CHM 301 2018 Lecture 6 1 Review: Parameters influencing acidity, Ka or pKa pKa = - log Ka Higher pKa= weaker acid Approximation: acidity of H-A is primarily related to the stability of A- Higher pKa = less stable A- How does the structure relate to the stability of A- A. Electronegativity of the atom carrying the charge in the conjugate base B. Consider the series: C. Solvent: conjugate base (anion) is very sensitive to stabilization by solvent. pKa of HA depends on the solvent in which it is measured. H-A H+ + A- Keq = Ka = [H+][A-][HA]R3C < R2N < ROStability:EN: C < N < OCF3CH2O-H CF3CH2CH2O-H CF3CH2CH2CH2O-H CH3CH2O-HpKa 12.8 14.6 15.4 15.9A B C DCHM 301 2018 Lecture 6 2 Solvent effects on acidity: water vs DMSO Water is a "polar, protic" solvent High dielectric constant (80), Strong H-bonding Dimethyl sulfoxide is a "polar, aprotic" solvent. High dielectric constant (47) Good interaction with cations but NO H-bond donation, WEAK stabilization of anions So: CH3OH is a much ___________(?) acid in DMSO compared to CH3OH in water Cf. pKa table. Solvent steric effects: pKa 15.5 15.9 16.5 17 Handout on pKa (16) (17) (18) Text p 218 ROHRO+ Hif better solvation: more stable, stronger acidHCHHOHHCH3CHOHHCH3CH3COHH3CCH3CH3COH HCHHOHCHHOHHHOHOHOHsolvation of the conjugate base, an alkoxideSH3CCH3OSCH3H3COCHM 301 2018 Lecture 6 3 Consider the extremes: methyl alcohol t-butyl alcohol, 2-hydroxy-2-methylpropane, 2-methyl-2-propanol, t-butanol Not due to donation of charge by the methyl group. In fact, in the absence of solvent, the methyl group stabilizes the anion due to polarizability differences compared to an H substituent. HCHHOCCCCOHHHHHHHHHWhy is the methoxide anionrelatively more stable thanthe t-butoxide anion? (in water solution)HCHHOCCCCOHHHHHHHHHHHHHHHOHOHOHOHOHOHsolvation of the conjugate base, alkoxideHCHHOCCCCOHHHHHHHHHsmall or tightly heldelectron cloud is less polarizablelarger electron cloud candeform to accept more electron densityCHM 301 2018 Lecture 6 4 Another version of –OH acidity: effect of C=O nearby. pKa Inductive effect, again: C=O is a powerful inductive withdrawing group, through the sigma bonds. But… Something else—the electrons on the oxygen anion can be delocalized onto the other oxygen; through pi bonds Good question: where are the electrons? Difficult to write conventional bonds to express this feature. The actual molecule is neither of the two "localized" structures, but the "average" of the two. Note the arrangement of p orbitals available to the carboxylate anion: We say that one lone electron pair is “conjugated” with the pi bond. OOOOOOOOHOHOHOHOOO-H++H+-H++H+-H++H+-H++H+CHM 301 2018 Lecture 6 5 Rules for creating and evaluating resonance structures: 1. Resonance structures require a set of adjacent p-type orbitals (cannot include carbon with four substituents in the delocalization) 2. The actual molecule is the average of the resonance structures, weighted according to stability: More stable resonance structure contributes more strongly; the actual molecule is closer in structure to the more stable resonance contributors. Ideal case: two or more energetically equal structures 3. The arrow formalism is useful in interconverting resonance structures. 4. All of the contributing structures must have: a. the same number of paired and unpaired electrons b. the same geometry/spatial arrangement of all atoms (bond distances, bond angles) 5. "good" structures will: a. have more bonds (fill the octet) b. minimize charge separation c. avoid concentrating charge on atoms with the "wrong" electronegativityCHM 301 2018 Lecture 6 6 The carbonyl group: NOTE on ethylene and other simple C=C pi bonds: No significant resonance structures (too high energy) NOTE: hybridization is adjusted to optimize delocalization Examples of resonance delocalization effects on acidity. Compare: Rationalize pKa differences by (a) EN of atom supporting the (-) charge (b) delocalization of the (-) in the conjugate bases The allyl anion: OOOOHCHHHHHHHCHHHH..HHHHHHHHHH-HHHHHH0.50.51.5 bonds OOHROHRHROHcarboxylic acid ketone alkene phenolCHM 301 2018 Lecture 6 7 The Enolate Anion: ketone Compare A and B: a. # bonds: b. #charges: c. location of charge: The conjugate base (an enolate anion) is a combination of the two structures, averaged proportional to estimated stability. The result is an anion that more resembles A or B? Partial double bond character between the two carbon atoms). Note reverse reaction: ROCHHHROHCHH..ROHHROHHA B-HHOHHHOHHABHOC HHHHOHHHCHM 301 2018 Lecture 6 8 Phenols: Benzene: Extended pi systems: OHphenol-H+Ophenoxide anionOOOOHHHHHHHH•••HHHHHHHHHHHHHHHHHHHHHHHHHHHHδδδCHM 301 2018 Lecture 6 9 HYBRIDIZATION: "s character" effects CHHHHH +CHHHH +C CHHHHC CHHHH +HRHH +RCHM 301 2018 Lecture 6 10 Grandaddy of stabilized hydrocarbon anions: _______________________________________________________________________________ Note: Similar analysis for cation stability. But: Highly unstable—never been observed. MO theory to the rescue—in 302/304 HH-H+HHHHHδ-δ-δ-δ-δ-completely delocalized 5 equivalent resonance structuresHHHHHHHHHHdelocalized cation; stabilizedHHHHHδ+δ+δ+δ+δ+completely delocalized 5 equivalent resonance structuresCHM 301 2018 Lecture 6 11 Summary: Steric, resonance, inductive (polarizability) and hybridization effects rationalize acidity of organic compounds. Inductive effects depend on electronegativity of attached or nearby atoms Polarizable atoms or groups can also allow charge