Your goal should be to understand, not memorize, the material presented in your organic
chemistry course. The following principles should be learned as you begin your study of
organic chemistry, then used as a solid foundation for building your understanding
throughout the course. These simple ideas explain a great deal about the structures and
properties of organic molecules, as well as the characteristic ways in which they react.
Thoroughly understanding the following three key principles and related ideas will allow
you to develop an intuitive feel for organic chemistry that avoids the necessity of
resorting to the far less effective use of extensive memorization.
Predicting Structure and Bonding
1. In most stable molecules, all the atoms will have filled valence shells. This
means that C, N, O and the halogens will have 8 electrons in their valence shells,
and H atoms will have 2 electrons in their valence shells. This simple yet
powerful principle predicts the type of bonds created (single, double or triple) and
how many lone pairs are found around the different atoms of a molecule. In
general, an atom surrounded by 4 atoms/lone pairs will have a tetrahedral
geometry, an atom surrounded by 3 atoms/lone pairs will have a trigonal planar
geometry and an atom surrounded by two atoms/lone pairs will have a linear
geometry. You will encounter a small number of molecules containing an atom
such as a C atom with only 6 or 7 electrons in its valence shell. Atoms such as
this with only a partially filled valence shell are noteworthy and highly reactive.
Note, however, that you can never overfill the valence shell of any atom in a
molecule such as placing more than 8 electrons in the valence shells of C, N, or O.
Study the structure of the anti-epilepsy drug Vigabatrin below. Each atom is
surrounded by a filled valence shell (8 electrons for C,N,O and 2 electrons for H)
when all the bonds and lone pairs of electrons are considered. All atoms
surrounded by four bonds or lone pairs are tetrahedral in geometry, and atoms
surrounded by three bonds (double bonds count once for this) are trigonal planar.
O
C
C
C
C
C
C
O
H
H H
H H
H
H
H
N
H
H
H
H
O
C
C
C
C
C
C
O
H
H H
H H
H
H
H
N
H
H
H
H
Vigabatrin - A drug used to treat epilepsy CHCH2
CH2
C
O
HO CH
NH3
CH2
Tet = Tetrahedral geometry
TP = Trigonal Planar geometry
Tet
TP
Tet Tet Tet
Tet
TP
TP
