chapter 3 chem

chapter 3 chem - Chapter 3 Molecular Shape and Structure...

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1 Chapter 3 Molecular Shape and Structure Introduction to Stereochemistry and Advanced Bonding Theory Lewis structures were able to provide a 2D picture of molecule; while good, it is incomplete! All molecules can’t be flat! When present, lone pairs must have ‘special’ rules for placement; Large atoms vs. . small atoms: does it make a difference how they are placed? ALL OF THESE HAD TO BE RESOLVED Introduction to Structure 3-1
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2 Consider CH 2 Cl 2 : if flat, TWO possible isomers must exist: The Dichloromethane Dilemma 3-2 C Cl H Cl H C H H Cl Cl cis trans HOWEVER, only ONE CH 2 Cl 2 structure is seen! V alence S hell E lectron P air R epulsion Theory • VSEPR adds 3D component that Lewis lacked • Based on repulsion of electrons and groups ( X ) around central atom ( A ); Thesis : items ( X ’s) around A will try to get as far from one another as possible; • Both multiple bonds and unshared electrons are treated the same as single bonds to arrive at 3D structure 3-3
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3 VSEPR: Electronic Geometry AX 2 AX 3 AX 4 AX 5 AX 6 3-4 VSEPR: Electronic Geometry (examples) 3-5
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4 VSEPR: Molecular Geometry (Shape) • VSEPR provides 3D component based on Lewis structures; • Electronic geometry ( aka VSEPR geometry) based on AX n ; • If central atom has lone pairs, structure observed will differ from predicted (lp’s can’t be seen); • Observed structure, the MOLECULAR GEOMETRY or SHAPE , based on VSEPR geometry. 3-6 VSEPR: Molecular Geometry (Shape) 2 From AX 3 3 From AX 4 4 From AX 5 5 From AX 6 1 From AX 2 1 2,3 2 3 3 4 4 5 5 5 4 3-7
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5 VSEPR: Molecular Geometry (Examples) 3-8 Lewis Structure Electronic Geometry Shape 3-9
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6 Lone Pair Effects Unshared electrons, since not involved in a bond, can be thought of as “taking up more space” than a bonding pair of electrons; this leads to bond angle anomalies: 3-10 For repulsions: lp-lp > lp-bp > bp-bp Cl C Cl Cl Cl Theory: 109.5 Actual: 109.5 H N H H Theory: 109.5 Actual: 107 H O H Theory: 109.5 Actual: 104.5 lp-lp Repulsion = angle expansion lp-bp Repulsion = angle compression Lone Pair Effects: The Strange Case of AX 5 Note that trigonal bipyramid has TWO different sets of bond angles and two different sets of positions; because of this, positions will have different energies 3-11 X A X X X X axial axial equitorial equitorial equitorial So: WHERE do lone pairs go?
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