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1. understanding reactivity is imperative in optimizing yield and elimination waste 1.1. .The following is an industrial process for making a pesticide precursor by Wacker Chemie, Munich (C &E News July 14, 1997, p51, July 28, 1997, p6). CH 3 OH O CH 2 C O heat CH 2 O O CH 3 OH CH 3 C CH 2 C OCH 3 O O Ca(OH) 2 CH 3 C CH C OCH 3 O-^ O (Ca+2) 0. 5 CH 3 O COCl CH 3 C CH C OCH 3 O O O OCH 3 C CH 2 C OCH 3 O O MeO NH 4 OH H+
- 1.2. Knowing a molecules reactivity (possible reactions) answers the following questions: 1.2.1. What possible side products are formed? 1.2.2. What are the optimum conditions? 1.2.3. Best reagents? 2. What is a reaction? 2.1. transformation of one or more compounds into new compounds A B + C D A C +^ B D 2.2. requires the breaking and/or making of bonds 2.3. Reactions involve **movement of electrons and nuclei.
- Chemistry is about electrons: Understand electrons understand reactivity** 3.1. electron position determines structure 3.1.1. near certain nuclei 3.1.2. orbital occupation 3.2. electron movement determined by orbital properties 3.2.1. energy, size, symmetry, nodes 3.3. nuclei follow electrons 4. Why do electrons move? (Why do reactions occur?) 4.1. predict products based on structure 4.1.1. Understand structure 4.1.2. correlate structure with reactivity
4.1.3. for example, we have carbonyl groups? , , * *, lone pairs, polarized bond
C O
5. electronic structure 5.1. only valence electrons move during reaction 5.2. bonds are based on atomic orbitals – s, p, d, f (how do they differ?) 5.3. orbital contraction – effective nuclear charge (C 2s v. O 2s) 5.4. bond formation – atomic orbital mixing 6. Molecular Orbitals: MO theory 6.1. bonds are MO’s: orbital overlap of similar symmetry and energy along bonding axis 6.2.
s s
B B B B 6.3.
s pz
B (^) B (^) B B 6.4. Extent of ovelap depends on distance and orbital symmetry 6.5. s and px does not work B (^) B B^ B 6.5.1. ++, +- overlap regions equally unlike s and px 6.5.2. s has axial symmetry along bonding axis, px has mirror plane 6.5.3. no net overlap, non-bonding interaction 6.6. Conservation of orbitals
6.6.4. no electron density at node 6.6.5. electron density between nuclei hold nuclei together (bonding) 6.6.6. electron density outside pulls apart (antibonding orbital) (what happens when H 2 or RBr reduced show energy diagrams) 6.7. electron energies correlate with orbital occupation 6.7.1. bonding < AO, (non-bonding) < antibonding 6.7.2. energy diagram of 2 electrons versus two p electrons 6.7.3. MO correlates with AO: C-O < C-C. Why? (homework format, names, structures) 6.8. Huckel MO’s for polyenes: http://www.chem.ucalgary.ca/SHMO/ 6.8.1. ethene, allyl, butadiene, pentadienyl 6.8.2. Huckel low level calculation, easy to determine 6.8.3. #AOs = #MOs (only adjacent orbitals interact) 6.8.4. MO’s have alternate plane and C2 symmetry 6.8.5. # nodes = 0, 1, 2, 3 etc from lowest to highest orbital: odd nodes means ends have opposite signs 6.8.6. Which are bonding and antibonding? Compare with p AO or overlap 6.8.7. #bond = #antibonding, related with respect to energy, bonding interaction, density (coefficient) 6.8.8. butadiene 1 < 1 ethene < butadiene 2 6.8.9. allyl http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/orbitals/molecular/allyl.html * 2 1 3 2 3 1 4 2 3 4
6.8.10. butadiene: http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/ orbitals/molecular/butadiene.html
7. Huckel MO’s for cyclic polyenes (contiguous p orbital/ planar) (move to polyenes) 7.1. inscribe regular polygon in circle of 2 radius 7.2. one vertex down, energy is 2 7.3. is energy of electron in ethene orbital 7.4. analytical geometry for calculation of energies: cyclopropyl example 7.5. horizontal diameter is 0 bonding energy, same as AO 7.6. 4 member ring 7.6.1. 2 electrons in lowest MO, two in each degenerate MO 7.6.2. unpaired, non-bonding, radicals 7.6.3. total energy is 4 relative to AO (p orbital) 7.7. 5 member ring 7.7.1. anion has 6 electrons 7.7.2. all paired in bonding orbital 7.7.3. follows 4n+2 rule 7.7.4. explains acidity of cyclopentadiene HC HC C H CH 2 H C HC HC C H CH H C H
pka = 15
8. Delocalization, resonance, conjugation (electron not nuclei move!) 8.1. more effective in systems than (show poor resonance form, butadiene equivalent!, how many electrons in each system?) O O^ O^ O O O O 8.2. orbitals must be collinear, structures with charge separation are not as important 9. hybridization
11. review - atomic/molecular properties 11.1. electronegativity – inductive and field effects F O
O 2 CCH 2 CH 2 NH 3
11.2. size – orbital overlap – mismatch in size 11.3. delocalization 11.4. steric effects – inhibit approach of atoms for favorable overlap H 3 C Cl CH 3 N H 3 C CH 3 C H 2 Cl N H H^ loss of chloride inhibited by steric effect
12. Types of reagents 12.1. Lewis acids – electrophiles: empty (or potential empty) orbital, electron deficient 12.1.1. cations: H+, PhCH 2 +, NO 2 +, Na+ 12.1.2. neutrals: AlCl 3 , SO 2 , CH 3 Br, Cl 2 , FeCl 3 , CH 3 CHO, ROH (carbonyls in general) 12.1.3. low energy orbitals available to accept electrons 12.2. Lewis bases – nucleophile: lone pairs, and electrons, electron donors 12.2.1. anions: AlH 4 - , HO-, CN-, RMgBr, RS-, Cl- 12.2.2. neutral: RNH 2 , H 2 C=CH 2 , ROH, R 3 Si-H, R 3 C-H 12.2.3. high energy orbitals for donation
