Lecture #12
 
  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Molecular Structure

Exceptions to the Octet Rule

Odd electron numbers

Expanded octets (hypervalency)

Examples

Resonance

Equivalent preferred contributors

Bond order
Another exception to the octet rule occurs in molecules that have an odd number of electrons. You can't get an even number (eight) working with an odd number.
There is also the possibility of having more than eight electrons around an atom once the d-levels become accessible. This happens in the third row of the periodic table, most significantly for elements with Z greater than 13. (The term "hypervalency" is sometimes used to describe this effect.)
Benzene is the cyclic hydrocarbon C6H6. The two Lewis structures drawn here look to be structural isomers. On the left, the chlorines are adjacent to a double bond. On the right, they are adjacent to a single C-C bond. Yet only one isomer exists. Why? The answer lies in the phenomenon called "resonance."
We could estimate the energy involved in the reaction shown by looking at bonds broken and bonds formed during the electrons' rearrangements. (These energies are called reaction heats and will be the subject of further discussion in the next lecture.)
But the value obtained differs significantly fromwhat is actually measured. Why? ..."Resonance!"
Despite a Lewis structure indicating alternating single and double bonds...alternating long and short carbon-carbon separation distances, all C-C bond lengths are identical. Why?..."Resonance!"
What, then, is resonance?
Benzene is the cyclic hydrocarbon C6H6. The two Lewis structures drawn here look to be structural isomers. On the left, the chlorines are adjacent to a double bond. On the right, they are adjacent to a single C-C bond. Yet only one isomer exists. Why? The answer lies in the phenomenon called "resonance."
We could estimate the energy involved in the reaction shown by looking at bonds broken and bonds formed during the electrons' rearrangements. (These energies are called reaction heats and will be the subject of further discussion in the next lecture.)
But the value obtained differs significantly fromwhat is actually measured. Why? ..."Resonance!"
Despite a Lewis structure indicating alternating single and double bonds...alternating long and short carbon-carbon separation distances, all C-C bond lengths are identical. Why?..."Resonance!"
What, then, is resonance?
Resonance is condition associated with two or more arrangements of valence electrons. We will be concerned with those that give equivalently preferred Lewis structures.
 First illustration of resonance among equivalent Lewis structures. The Lewis structure is represented by this mixture; not by the three arrangements, but by a single arrangement of electrons too complicated to be drawn on one picture, and so we resort to symbolizing that one picture as a superposition of three pictures we can draw.  
Previously, we had single, double, and triple bonds corresponding to bond orders of one, two and three, respectively. But with resonance structures, we need a modification of our definition of bond order to accommodate intermediate possibilities. Here we illustrate resonance structures and bond order using ozone (O3) as an example.
The carbon-oxygen bond order in the carbonate ion is 1.33 for each of the three CO bonds.
 For benzene, the carbon-carbon bond order is exactly halfway between a pure single bond and a pure double bond.  
Any reference to a complete, preferred Lewis structure will imply that formal charges must be indicated where necessary and all resonance contributions of equivalently preferred structures are needed as well.
The nitrate ion preferred structure.
Illustrating what is meant by equivalent structures using an example where structures are not equivalent.
This is not a pair of resonance structures for CH3CHO. Check the rule for resonance structures several slides back.