Chapter02&03 - 2.4 Resonance CHM 151 Background for...

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1 1 CHM 151 – Background for organic chemistry Based on parts of Ch 2 and Ch 3, McMurry., “Organic Chemistry” Ronald Kluger, University of Toronto 2 2.4 Resonance ± Some molecules are have structures that cannot be shown with a single representation ± In these cases we draw structures that contribute to the final structure but which differ in the position of the π bond(s) or lone pair(s) ± Such a structure is delocalized and to is represented by resonance forms (based on the concept of resonant waves) ± The resonance forms are connected by a double-headed arrow 3 Resonance Hybrids ± A structure with resonance forms does not alternate between the forms ± Instead, it is a hybrid of the two resonance forms, so the structure is called a resonance hybrid ± For example, benzene (C 6 H 6 ) has two resonance forms with alternating double and single bonds ± In the resonance hybrid, the actual structure, all its C-C bonds equivalent, midway between double and single 4 2.5 Rules for Resonance Forms ± Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure) ± Resonance forms differ only in the placement of their π or nonbonding electrons ± Different resonance forms of a substance don’t have to be equivalent ± Resonance forms must be valid Lewis structures: the octet rule applies ± The resonance hybrid is more stable than any individual resonance form would be
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2 5 Curved Arrows and Resonance Forms ± Imagine that electrons move in pairs to convert from one resonance form to another (they really don’t) ± A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow 6 2.6 Drawing Resonance Forms ± Any three-atom grouping with a multiple bond has two resonance forms 7 Different Atoms in Resonance Forms ± Sometimes resonance forms involve different atom types as well as locations ± Illustration: The “enolate” derived from acetone 8 2,4-Pentanedione ± The anion derived from 2,4-pentanedione ± Three resonance structures result
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3 9 2.7 Acids and Bases: The Brønsted– Lowry Definition ± Arrhenius definition – an acid makes a solution acidic, a base makes it basic ± Not relate to reactions or chemical potential ± Dependent only on concentration ± Brønsted–Lowry definitions – acids are proton (H + ) donors and bases are acceptors ± Refers to the potential to accept or donate 10 Brønsted Acids and Bases ± An acid is a substance that donates a proton to a base (Brønsted, 1923) ± A Brønsted base is a substance that accepts the H + from an acid ± This concept is useful within the principles and applications of thermodynamics His firm opposition to Nazism during World War II won him election to the Danish Parliament (1947), but illness prevented him from taking his seat.
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This note was uploaded on 04/18/2011 for the course CHM 151 taught by Professor Dong during the Winter '08 term at University of Toronto- Toronto.

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Chapter02&03 - 2.4 Resonance CHM 151 Background for...

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