Lecture 2

Lecture 2 - Resonance! For many molecules and ions, no...

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1-1 Resonance For many molecules and ions, no single Lewis structure provides a truly accurate representation Ethanoate ion (acetate ion) C O O H 3 C C O O H 3 C - - and + + Formiminium ion and
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1-2 Resonance Linus Pauling - 1930s many molecules and ions are best described by writing two or more Lewis structures individual Lewis structures are called contributing structures connect individual contributing structures by double- headed (resonance) arrows: the real structure is a hybrid of contributing resonance structures resonance allows formal delocalization of electrical charge : - a molecule represented by resonance structures is more stable than its fictional resonance contributors
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1-3 Resonance Examples: Acetate ion (equivalent contributing structures) Nitrite ion (equivalent contributing structures) : : : : N O - O : N O O - : : : : : : : : C O - O CH 3 C O O - CH 3 : : Formiminium ion
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1-4 Curved Arrow Formalism Movement of electrons can be shown with curved-arrow formalism from electron donor to electron acceptor Curved arrow: a symbol used to show the redistribution of valence electrons In using curved arrows, there are only two allowed types of electron redistribution: from a bond to an adjacent atom from an atom to an adjacent bond Electron pushing is a survival skill in organic chemistry: learn it well! X Y + X Y
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1-5 Curved Arrow Formalism Question: Which of the following curved arrow schemes accounts correctly for the difference between the two resonance structures? A B C D H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H
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1-6 Curved Arrow Formalism Answer: A B C D H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H H 2 C C H O H 2 C C O H
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1-7 Resonance Key Rules: 1. Resonance contributors must be valid Lewis structures 2. Only electrons can be delocalized (nuclei do not move, and bond angles remain the same) H C H C H C H H H H C H C H C H H H H C H C O H H H C H C O H H H C H C O H H H C H C O H H H C H C H C H H H C H C H C H H
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1-8 Resonance The nitrate ion, for example, is a hybrid of three equivalent contributing structures All the bond lengths are the same (neither single, nor double bond character). the negative charge is distributed equally among the three oxygens (-2/3 electrical charge per O) N O O O _ _ N O O O _ N O O O N O O O + 2/3 – 2/3 – 2/3 =>
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1-9 Resonance The formiminium ion is a hybrid of two nonequivalent contributing structures major contributor minor contributor, carbon does not have octet
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1-10 Resonance Preference 1: Flled valence shells structures in which all atoms have Flled valence shells contribute more than those with one or more unFlled valence shells
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1-11 Resonance Preference 2: maximum number of covalent bonds structures with a greater number of covalent bonds contribute more than those with fewer covalent bonds
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Lecture 2 - Resonance! For many molecules and ions, no...

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