M. Shair. D. Evans Carbocation Generation Stability Chem 206 Carbocation Stability: The pKR+ valt Removal of an energy-poor anion from a neutral precursor via Lewis Acids efinition R+H2O一ROH+H R3C-X LA R3C KR aR+·aH20 LA: Ag, AlCl3, SnCl4, SbCls, SbF5, BF3, FeCl3, ZnCl, PCI, PCl5, POCl3 pKR+ =-log KR arey Sundberg, A, p 277 X: F CL Br L OR Table: pKa+ values of some selected carbenium salts Acidic dehydratization of secondary and tertiary alcohols (4-Meo-CsHg 0.82 6.63 110 R3CoH+Hx120R?c④+xe R: Aryl other charge stabilizing substituents Co2(co)6 X: SO CIO4, FSO3, CF3SO3 From neutral precursors via heterolytic dissociation(solvolysis)-First step in SN1 or E1 reactions E>-CH R3C-X 3C+X⊙ 7.2 Carey Sundberg, A, pp 276- Ability of X to function as a leaving group -N2>-0So2R>-0PO(OR)2>->-Br> CI> OH Carbocation Generation Hydride abstraction from neutral precursors Addition of electrophiles to T-systems R R2C-H Lewis-Acid RaCo R2n RoC-H C Provide a mechanism of this transformation h& B,. be HC=-CH2OH Jcs,cc1971,556CH2 CHPh R3C R3C R3C R3C X X Fe R3C H H7C3 H7C3 C3H7 R3C H H H Fe Ph3C H H (3-Cl-C6H4 )3C Cr(CO)3 CHPh RS RS H H R2N R2N H H Ph2CH R CPh2 Co2 (CO)6 HC CH2OH R R R3C OH R3C X R R R R R3C X Br H–X H H H2SO4 R R R R H R H R Me O M. Shair, D. Evans Carbocation Generation & Stability Chem 206 Carbocation Stability: The pKR+ value Definition: R + + H2O ROH + H+ KR+ = aROH × aH+ aR+ × aH2O a = activity pKR+ = – log KR+ (4-MeO-C6H4 )3C + 0.82 – 6.63 – 11.0 – 13.3 0.40 0.75 –10.4 –7.4 7.2 4.77 Table: pKR+ values of some selected carbenium salts Hydride abstraction from neutral precursors + Lewis-Acid = etc. Lewis-Acid: Ph3C BF4, BF3, PCl5 Removal of an energy-poor anion from a neutral precursor via Lewis Acids + LA LA–X LA: Ag , AlCl3, SnCl4, SbCl5, SbF5, BF3, FeCl3, ZnCl2, PCl3, PCl5, POCl3 ... X: F, Cl, Br, I, OR Acidic dehydratization of secondary and tertiary alcohols - H2O R: Aryl + other charge stabilizing substituents X: SO4 2-, ClO4 - , FSO3 - , CF3SO3 - From neutral precursors via heterolytic dissociation (solvolysis) - First step in SN1 or E1 reactions solvent Ability of X to function as a leaving group: -N2 + > -OSO2R' > -OPO(OR')2 > -I ³ -Br > Cl > OH2 + ... Carbocation Generation + + + + Addition of electrophiles to -systems chemistry chemistry J.C.S.,CC 1971, 556 Provide a Mechanism of this transformation Carey & Sundberg, A, p 277 Carey & Sundberg, A, pp 276-