ERM Ch. 2 Sorrell Su19.ppt - Chapter 2 Bonding in Organic...

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Chapter 2 Bonding in Organic Molecules
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2 Formal charge A formal charge is a positive or negative charge on an individual atom The sum of formal charges on individual atoms is the total charge of the molecule or ion The formal charge is calculated by subtracting the assigned electrons on the atom in the molecule from the electrons in the neutral atom (valence electrons for that particular element minus electrons it actually has) Formal charge is calculated as follows: Note: For each line (bond), we count only 1 electron since the other electron comes from the other atom to which is bonded to Electrons in bonds are evenly split between the two atoms; one to each atom Lone pair electrons belong to the atom itself
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3 Examples Ammonium ion (NH 4 ) + Nitrate ion (NO 3 ) -
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4 An atom will always have the same formal charge depending on how many bonds and lone pairs it has regardless of which particular molecule it is in For example a singly bonded oxygen with 3 lone pairs will always have a negative charge and an oxygen with three bonds and one lone pair will always have a positive charge Knowing these forms of each atom is invaluable in drawing Lewis structures correctly and rapidly Summary of Formal Charges
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5 Resonance For many molecules and ions, no single Lewis structure provides a truly accurate representation The real structure for carbonate ion is not represented by any of the structures 1,2 or 3 Experimentally carbonate is known not to have two carbon-oxygen single bonds and one double bond; all bonds are equal in length (1.28 Å) and the charge is spread equally over all three oxygens Average Bond Lengths for atoms in organic molecules
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6 Resonance structures only exist on paper. The real structure is a resonance hybrid or mixture of all three Lewis structures Resonance structures are useful because they allow us to describe molecules and ions for which a single Lewis structure is inadequate Double headed arrows are used to show that the three Lewis structures are resonance contributors to the true structure The use of equilibrium arrows is incorrect since the three structures do not equilibrate ; the true structure is a hybrid (average) of all three Lewis structures One resonance contributor is converted to another by the use of curved arrows which show the movement of electrons A calculated electrostatic potential map of carbonate clearly shows the electron density is spread equally among the three oxygens
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7 The energy of the actual molecule is lower than the energy of any single contributing form The lowering of energy is called resonance stabilization Equivalent resonance forms make equal contributions to the structure of the real molecule Structures with equivalent resonance forms tend to be greatly stabilized Each carbon-carbon bond is 1.39 Å, which is between the length of a carbon- carbon single bond between sp 2 carbons (1.47Å) and a carbon-carbon double bond (1.33 Å)
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