; since only two bonds are shown it must be a CH2. Let’s look at some
examples, using some of the molecules we have already used:
H
H
H
H
H
C
C
C
C
H
H
H
H
H
is
CH2 CH2 Cl
is
CH3CH2CH2NH2
is
CH3
CH3 C
Cl
CH3
CH2
CH2
CH2
CH2
C
NH2
CH
CH
is
CH2
O
O
C
CH3
OH
is
OH
Let’s look in a little more detail at how a correlation is drawn between a line-angle structure and a
condensed or extended structure. Using the Cl-containing compound above, it maps out as follows:
C with 2 bonds shown, one to a Cl
must be a CH2
End of a line; only one bond shown;
must be a CH3
Cl
C with 4 bonds shown: C
CH3
CH3 C
CH2 CH2 Cl
CH3
This method of drawing structures is extremely fast once you have practiced it a little bit. Line
angle is the preferred method of representing organic molecule structure.
REAL-WORLD STRUCTURAL REPRESENTATIONS
What has been laid out for you here are three different, yet equivalent, ways of drawing organic
molecules. In reality, ALL THREE representations are used to some degree or another. A molecule
may, for the most part, be drawn in line angle but have portions that are done in condensed and/or
extended format. Some examples are shown below:
Br
H
O
H
H
C
C
H
CH3
CH3
H
C(CH3)3
NH2
Notice that in all of the structures shown there is SOME of each motif. Now, why would we mix
our formats? To answer that question, look quickly at the structures above. What is the first thing that
catches your eye? Odds are it is the portion of the molecule drawn in either extended or condensed
format. This is one of the reasons for mixed format: to draw your attention to a part of the molecule
that bears some importance in a discussion that follows. Other reasons exist but this is the most
important for purposes of this discussion. Whatever format or mixed format you like is your own
business, but you do need to be able to move back and forth between the three formats on a regular
basis.