lecture08_s05

lecture08_s05 - 5.12 Lecture 8 VII. Overview of Chemical...

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Unformatted text preview: 5.12 Lecture 8 VII. Overview of Chemical Reactions A. Chemical Reactions B. Classifications C. Radical Mechanism D. Polar Mechanism 1. Nuts and Bolts 2. Nucleophile 3. Electrophile 4. Describing Polar Reactions a. Arrow Pushing b. Molecular Orbital Suggested Reading: Chapter 5 Suggested Problems: 5.24-5.33, 5.39-5.47 5. Example A. Chemical Reactions What: Changes in electron configuration Bonds broken/bonds formed Why: Attain a stable state High energy structures spontaneously react Lower energy structures need external energy to react How: Collisions (between atoms, molecules, ions) For a collision to result in a reaction, the molecule must have: 1) sufficient kinetic energy 2) proper orientation 1 B. Classifications of Reactions 1. Overall Tranformation (relationship of reactant to product) a. Addition H A+B C H Br H H H base HH c. Substitution HCCH HH HCCH B + C A-B + C-D H H Br b. Elimination A H H Br H H H Br H A-C + B-D H Cl Cl HCH H C Cl H H Cl H H3CH2C H H d. Rearrangement light H acid cat. H3C H H CH3 B. Classification of Reactions 2. Mechanism • • a. overall description of how a reaction occurs (changes in elec. configuration) which bonds are broken and formed Free Radical Movement of one electron, usually proceeds by chain reaction b. Polar Movement of electron pairs from areas of high electron density (nucleophile) to areas of low electron density (electrophile) c. Pericyclic Movement of electrons in a cyclic fashion d. Metal-mediated and Metal-catalyzed (need transition metal) (If the transition metal complexes act only as Lewis acids or 1 electron transfer agents, the reactions are better classified as polar, pericyclic or radical) 2 Ways to Form and Break Covalent Bonds: 1. Symmetrical - 1 electron goes to each atom in bond Homolytic cleavage: A Homogenic formation: A A B + B + B AB Use fishhook arrows! 2. Unsymmetrical - Both electrons go to one of the atoms in the bond Heterolytic cleavage: A Heterogenic formation: A + A B B + B AB Use double-headed arrows! Nucleophiles and Electrophiles Involved in polar reaction bond formation donate Nucleophiles ______ electrons. Identifying nucleophiles •Lone pairs •(-) charges •� bonds Nucleophilic atom either starts neutral and becomes positive, or starts negative and becomes neutral Electrophiles accept electrons. Identifying electrophiles •Empty orbitals (incomplete octet) •(+) charges •� bonds (polarized) Electrophilic atom either starts neutral and becomes negative, or starts positive and becomes neutral Courtesy of Jeffrey S. Moore, Department of Chemistry, University of Illinois at Urbana-Champaign. Used with permission. Adapted by Kimberly Berkowski. 3 Still Having Trouble Identifying Nu: and E+? 1. Look for regions of electron density and electron deficiency 2. Draw in all lone pairs 3. Draw as many resonance structures as you can (often, the 2nd best resonance structure shows the electrophilic and nucleophilic sites in a molecule) The terms “nucleophile” and “electrophile” can mean the entire molecule or specific atoms and functional groups. Don’t let the dual meaning confuse you! A Few Notes About Electron Pushing • A curved arrow shows the “movement” of an electron pair source • The tail of the arrow shows the ______ of the electron pair, which is a ____ orbital. This will be a: filled • lone pair • p-bond • s-bond • The head of the arrow indicates the destination (sink) of the ______________ electron pair which will be: • An electronegative atom able to support negative charge empty • An ______ orbital when a new bond is formed • Overall charge is conserved. Check that your products obey this rule. Courtesy of Jeffrey S. Moore, Department of Chemistry, University of Illinois at Urbana-Champaign. Used with permission. Adapted by Kimberly Berkowski. 4 Electron Pushing to Uncharged C, H, N, or O If you make a new bond by pushing an arrow to an uncharged C, H, N, or O, you must also break one of the existing bonds in the same step. Courtesy of Jeffrey S. Moore, Department of Chemistry, University of Illinois at Urbana-Champaign. Used with permission. Adapted by Kimberly Berkowski. Addition of Nucleophiles to Electrophiles “Arrow Pushing” Description ved cur ow arr lone pair empty orbital n tio eac ow r arr new bond Molecular Orbital Description ing ond ion ib ant eract int filled orbital new MOs empty orbital ing ond ction ba r inte Courtesy of Jeffrey S. Moore, Department of Chemistry, University of Illinois at Urbana-Champaign. Used with permission. Adapted by Kimberly Berkowski. 5 Molecular Orbital Diagram: H2 H-H Antibonding MO (unfilled) s* H 1s orbital H 1s orbital s H-H Bonding MO (filled) Constructive (+/+) or (-/-) combination of atomic orbitals; lower energy MO Diagrams: Pi Bonding (p) py + py p* Energy Energy node Destructive (+/-) combination of atomic orbitals; higher energy py py p 6 Orbitals of Nucleophiles and Electrophiles Nucleophiles Electrophiles 1. Filled nonbonding orbital n 1. Empty nonbonding atomic orbital a Nu 2. Filled p bonding orbital 2. Empty pi antibonding orbital C p 3. Filled s bonding orbital C O p* C s C C 3. Empty sigma antibonding orbital s* M C X Filled Nonbonding + Empty Nonbonding n+a CH3 CH3 Br H3C C Br CH3 C CH3 CH3 CH3 Br H3C n C CH3 a 7 Filled Nonbonding + Sigma Antibonding n + s* H HO C H H H HO H C Br H H C HH HO Br Br s* n Filled Nonbonding + Pi Antibonding n + p* O O C NC NCC H H H H O NC C H H n p* 8 Pi Bonding + Empty Nonbonding p+a CH3 H3C H CH3 CH3 H3C H C H3C CC CH3 CH3 HH CH3 CH3 CH3 C C H H CH3 H3C C C H3C CH3 a p Pi bonding + Sigma Antibonding p + s* H3C CH3 CH3 H3C C D Cl H CD Cl HH H H3C CH3 C D C H Cl H p s* 9 Sigma Bonding + Empty Nonbonding and s* s+a s + s* H3C H H H C CC H3 CH H 3 HH H H hyperconjugation H H3C H H H3C H3C s H C H a CH H s H s* 10 ...
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This note was uploaded on 03/02/2012 for the course CHEM 512 at MIT.

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