Section_2.3_Resonance_post

Section_2.3_Resonance_post - Resonance As seen in the...

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1 As seen in the previous examples you may be able to draw molecules in which there are different combinations of bonding. Which is the correct structure of the carbonate ion (CO 3 2- )? You can draw the molecule three different ways: O O O O O O O O O Experimentally carbonate does not to have two single carbon-oxygen bonds and one double carbon-oxygen bond. In fact, all of the bonds are equal in length and the charge is spread equally over all all three oxygens. The real structure is a resonance hybrid or a mixture of all three resonance structures A calculated electrostatic potential map: areas which are red are more negatively charged while areas that are blue have relatively less electron density O C O O 2 3 2 3 2 3 Resonance Sec. 2.3 - Resonance
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2 Resonance structures represent the true structure of a molecule. One resonance contributor is converted to another by the use of curly arrows which show the flow of electron density in a resonance structure. When we use arrows later in reaction mechanisms, the arrows will show us the movement of electrons between molecules. The use of these arrows serves as an electron bookkeeping device and to help us think about where the electrons can possible flow. Tail-end shows where the electrons are coming from Front-end shows where the electrons are going to A double headed arrow shows the movement of two electrons A single headed arrow shows the movement of one electron (used for radical reations) Resonance Sec. 2.3 - Resonance
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3 Rules for Resonance For this structure to be resonance two hydrogens had to move: had to break two C-H sigma bonds How did the electrons move in these resonance structures? + + + H 3 C C C H H C H H H 3 C C C H H C H H + + Only electrons can move between resonance structures The position of the atoms does not change!! The only electrons that can participate in resonance are non-bonding electrons (lone pairs) and pi electrons (double or triple bonds) Sigma bonds cannot participate in resonance (cannot break sigma bonds) Sec. 2.3 - Resonance
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4 All structures must be proper Lewis structures What is wrong with following resonance structures? Rules for Resonance + - Carbon has 5 bonds!! Nitrogen has 4 bonds and 1 lone pair of electrons When drawing resonance structures remember the following: You cannot exceed the octet rule for elements in rows 1 and 2 (8 electrons max) The OVERALL charge of each resonance structure MUST be the SAME!! THESE ARE NOT PROPER LEWIS STRUCTURES!!
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This note was uploaded on 11/14/2010 for the course CHEM CHEM266 taught by Professor Forsey during the Fall '10 term at Waterloo.

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Section_2.3_Resonance_post - Resonance As seen in the...

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