D. A. Evans Acidity Trends for Carbonyl Related Compounds Chem 206 Substituent Effects Electrons in 2S states see"a greater effective nuclear charge than electrons in 2P states Electronegativity e.g. Compare Carboxylic Acids vs. Ketones This becomes apparent when the radial probability functions for S and P-states are examined: The radial probability functions for the hydrogen atom s& p states are shown below 0- more stabile 100% 100% enolate because electronegative than C pKA=4.8 pKA≈19 (DMSO) O Orbital 2 S Orbital Hybridization -S-character of carbon hybridization 〓2 P Orbital sp-orbitals 25%s-character sp-orbitals 33%S-character sp-orbitals 50%S-character 3 S Orbital O> 3 POrbital C∧ Carbon acids S-states have greater radial penetration due to the nodal properties of the wave function. Electrons in s states see a higher nuclear charge The above observation correctly implies that the stability of nonbonding electron pairs is directly proportional to the of S-character in the doubly occupied orbital Hybridzation Carbanions (DCsP3 (DCsP2 DCsp Bond angle 180° 20° ≈120 Least stable Most stable pK(DMSO) 23 Carbenium ions Most stable <t Least stable The above trends indicate that the greater the of S-character at a give atom, the greater the electronegativity of that atomsp3 -orbitals 25% s-character sp2 -orbitals 33% s-character sp-orbitals 50% s-character CSP3 CSP2 CSP CSP2 1 S Orbital 2 S Orbital 3 S Orbital 2 S Orbital 2 P Orbital 3 P Orbital CSP3 CSP R C O – CH2 H R R R R R H C O O – R H R R R C C O CH2–H H H H (DMSO) R H H C H H C O O H (DMSO) D. A. Evans Acidity Trends for Carbonyl & Related Compounds Chem 206 Substituent Effects Electronegativity e.g. Compare Carboxylic Acids vs. Ketones pKA = 4.8 pKA » 19 Carboxylate ion more stabile than enolate because O more electronegative than C Hybridization - S-character of carbon hybridization Remember: Hybridzation pKa (DMSO) Bond Angle sp sp2 » sp2 sp3 180° 120° 109° 23 32 » 39 50 » 120 Carbon Acids Carbenium ions Carbanions Most stable Least stable Least stable Most stable S-states have greater radial penetration due to the nodal properties of the wave function. Electrons in s states see a higher nuclear charge. The above observation correctly implies that the stability of nonbonding electron pairs is directly proportional to the % of S-character in the doubly occupied orbital. Electrons in 2S states "see" a greater effective nuclear charge than electrons in 2P states. Å Radial Probability 100 % This becomes apparent when the radial probability functions for S and P-states are examined: The radial probability functions for the hydrogen atom S & P states are shown below. 100 %Radial Probability Å The above trends indicate that the greater the % of S-character at a given atom, the greater the electronegativity of that atom