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 ALKENE HYDROCARBONS  


Alkenes, containing a carbon-carbon double bond, have the trigonal planar geometry as a primary feature.

A portion of the molecule is flat and only 2-dimensional.

Alkenes - 3 atoms and 1 double bond = trigonal planar

Ethene or ethylene, H2C=CH2, is the simplest alkene example.

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Since a double bond is present and each carbon is attached to 3 atoms (2 H and 1 C), the geometry is trigonal planar. Two overlapping triangles are present since each carbon is the center of a planar triangle.

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

 Example - propene:

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What is the geometry for each carbon in CH2=CHCH3? Draw a 3-D representation.

Solution:
Carbons #1 and 2 both have only 3 other atoms attached and one double bond, therefore both are centers of trigonal planar geometry. Carbon # 3 has 4 other atoms attached and all single bonds, therefore it is the center of a tetrahedron.

 

Quiz:

What is the geometry for each carbon in 2-methyl-2-butene shown on the left?

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Draw a 3-D representation. Then check the solution using the pulldown menu.

Solution:
C # 1
C # 2
C # 3
C # 4
C # 5