Chem 161-2010 Lecture 20

Chem 161-2010 Lecture 20 - CHEMISTRY 161-2010 LECTURE 20...

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CHEMISTRY 161-2010 LECTURE 20 ANNOUNCEMENTS ATTENDANCE QUIZZES EXAMS/QUIZZES Surprise quiz? THANKSGIVING WEEK Regular class Tuesday, Nov. 23 rd No class Friday, Nov. 26 th Chem 161-2010 Lecture 20 1
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PLAN FOR TODAY : CHAPTER 10 MOLECULAR STRUCTURE (VSEPR) - Bond angles - Polarity of molecules Chem 161-2010 Lecture 20 2
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VSEPR Valence Shell Electron Pair Repulsion Electron pairs surrounding a central atom keep as far away from each other as possible in order to minimize electron repulsion. Lewis structures provide two-dimensional geometry. VSEPR provides three-dimensional geometry, including molecular polarity. Similar molecular shapes often govern chemical and biological activity. e.g., Sulfanilamide and para-aminobenzoic acid: both antibiotics e.g., Colgate-Palmolive research Triclosan 2-Isopropoxyphenol: model compound for triclosan Chem 161-2010 Lecture 20 3 OH O Cl Cl Cl OH O
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A Strategy for Applying the VSEPR Method (1) Draw a Lewis structure of the molecule or polyatomic ion. (2) Determine the number of effective and non-effective electron groups around the central atom. Effective groups Bonding electron pair (sigma bond) Non-bonding electron pair (lone pair) (Also, non-bonding electron (free radical)) Non-effective groups The π bond of a double bond. The two π bonds of a triple bond. (3) Identify the electron group geometry, based on the number of effective electron groups (4) Identify the molecular geometry, based on the atom positions (not based on the lone-pair electrons Chem 161-2010 Lecture 20 4
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Chem 161-2010 Lecture 20 5
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Apply VSEPR on BeH 2 vs. BH 3 Discuss electron group geometries and bond angles. Apply VSEPR on CH 4 square planar vs. tetrahedral Discuss “effective electron groups” demonstrating almost equal electron group geometry CH 2 -O (single bond) CH 2 - (non-bonding electron group) CH 2 =O (double bond) Single bond is the reference point.
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