2012 HL Delocalization of Electrons

1. LESSON Delocalization IB Chemistry Power Points Topic 14 (AHL) Bonding www.pedagogics.ca

2. Delocalized Electrons – classic example Benzene Benzene has the formula C 6 H 6 and consists of a six carbon ring structure There are 3 double bonds shown here by the pi orbital overlap. σ bonds

3. Problem : there were two possibilities for location of the C=C double bonds Neither structure is correct. Studies show all bonds are of equivalent length (but shorter than a single bond and longer than a double bond). A Lewis structure cannot show the true nature of this molecule.

4. sigma bond pi bond F igure A: bonding in benzene with static single and double bonds (not the true model)

5. Delocalization accounts for all the C-C bonds in benzene being the same length and same strength. The pi bond electrons “delocalize” over the entire structure instead of above and below alternating bonds. It is said that the original structure of Benzene (alternating single and double bonds) came to Freiderich Kekule in a dream!

6. sigma bond delocalized region in pi bonds figure B: delocalized bonding in benzene (the true model)

7. the bond lengths of CHO 2 – predicted by the Lewis structure are incorrect (draw it) The double CO bond should be shorter, and possess a greater bond energy (due to the higher concentration of electrons in a double bond) Yet, experimentally, both bonds are the same Answer . . . Delocalization: the pi bond spreads out over both C-O bonds H C O O – [ ] Electron delocation explains resonance structures

8. Sometimes textbooks refer to “resonance” theory--the idea that the double bond flips back and forth. Diagram 1 is the type used to show the old resonance theory. 1 2 H C O O – [ ] H C – O O [ ] H C O – [ ] O Delocalization is more widely accepted. Diagram 2 shows how delocalized bonds are often represented.

9. Practice: Draw the delocalized structure of the carbonate anion