chm115_lecture16

chm115_lecture16 - 16-1Chemistry 115Lecture 16...

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Unformatted text preview: 16-1Chemistry 115Lecture 16 OutlineChapter 8Finish Lewis DotsCovalent BondsElectronegativityEXAM II:Thursday, March 108:00pm, Elliott Hall of MusicChapters 5, 6, 7Review Session: Wed 7:30 pm WTHR 200Recitation: ElectronegativityVSEPR, molecular shapesMolecular polarityStill follows octet ruleThe steps in converting a molecular formula into a Lewis structureH -C -C -O -HH HH H••••(Lewis Dots, continued)Note that many larger molecules have more than one central atom. Example, Ethanol: CH3CH2OH.Place carbons, and oxygen next to one another. The hydrogen atoms can only have one bond each.Add multiple bonds as necessary to follow octet ruleResonance StructuresSome molecules have more than one Lewis Dot structure.ORBoth are correct. These are called resonance structures. We use a ↔ to depict this situation. One of the most famous cases is benzene, C6H6. You can convert from one form to the other by moving an electron pair, or a bond.Neither is correct by itself. Instead, the actual molecule is an average of the two:The dotted lines indicate a partial bond. Each bond has a bond order of 3/2. (3 bonded electron pairs/2 bond linkages).What do you do in a case such as ozone O3?••••••O = O – O••••••••••••O – O = O••••••••••O – O – O ••••How about for CO3-2?Each is equally likely.OCO O-2••••••••••••••••O O-2CO••••••••••••••••O O -2CO••••••••••••••••Example 3:Nitrogen dioxide anion,NO2-••••N•5 + 12 + 1 = 1818 - 4 = 14••••••••••••••NOO-NOO••••••••••••-NOO••••••••••••-orWhich one is correct?Both. Electrons are delocalized over the 2 bonds.N-••••O••••O•••••••O•2Insufficient electrons for octet on N. Fix with double bond:-2A B CRules:Smaller FC preferredLike charges on adjacent atoms to be avoidedMore neg. FC should be on more EN atoms+1-1-1So, from FC, first structure (A) not important. Cpreferred over B.In some cases, certain resonance structures are preferred. We can predict this using the formal charge on each atom:The formal charge is given by:#valence e--(# lone pair e-+ 1/2 # bonded e-)Formal Charge••[ N – C ≡ O] -↔ [N = C = O] -↔ [ N ≡ C – O ] -••••••••••••••••••••••[:N – C ≡ O]...
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This note was uploaded on 04/09/2011 for the course CHM 115 taught by Professor Towns during the Spring '08 term at Purdue.

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chm115_lecture16 - 16-1Chemistry 115Lecture 16...

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