D. A. Evans Three-center Bonds Chem 206 Consider the linear combination of three atomic orbitals. The resulting molecular orbitals(MOs) usually consist of one bonding, one nonbonding Case 3: 2 p-Orbitals: 1 s-orbital and one antibonding MO antibonding Case 1: 3 p-Orbitals pI-onentation antibonding 2 nonbonding I Case 4: 2 s-Orbitals: 1 p-orbital Do this as bonding Note that the more nodes there are in the wave function, the higher its energy Examples of three-center bonds in organic chemistry H2C=CH-CH2 Allyl carbonium ion: both pi-electrons in bonding state A. H-bonds:(3-center, 4 electron) 0-H-0 The acetic acid dimer is H2c=CH—CH2Alyl 2 electrons in bonding obital plus 3 stabilized by ca 15 kcal/mol nonbonding MO B. H-B-H bonds:(3-center, 2 electron) Case 2: 3 p-Orbitals sigma-orientation antibonding i diborane stabilized by 35 kcal/mol nonbonding C. The SN2 Transition state: (3-center, 4 electron) The SN 2 transition state approximates a case 2 situation with a central carbon p-orbita The three orbitals in reactant molecules 1 nonbonding MO from Nucleophile (2 electr bonding 1 bonding Mo o C-Br(2 electrons) 1 antibonding MOσC-BConsider the linear combination of three atomic orbitals. The resulting molecular orbitals (MOs) usually consist of one bonding, one nonbonding and one antibonding MO. Case 1: 3 p-Orbitals 3 Energy bonding nonbonding antibonding Note that the more nodes there are in the wave function, the higher its energy. + Allyl carbonium ion: both pi-electrons in bonding state ● Allyl Radical: 2 electrons in bonding obital plus one in nonbonding MO. – Allyl Carbanion: 2 electrons in bonding obital plus 2 in nonbonding MO. antibonding nonbonding bonding Energy 3 Case 2: 3 p-Orbitals pi-orientation sigma-orientation 2 + Case 3: 2 p-Orbitals; 1 s-orbital Examples of three-center bonds in organic chemistry A. H-bonds: (3-center, 4 electron) The acetic acid dimer is stabilized by ca 15 kcal/mol B. H-B-H bonds: (3-center, 2 electron) diborane stabilized by 35 kcal/mol C. The SN2 Transition state: (3-center, 4 electron) The SN 2 transition state approximates a case 2 situation with a central carbon p-orbital The three orbitals in reactant molecules used are: 1 nonbonding MO from Nucleophile (2 electrons) 1 bonding MO s C–Br (2 electrons) 1 antibonding MO s* C–Br D. A. Evans Three-center Bonds Chem 206 H2C CH CH2 H2C CH CH2 H2C CH CH2 O H H O O O CH3 CH3 B H B H H H H H B H B H H H H H C H H H Nu Br bonding nonbonding antibonding Case 4: 2 s-Orbitals; 1 p-orbital Do this as an exercise