Delocalized Electrons – classic example BenzeneBenzene has the formula C6H6 and consists of a six carbon ringstructure σ bonds There are 3 double bonds shown here by the pi orbital overlap.
Problem : there were two possibilities for location ofthe C=C double bondsNeither structure is correct.Studies show all bonds are of equivalent length (but shorterthan a single bond and longer than a double bond).A Lewis structure cannot show the true nature of thismolecule.
pi bond sigma bondFigure A: bonding in benzene with static single anddouble bonds (not the true model)
Delocalization accounts for all the C-C bonds inbenzene being the same length and samestrength. The pi bond electrons “delocalize”over the entire structure instead of above andbelow alternating bonds. It is said that theoriginal structure of Benzene (alternating singleand double bonds) came to Freiderich Kekule ina dream!
delocalized region in pi bonds sigma bondfigure B: delocalized bonding in benzene (the true model)
Electron delocation explains resonance structures –• the bond lengths of CHO2 predicted by the Lewis structure are incorrect (draw it) [ ] O – H C O• 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
Sometimes textbooks refer to “resonance” theory--the idea thatthe double bond flips back and forth. Diagram 1 is the typeused to show the old resonance theory. [ ] – [ ] 1 O O – H C H C O O [ ] O – 2 H C O Delocalization is more widely accepted. Diagram 2 shows how delocalized bonds are often represented.
Practice: Draw the delocalized structure of the carbonate anion
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