ORGANIC MOLECULE GEOMETRY

Molecular Geometry

Molecular Geometry Types ||| Types II
 Elmhurst College
Alkanes Alkynes  Optical or Chiral  Chemistry Department
Alkenes ||| Cis / Trans Alkenes All Functional Groups Rings  Virtual ChemBook

 

GEOMETRY OF SIMPLE ALKANE HYDROCARBONS  


As soon as more complex molecules are encountered, more complex molecular geometries result. In these cases each atom must be examined as a center for a particular geometry. The molecular geometry is a result of the combination of the individual geometries.

If the formula of the compound is given, then count the number of atoms attached to each carbon and the type of bonds - single, double, or triple. The geometry can be easily determined:
4 atoms = tetrahedral
3 atoms and 1 double bond = trigonal planar
2 atoms and 1 triple bond = linear

Alkanes - 4 atoms = tetrahedral

Methane is the simplest alkane example and shows that four hydrogens bonded to carbon give a tetrahedral geometry about the carbon.

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Example - Ethane:

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What is the geometry for each carbon in CH3-CH3? Draw a 3-dimensional
(3-D) representation.

Solution:
Carbon #1 has 4 atoms attached to it (3 H and 1 C) and carbon #2 has 4 atoms attached to it. Therefore, around each carbon will be tetrahedral . Since 3-D drawings are hard to make on a flat surface as shown by the easy drawings on the upper and middle left. These do not really tell you anything about the 3-D qualities of the molecule.


To make a 3-D drawing on paper, the molecule on the lower left is a good representation. Bonds which "stick" out in front or behind the plane of the paper are indicated with "wedges" or dashed lines. "Thin" lines are in the plane of the paper.

Finally look at the ball and stick model to see how the two tetrahederals centered on each carbon overlap each other.

 

 Example:

What is the geometry for each carbon in CH3CH2CH3?

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Draw a 3-D representation.

Solution:
All single bonds are present, therefore each carbon is the center of tetrahedral geometry. To draw the molecule start with C # 2 as the center of a tetrahedron. Add carbons # 1 and 3 as part of the main tetrahedron. Finish with hydrogens on all carbons as further centers of two more tetrahedrons.