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Lecture 12

CHEM H90 Lecture 12: Molecular Shapes and Organic Chemistry (04/28/17)

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LECTURE 12 Thursday, April 28, 2017 Molecular Shapes and Organic Chemistry Condensed structural formulas • Some molecular formulas could have more than one Lewis structure o Several arrangements that all make sense and work o Different molecules drawn in different ways • A condensed structural formula tells you which Lewis structure to draw (better than a chemical formula) o Instead of the pure chemical formula, write the structural formula o (e.g.) C3 4 ▪ CH 2CH 2 ▪ CH 3CH Line structures in organic chem • Once you know the carbon backbone, the hydrogens fall into place o Each corner or end of a line is a carbon atom o Hydrogens bonded to carbon are not shown, but it is assumed that there are enough of them to make sure each carbon has 4 bonds Line structures (molecular forms) • Figure out where the carbons and hydrogens are Bee pheromones • Line structures make it easy to see small differences between similar molecules o Queen bee v. Worker bee • A small difference in the structure may lead to a large difference in biological activity Dyes • Napthoquinone (colorless) • Juglone (brown) o From walnuts • Henna (reddish-orange) o Used at weddings • The chemical formula for juglone and henna are exactly the same, so it’s important to know where the OH is Shapes of molecules • Lewis structures are useful for keeping track of the number and placement of electrons in a molecule o They do not tell us anything about the geometry • You need to figure out how they’re connected, where the bonds are • Many of the interesting properties of water result from its shape and cannot be predicted from the Lewis structure VSEPR theory • Valence Shell Electron Pair Repulsion (Electron domain model) o Why a molecule is like a junior high dance • Because electrons are negatively charged, they repel each other • In a molecule, bonds and lone pairs stay as far apart as possible o Tetrahedron angle = 109.5 degree angle, as big as possible VSEPR theory – RULES • The arrangement of bonds and lone pairs around the central atom is controlled by electrostatic repulsion o Arrange all bonds and lone pairs as far apart as possible o Double and triple bonds can be treated approximately like single bonds ▪ Count them all, that’s the steric number, and determines basic shape o If a molecules has resonance structures, VSEPR can be applied to all of them o Number of attachments to central atom = steric number (SN) Perfect shapes • The basic shapes determined by this principle Angles of perfect shapes • Linear • Trigonal planar o A triangle in a plane • Tetrahedral o Four with 109.5 degrees • Trigonal bipyramidal o It’s like two pyramids, one on top of another • Octahedral Distortions • Lone pairs (or double bonds) have more elect
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