ORGANIC MOLECULE GEOMETRY

Molecular Geometry

Molecular Geometry Types ||| Types II
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 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.

 Methane

 All single bonds

Graphic
 Alkene

 2 single, 1 double

Graphic
 Alkyne

 1 single, 1 triple

Graphic
 Alcohol

 All single

Graphic
 Ether

 All single

Graphic
 Aldedyde

 1 double

Graphic