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resonanceg - Copyright Arizona State University Delocalized...

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Resonance 1 Copyright, Arizona State University Copyright, Arizona State University Resonance Delocalized Electrons (more...) 1 Delocalization Lewis structures assign ALL ELECTRONS to specific bonds or as non-bonding pairs localized on specific atoms . The Lewis structure model is very powerful, but this strict localization of electrons to specific bonds and atoms is sometimes just not correct. To properly describe some structures we need to take into account the fact that some of the electrons are DELOCALIZED, i.e., are not "stuck" on specific atoms or in specific bonds. For some these a SINGLE LEWIS STRUCTURE is INSUFFICIENT • At this point we could go to more sophisticated molecular orbital models of bonding, but these are often too detailed for solve simple organic chemistry problems. Instead, we go to the next "level" of bonding model, one that retains the simple Lewis picture, but expands it to take delocalization into account by describing some structures in terms of a MIXTURE of MORE THAN ONE LEWIS STRUCTURE . This is RESONANCE Q. Which of the following two Lewis structures is correct? (important new terminology and notation alert!) A. Neither are wrong but neither give a proper description of where the electrons are in the molecule , a proper description of CH 3 NO 2 requires a CONTRIBUTION from BOTH LEWIS STRUCTURES • typical N–O bond ~1.3Å and N=O bond ~1.16Å, however, both N–O bonds are same length, ~1.22Å, i.e. "average of the two" • the "actual" structure shows two partial pi bonds and a half negative charge on each oxygen, but the "actual" structure IS NOT a Lewis structure, since Lewis structures can't have "dashed bonds" and electrons that are not specifically associated with a particular atom or a particular bond Origin of Resonance (Electron Delocalization) Phenomenon Example 1: Allyl Cation • Each carbon of the allyl cation (below) is sp2 hybridized • Add the p A.O.s that make the pi-bond and the "empty" p A.O. on the carbon with the positive charge • Which 2 p A.O.s make the pi bond and which is empty? the molecule isn't sure either!! • Neither of these two Lewis structures are, on their own, correct. The correct (actual) structure is a RESONANCE MIXTURE or HYBRID of the two Lewis structures, they are RESONANCE CONTRIBUTORS that when MIXED (hybridized) together properly describe the overall structure Resonance contributors are contained within resonance brackets , and joined by resonance arrows, each resonance contributor is a correct Lewis Structure two resonance CONTRIBUTORS H C H H N O O H C H H N O O H C H H N O O = resonance arrow resonance brackets 1/2 "actual" structure resonance hybrid NOT a Lewis structure 1/2 50% + 50% these 2 make the π -bond why don't these 2 make a π -bond? they do!!
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