Evans Acidity Trends for Carbonyl Related Compounds Chem 206 Hybridization Vs Electronegativity Substituent Effects There is a linear relationship between %S character Pauling electronegativity I Alkyl Substituents on Localized Carbanions are Destabilizing: Steric hinderance of anion solvation JAcs1975,97,190) PhSOz- CH-H 29 31.1 383 a Heteroatom-Substituents:- 1st row elements of periodic table pKA (DMSO) 253035 Inductive stabilization versus %S-Character PhSO2-CH-OCH3 30.7 Lone Pair Repulsion l vS+M-Effect) There is a direct relationship between %S character H PhSo2-CH-OPh 27.9 PhSo2-CH-NMe3 19.4 Inductive Stabilization H Seomx a Heteroatom-Substituents:-2nd row elements of periodic table c6H(4 Strong carbanion stabilizing effect pKA ( DMSO) PhCC-H(29) PhSo2-CH-H PhSO2- CH-SO2Ph 12.2 PhSoz CH-SPh 20.5 PhSo2-CH-PPh2 20.52 2.5 3 3.5 4 4.5 5 Pauling Electronegativity 20 25 30 35 40 45 50 55 % S-Character CSP3 CSP2 CSP NSP3 NSP2 NSP 25 30 35 40 45 50 55 60 Pka of Carbon Acid 20 25 30 35 40 45 50 55 % S-Character CH4 (56) C6 H6 (44) PhCC-H (29) S S H H S S Me H PhSO2-CH-OCH3 H PhSO2 -CH–H H PhSO2 -CH–Me H PhSO2-CH-OPh H PhSO2-CH-NMe3 H PhSO2-CH-H H PhSO2-CH-SPh H PhSO2-CH-SO2Ph H PhSO2-CH-PPh2 H D. A. Evans Acidity Trends for Carbonyl & Related Compounds Chem 206 There is a linear relationship between %S character & Pauling electronegativity Hybridization vs Electronegativity There is a direct relationship between %S character & hydrocarbon acidity Substituent Effects ■ Alkyl Substituents on Localized Carbanions are Destabilizilng: Steric hinderance of anion solvation pKA (DMSO) 29 31 pKA (DMSO) 31.1 38.3 Compare: (JACS 1975, 97, 190) Inductive Stabilization versus Lone Pair Repulsion (-I vs +M -Effect) pKA (DMSO) 30.7 27.9 19.4 Inductive Stabilization ■ Heteroatom-Substituents: - 1st row elements of periodic table ■ Heteroatom-Substituents: - 2nd row elements of periodic table pKA (DMSO) 29 20.5 12.2 Strong carbanion stabilizing effect 20.5 pKA (DMSO)