D. A. Evans Substitution Reactions: General Considerations Chem 206 Why do sn2 Reactions proceed with backside displacement? Electrophilic substitution at saturated carbon may occur with either inversion of retention R Inversion x—-|№u-2-x Ra Given the fact that the LUMo on the electrophile is the C-X antibonding orbital, Nucleophilic attack could occur with either inversion or retention Retention Inversion Retention M E(+) 白 HOMO LUMO Constructive overlap between Overlap from this geometry results Ra nu &o*C-x in no net bonding interaction M○ HOMO Expanded view of o*C-X Retention LUMO Examples HOMO 人2,丈2人 Nu Fleming page 75-76 predominant inversion predominant retention Stereochemistry frequently determined by electrophile structureD. A. Evans Substitution Reactions: General Considerations Chem 206 Why do SN2 Reactions proceed with backside displacement? d– d– ‡ Nu: – X: C – H H R Nu X C H H R Nu Given the fact that the LUMO on the electrophile is the C–X antibonding orblital, Nucleophilic attack could occur with either inversion or retention. Nu Inversion C X R H H C X R H H Constructive overlap between Nu & s*C–X C X R H H Retention Nu Overlap from this geometry results in no net bonding interaction Expanded view of *C–X C X Nu HOMO LUMO LUMO antibonding bonding ●● ●● ●● HOMO Electrophilic substitution at saturated carbon may occur with either inversion of retention d+ d+ ‡ El(+) C R H b Ra Nu M C H Rb Ra C M Nu Ra Rb H C M Ra Rb H LUMO El(+) Retention C M Ra Rb H El(+) Inversion HOMO ●● ●● Inversion Retention ‡ El(+) C M Ra Rb H C Ra M Rb H El d+ d+ C El Ra Rb H Fleming, page 75-76 Li H Br2 H Br predominant inversion CO2 CO2Li H predominant retention Examples Stereochemistry frequently determined by electrophile structure M+ M+