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Review:
A HYDROCARBON is a compound containing only hydrogen and carbon

UNBRANCHED (STRAIGHT CHAIN) ALKANES
An Alkane is a hydrocarbon in which all the carbon atoms are connected by
single bonds.

(1)The name of the Alkane end in "ane".
(2)Alkane is also called a "SATURATED".


For example:
tpchem_04_05[1].png


condensed structure:
pentane = C5H12 = CH3- CH2-CH2-CH2-CH3 (CH3CH2CH2CH2CH3)
hexane = C6H14 = CH3-CH2-CH2-CH2-CH2-CH3 (CH3CH2CH2CH2CH2CH3)


Exercise:
Draw the full structure and condensed structure for heptane, octane, nonane and decane.




THE GEOMETRY OF ALKANES




All the bonds have equal lengehs and all the H-C-H angles are 109 degree.


For example:
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.


208methanemg[1].jpg

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.


209ethane[1].gif

What is the geometry for each carbon in CH3CH2CH3?
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.


209propane[1].gif