Structure

CH 106 - Lesson 2

Structure

The important structural feature of an alkene is the double bond. You should remember that a double bond consists of a sigma bond and a pi bond.

This diagram should remind you that the electrons in the pi bond are not located between the two carbon atoms but off to both sides of a line joining the carbon atoms. Because these electrons stick out, the pi bonds are generally very reactive. We will deal with that reactivity later.

The molecule shown here happens to be ethene. The red cloudy regions shown on the left are the hybrid orbitals of the carbon atoms overlapping with one another and with the orbitals of hydrogen to form sigma bonds. In the middle, the molecule with its "red" sigma bonds has been turned so that the carbon and hydrogen atoms are in a horizontal plane. You can also see the "grey" p orbitals for the carbon atoms which have not yet been shown to have formed a bond.

On the right the grey shaded regions now show how there is a pi bond between those two carbon atoms which has resulted from the overlap of two p orbitals. This sequence from left to right is intended to show you what happens as the pi bond forms.

Again, make note of how the electrons in that pi bond (the grey shaded region) are quite a bit more accessible to other chemicals than are the electrons that are in the red shaded regions which represent the sigma bonds.

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E-mail instructor: Eden Francis

Clackamas Community College
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