Chapter 7 Acids and Bases; Intermolecular Attractions

Read (no lecture) Section 7.11, 7.12 and 7.13 Problems for Chapter 7 Skip section 7.4, 7.10, and 7.14 Read (no lecture) Section 7.11, 7.12 and 7.13 In Text: 1 a, b, g, h, 2b, 6b, 8a, 11b, c 14, 15, 18 through 22 24, 25, 27, 28, 31, 32, 33, 34 End of Chap: 36, 38, 42, 43, 45 a, b, e, 48 through 52 54, 55, 56, 58, 59, 62, 63

HA + B- A- + BH Section 7.1 Bronsted-Lowry Theory acid = proton donor conjugate base conjugate acid acid base HA + B- A- + BH -H+ +H+ acid = proton donor base = proton acceptor Conjugate Acid = Base + Proton Conjugate Base = Acid - Proton

Section 7.2 pKa pKa = - log Ka Strong acid = large Ka = small pKa Weak acid = small Ka = large pKa

Sect. 7.2 Relative Acid and Base Strengths

Acid Strengths Strong Acid Conjugate base is weak pKa is small Weak Acid Conjugate base is strong pKa is large

B: A Section 7.3: Lewis Acid-Base theory more general than Bronsted theory BASE B: electron-pair donor A ACID electron-pair acceptor Some Lewis Acids: Fe3+ BF3 H3O+ NH3 H2O Some Lewis Bases:

Section 7.5 Electronegativity and Size Effects

Sect. 7.5: Electronegativity and Size

Effect of Atomic Size on Acidity increasing atom size pKa Values 3.5 16 5 -7 7 -9 4 -10 3 I - Br - F - Cl - 1.36 A 1.81 A 1.95 A 2.16 A

Effect of Electronegativity on Acidity increasing electronegativity pKa Values >45 45 20 34 35 15 16 18 5 3.5

Section 7.6 Resonance Effects

Take Away Lession: Resonance strengthens acid Resonance weakens base

Resonance in the Acetate Ion equivalent structures charge on oxygens O C H C 3 O + O -H _ C H C O H 3 base _ acetic acid O C H C 3 O acetate ion

- _ _ Phenolate ion Resonance O non-equivalent structures charge on carbon and oxygen

Nitrophenols Placing a nitro group on the benzene ring of a phenol increases its acidity. The effect is largest when the nitro group is placed in an ortho or a para position on the ring, and considerably smaller for the meta position. Multiple nitro groups at the ortho and para positions can increase the pKa of a phenol to the point that it becomes a very strong acid.

pKa Values of Nitrophenols 7.2 10 9.3 4.0 0.4 7.3

_ _ Resonance in p-Nitrophenol O N extra structure When in an ortho or para position a nitro group can participate in resonance. _ O N 2 + extra structure +

Section 7.7 Alpha Hydrogen compounds

Take Away Lession: Resonance strengthens acid Resonance weakens base

Section 7.8 Inductive Effects

Resonance operates through the p bonding system Resonance operates through the p bonding system. It doesn’t drop off with distance. Inductive effect operates through the sigma bonding system. It drops off with increasing distance.

Cl d- d+ C d+ d- CH3 C Types of Inductive Effects R, CH3 ELECTRON WITHDRAWING GROUPS ELECTRON DONATING GROUPS Cl d- d+ C d+ d- CH3 C R, CH3 F, Cl, Br, NO2 , NR3+ electronegative elements take electron density from cabon. This strenthens the acidity. alkyl groups donate electron density to carbon. This weakens the acidity.

O d- d+ Cl C O O d- d+ Cl C C C O Inductive Effects on Haloacids d- d+ Chlorine helps to stabilize -CO2- by withdrawing electrons Cl C O O d- d+ d- d+ d- d+ Cl C C C O The effect diminishes with distance - it carries for about 3 bonds.

Inductive Effects pKa Values 3.13 4.75 2.87 2.81 2.81 1.29 2.66 0.65 increasing electronegativity multiple substituents pKa Values 3.13 4.75 2.87 2.81 2.81 1.29 2.66 0.65

Inductive Effects pKa Values 3.75 4.8 4.5 4.75 4.0 4.87 4.81 2.9 5.02 distance 5.02 When the chlorine atom is moved further away from the carboxyl group, acidity decreases Alkyl groups release electrons. This decreases acidity

Section 7.9 Hybridization Effects

Effect of Hybridization pKa values sp3 ca. 50 sp orbitals are more electron withdrawing than sp3 orbitals (sp orbitals have more s character). sp > sp2 > sp3 sp2 35 sp 25

Section 7.11 Solvent Effects

Solvation Effects - 4.75 5.02 4.87 4.81 C H C O O H C O H C H C H C O 3 3 5.02 C H C H C O O H 4.87 3 2 C H C H C H C O O H 4.81 3 2 2 Notice that these are all similar ….but this one has a larger pKa This is probably a solvation effect. Solvation lowers the energy of the ion. steric hindrance C O - H Stronger Acid The bulky t -butyl group is not as well solvated. unbranched Weaker Acid

Sect. 7.12: Intermolecular Attractions hydrogen bonding dipole-dipole attractions London forces or van der Waals attractions Sect. 7.13: Solubility

SUMMARY

Electronegativity of atom bearing acidic hydrogen; more electronegative = more acidic Size of atom bearing acidic hydrogen; larger = more acidic. Resonance: greater charge delocalization in conjugate base = more acidic. alpha-hydrogens: carbonyls, nitro, cyano, etc. Greater charge delocalization in conjugate base = more acidic. Inductive effect: electronegative atoms withdraw electrons making the acid more acidic. Hybridization; more s-character = more acidic

Classification of Weak and Strong Acids by Functional Group weak acids strong acids 40 20 10 5 0 pKa di- and tri- nitrophenols alkanes alkenes amines alcohols ketones amides alkynes phenols carboxylic acids nitrophenols inorganic acids oxyacids b-diketones

Electron-Withdrawing Effects Strengthen Acids (-) (-) - - - - - Any effect that “bleeds” electron density away from the negatively-charged end of the conjugate base will stabilize (lower the energy) of the conjugate base and make it a weaker base. The parent acid will be a stronger acid.

Electron-Donating Effects Weaken Acids Conversely ….. Electron-Donating Effects Weaken Acids - (-) (-) - - - - - Any effect that “pushes” extra electron density toward the negatively-charged end of the conjugate base will destabilize (increase the energy) of the conjugate base and make the base weaker. The parent acid will be stronger.