Organic Structure - DRAWING ORGANIC MOLECULES Drawing...

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 (CH
4
), ethane (C
2
H
4
) and acetylene;
H
C
H H
H
C C
H H
H H
C C
H H
the carbons are all neutral, as each has a TOTAL of four bonds. Likewise, in methanol (CH
3
OH),
methyl amine (CH
3
NH
2
), acetonitrile (CH
3
CN) and formaldehyde (CH
2
O):
H
C
H H
O
H
H
C
H H
N
H
H
H
C
H H
C N
O
C
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 (C
4
H
10
) this can be demonstrated by the structures below:
C
C
C
C
H
H
H
H
H
H
H
H
H
H
C C C C
H
H
H
H
H
H
H
H
H
H
A
B
The first one ( A