Molecular Orbitals - Organic Chemistry II - Lecture Slides, Slides of Organic Chemistry

Download Molecular Orbitals - Organic Chemistry II - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Molecular Orbitals - Conservation of Orbital Symmetry in Concerted Processes docsity.com Quantum mechanics: application of mathematics and physics to describe phenomena that exhibit quantized functions. eg. Electrons in atoms behave like waves. Wave mechanics can be used to solve for energies and orbitals. The math is very complicated and time consuming. By making assumptions and approximations, it is possible to get solutions that are useful, if not exact. In fact, we do not need to do any math if we understand the results on a qualitative level. docsity.com Molecular Orbitals (MOs) Covalent bonds result from the overlap (combinations) of atomic orbitals to produce molecular orbitals. Molecular orbitals result from Linear Combinations of Atomic Orbitals. LCAO  wave mechanics of MO’s φ = atomic wave function ψ = molecular wave function For molecule A—B ψ = φA  φB docsity.com Bonding when: a) appreciable overlap of atomic orbitals b) energies of atomic orbitals are ~ equal c) same symmetry Hydrogen H2 H:H LCAO of two AO’s  two MO’s ψ2 = φA - φB antibonding σ* • • one node ψ1 = φA + φB bonding σ • • no nodes • docsity.com π – molecular orbitals ethylene CH2=CH2 look only at π orbitals How many AO’s in the π system? p + p two How many MO’s result? also two How many electrons in the π system? 2 ψ = pz  pz docsity.com C C C C C C C C C C C C C C C C LCAOs MOs E   3  1 2 4  1,3-butadiene docsity.com C C C C C C C C C LCAOs MOs E   3 1 2  + allyl cation CH2=CH—CH2 3 AO’s  3 MO’s 2 π e - π* n π docsity.com Electrocyclic reactions: Δ or hv conjugated polyene cyclic compound The mechanism is concerted! docsity.com H CH3 H CH3 CH3 H H CH3 conrotatory conrotatory docsity.com In the concerted electrocyclic reactions, symmetry must be conserved for bonding to take place. The molecular orbital involved = highest occupied molecular orbital in the polyene. HOMO C C C C C C C C C C C C C C C C LCAOs MOs E   3  1 2 4  HOMO docsity.com heat HOMO =  2 motion must be conrotatory for bonding docsity.com a heat Ch trans, cis, trans - cis -5,6-dimethyl- 2,4,6-octatriene 1,3-cyclohexadiene ® docsity.com disrotatory! CH3 HH CH3 heat heat docsity.com HOMO =  3 disrotatory CH3 HH CH3 HOMO (polyene) = ? 6 AO 6 MO 6 e- docsity.com HOMO* = 3 motion must be disrotatory for bonding hv docsity.com hv CH3H3C disrotatory CH3 H CH3 H docsity.com Woodward – Hofmann Rules for Electrocyclic Reactions: conrotatory disrotatory disrotatory conrotatory thermal photochemical 4n 4n + 2 docsity.com Cycloadditions Diels-Alder diene + dienophile  cyclohexene [ 4 + 2 ] cycloaddition 1. diene must be sigma-cis 2. syn- addition + [ 4 + 2 ] docsity.com The Diels-Alder cycloaddition is a concerted reaction: Molecular orbital symmetry must be conserved. docsity.com C C C C C C C C C C C C C C C C LCAOs MOs E   3  1 2 4  C C C C LCAOs MOs E   1 2 LUMO HOMO LUMO HOMO CH2=CH2 CH2=CHCH=CH2 docsity.com C C C C LCAOs MOs E   1 2 LUMO HOMO LUMO HOMOsymmetry forbidden for supra-supra thermal: LUMO + HOMO docsity.com C C C C LCAOs MOs E   1 2 HOMO* LUMO HOMO*symmetry allowed for supra-supra photochemical = HOMO* & LUMO docsity.com Woodward – Hofmann Rules for Cycloadditions: supra-supra forbidden supra-supra allowed supra-supra allowed supra-supra forbidden 4n 4n + 2 Thermal Photochemical [ i + j ] docsity.com + [ 4 + 4 ] HOMO* = 3 LUMO =   3 hv allowed docsity.com Sigmatropic rearrangements ―no mechanism, no reaction – reaction.‖ Migration of an atom or group with its sigma bond within a conjugated π framework. G G | | C—(C=C)n  (C=C)n—C docsity.com G C C C C C C G[ 1,3 ] G C C C C C C [ 1,5 ] C C C C G G = H, R docsity.com [ 1,3 ] [ 1,5 ] [ 1,7 ] Suprafacial migration of H forbidden allowed forbidden docsity.com CH; CD. [1,5]-H c CHo CHD» docsity.com [ 1,3 ] [ 1,5 ] [ 1,7 ] Suprafacial migration of R allowed with inversion of configuration allowed with retention of configuration allowed with inversion of configuration docsity.com