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 ALKYNE HYDROCARBONS  


Alkynes, containing a carbon-carbon triple bond, have the linear or straight geometry as a primary feature.

A portion of the molecule is in a straight line and really only 1-dimensional.

Alkynes - 2 atoms and 1 triple bond = linear

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Ethyne or acetylene, HC=CH, is the simplest alkyne example.

Since a triple bond is present and each carbon is attached to 2 atoms (1 H and 1 C), the geometry is linear. This linear feature extends through a total of four atoms.

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 - propyne:

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

Solution:
Carbons #1 and 2 both have only 2 other atoms attached and one triple bond, therefore both are centers of linear 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 4-methyl-4-ene-2-hexyne 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
C # 6
C # 7