| |
Organic Molecular
Geometry
|
Introduction:
The specific three dimensional arrangement of atoms in molecules
is referred to as molecular geometry. Organic molecular
geometry is really just an application of the principles of simple
molecular geometry applied to molecules with many atoms.
There are various instrumental techniques such as X-Ray crystallography
and other experimental techniques which can be used to tell us
where the atoms are located in a molecule. Using advanced techniques,
very complicated structures for carbohydrates, proteins, enzymes,
DNA, and RNA have been determined. Molecular geometry is associated
with the chemistry of vision, smell and odors, taste, drug reactions
and enzyme controlled reactions to name a few.
|
| Molecular geometry is associated with the specific
orientation of bonding atoms. For organic molecules, we will
observe the same types of geometry
- linear, trigonal planar, tetrahedral, trigonal pyramid, and
bent. When a molecule consists of many atoms, each carbon, oxygen,
or nitrogen atom may be the center of the one of the geometries
previously listed. The molecule as a whole will be the sum of
all of the individual geometries to give an overall shape to
the molecule. |
|
ELECTRON AND BOND NUMBERS
|
Element |
Number of outer shell electrons |
Number of electrons to complete octet |
Number of bonds to other elements |
|
C |
4 |
4 |
4 |
|
N |
5 |
3 |
3 |
|
O |
6 |
2 |
2 |
|
S |
6 |
2 |
2 |
|
H |
1 |
1* |
1 |
Halogens
(F, Cl, Br, I) |
7 |
1 |
1 |
| |
|
*"octet" for H =
2 |
|
|
|
BONDING RULES:
Organic compounds consist mainly of the following elements:
carbon, hydrogen, oxygen, sulfur, halogens. The structures of
organic compounds are determined by the specific arrangements
of the above elements by simple rules of bonding.
First, consider how many outer shell electrons are present
in each of the above elements. See table on the left.
The number of outer shell electrons determines the number
of electrons needed to complete the octet of eight electrons.
Elements in organic compounds are joined by covalent bonds
- a sharing of electrons; each element contributes one electron
to the bond. The number of electrons necessary to complete the
octet determines the number of electrons that must be contributed
and shared by a different element in a bond. This analysis finally
determines the number of bonds which each element may enter into
with other elements.
|
| MEMORIZE the number of bonds that each element
may have. A knowledge of these numbers enables you to write and
comprehend organic structures. |
|
TYPES OF BONDING ARRANGEMENTS
|
Element |
Number of bonds |
All single bonds |
Other bond combinations |
|
C |
4 |
4 |
2 single, 1 double
1 single, 1 triple
|
|
N |
3 |
3 |
4 bonds w/ a positive charge |
|
O |
2 |
2 |
1 double |
|
S |
2 |
2 |
1 double |
|
H |
1 |
1* |
- |
Halogens
(F, Cl, Br, I) |
1 |
1 |
- |
|
|
VARIETY OF BONDING ARRANGEMENTS:
By knowing the number of bonds that are possible, then a number
of bonding arrangements are possible as listed in the table on
the left.
|
|
