D. A. Evans Acidity Concepts-1 Chem 206 Activation of Organic Molecules ■ Definition of ka Let H-x be any bronsted acid. In water ionization takes place ■ Base Activation H-X HOH H30+X base Nucleophile where HoH]=55.5 mol L H-baseo Ri where Keg: HH301X HX叶HOH Since [ HOH is, for all practical purposes, a constant value, the acid dissociation constant Ka is defined without regard to this entity. e. g pKa, describes quantitatively a molecule's propensity to act as an acid, ie to release a proton. H"+X- where H=H30 Medium effects Hence Structural effects (influence of substituents R1) From the above definitions, Ka is related to Keg by the relation Acid activation Ka(H-x)=55.5 Keq(H-x acid(protic or lewis acid R2 号 electrophile|■ Autoionizationofwater IOH+ HOH 5 H30+HO" X=e.gO, NR Eq C ■ The Aldol Example Since pKa is defined in the following equation base catalys Ka=-log1o[Kal The pKa of HOH iS 15.7 Keep in mind that the strongest base that can exist in water is HO Lets now calculate the acid dis constant for hydronium ion H3o H20 0 Ca 10*5 Activation SiMe obviously cid catalysis Ka[HOH]X Keg hence Ka=55.5 pKa=-log1o Ka=-1.7 The strongest acid that can exist in water is H3OR1 R2 X C R1 R2 R3 H O R M R H R O O R SiMe3 R H R O M O R R H R O C R1 R2 R3 R1 R2 X acid R R O R OH R R O R O M R R O R O SiMe3 HOH H–X H3O + HOH H–X HOH [H3O + ] [X– ] [H–X] [HOH] H2O [H+ ] [X– ] H3O + H + H3O + X– HO– X– H3O + (B) H2O (C) (A) D. A. Evans Acidity Concepts-1 Chem 206 Activation of Organic Molecules base - H-base pKa , describes quantitatively a molecule's propensity to act as an acid, i.e. to release a proton. acid (protic or lewis acid) Nucleophile Electrophile X = e.g. O, NR ... - Medium effects - Structural effects (influence of substituents R1 ) ■ Base Activation ■ Acid Activation + ■ The Aldol Example + + base acid Let H–X be any Bronsted acid. In water ionization takes place: + + where Keq = where [HOH] = 55.5 mol L-1 Since [HOH] is, for all practical purposes, a constant value, the acid dissociation constant Ka is defined wilthout regard to this entity. e.g. + where H+ = H3O + Hence [H–X] Ka = From the above definitions, Ka is related to Keq by the relation: Ka (H–X) = 55.5 Keq(H–X) ■ Autoionization of water + + Keq = 3.3 X 10–18 From Eq C: Ka = 55.5 Keq = 55.5(3.3 X 10–18) Hence Ka = 1.8 X 10–16 Since pKa is defined in the following equation: pKa = – log10 [Ka] The pKa of HOH is + 15.7 Keep in mind that the strongest base that can exist in water is HO– . ■ Definition of Ka pKa = – log10 Ka = –1.7 Ka = [HOH] x Keq obviously: Keq = 1 + Lets now calculate the acid dissociation constant for hydronium ion. + Ka = 55.5 The strongest acid that can exist in water is H3O + . hence base catalysis acid catalysis Ca 10+6 Activation