Recall that the trigonal planar molecular structure
had an unhybridized p orbital on the central atom. The remaining p orbitals on the central atom
hybridized with the s orbital to form three sp2 hybridized orbitals.
In molecules with double bonds, there are two atoms that similarly an unhybridized p orbital
and three sp2 hybridized orbitals. The unhybridized p orbitals
on each of the two atoms is a critical component in the formation of a double bond
between those atoms.
In the following 3D applet, there are two central white atoms that are bonded to each other.
Two separate sets of red atoms are bonded to each of the central white atoms. Note that the molecular
structure around each of the central white atoms resembles the
trigonal planar molecular structure.
Molecules with double bonds are described in Chapter 10 in your textbook.
Place the cursor over the molecule, press and hold the left button down, and move the mouse.
You should be able to spin the molecule in any direction, and examine the orientation of
the atoms. (If the image looks weird, like it is only partially shown or it looks like a double image,
click the refresh button of your browser. It is at the top in the toolbar.)
Convince yourself that all of the atoms, whether they are white or red, all lie in the
same geometric plane.
Is this consistent or inconsistent with the trigonal planar molecular structure?
The following 3D applet shows the same molecule with the unhybridized p orbitals on the central white atoms.
Ethylene, CH2CH2, is the simplest molecule with a double bond between the central atoms. Recall
in the trigonal planar molecular structure, boron was the central atom, and it did
not obey the octet rule.
In contrast, the carbon atoms in CH2CH2 obey the octet rule.
How many electrons from each central carbon atom would occupy the unhybridized p orbitals?
Hint, think Lewis formulas.
Note that unhybridized p orbitals are drawn in such a way that they are touching one other. They are
said to overlap each other. The electrons that occupy the unhybridized p orbitals on each central atom
are shared between the two p orbitals.
The type of bond that is formed by the sideways overlap of two unhybridized p orbitals on adjacent atoms
is called a pi bond.
There is also the bond shown by the blue line between the two central white atoms. This bond
is a sigma bond. (The bonds depicted by the blue lines between the white atoms and the
red atoms are also sigma bonds.)
What is the hybrid of the orbital that is depicted by the blue lines?
Hint, what hybridized orbitals were the three bonds in the
trigonal planar molecular structure.
A double bond consists of one sigma bond and one pi bond.
Rotate the molecule in the 3D applet.
Convince yourself that the sigma bond between the two central white atoms lie in the same
geometric plane as all the atoms.
Convince yourself that the pi bond has two lobes that are formed by the overlap of the
unhybridized p orbitals. One lobe is above the geometric plane of the atoms and the other
is below.
Which lobe do the electrons occupy in the pi bond? Hint, some students
consider this a trick question.
URL: http://www.rretc.com/chemistry/double_bonds.html
Copyright (c) 2006 with Michael Keyes (michael.keyes@rretc.com)
Feel free to use for single-user, educational purposes only. All other purposes, please email.