1/26/18 CHEM 109A Organic Chemistry https://labs.chem.ucsb.edu/zakarian/armen/courses.html Chapter 2 Acids and Bases Central to Understanding Organic Chemistry © 2017 Pearson Education, Inc. Draw the conjugate acid of each of the following: a. CH3CH2CH2OH b. CH3CH2O O c. O d. CH3NH2 e. HCl © 2017 Pearson Education, Inc. 1 1/26/18 Which is the stronger base? (by comparing pKas) a. CH3CH2O b. O O © 2017 Pearson Education, Inc. Approximate pKa Values p Ka ~ 35 RNH2 NH3 estimate the pKa values of the following compounds + © 2017 Pearson Education,CH Inc.3 CH2CH2NH2, CH3CH2OH, CH3CH2CH2NH3 2 1/26/18 The Acid Dissociation Constant (Ka) is a Measure of the Extent of Dissociation of an Acid Put the constants on the same side of the equation. © 2017 Pearson Education, Inc. General representation is of acid-base reaction simple formalization moving H+ reversible A H + B A + H B charge charge changes changes by -1 by +1 conj. base conj. acid © 2017 Pearson Education, Inc. 3 1/26/18 General representation is of acid-base reaction simple formalization moving H+ reversible A H + B A + H B charge charge changes changes by -1 by +1 conj. base conj. acid complicated reality still moving H+ but which one? A H + B H © 2017 Pearson Education, Inc. An Alcohol Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. © 2017 Pearson Education, Inc. 4 1/26/18 A Carboxylic Acid Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. © 2017 Pearson Education, Inc. An Amine Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. © 2017 Pearson Education, Inc. 5 1/26/18 Approximate pKa Values © 2017 Pearson Education, Inc. Which Reactant is the Acid? The stronger acid behaves as the acid. Water is the base. Water is the acid. © 2017 Pearson Education, Inc. 6 1/26/18 The Position of Equilibrium © 2017 Pearson Education, Inc. The Position of Equilibrium © 2017 Pearson Education, Inc. 7 1/26/18 The Position of Equilibrium © 2017 PearsonThe Education, equilibrium Inc. favors formation of the weaker acid. The Strength of an Acid is Determined by the Stability of Its Conjugate Base The more stable the conjugate base, the stronger the acid. Stable bases are weak bases. © 2017 Pearson Education, Inc. 8 1/26/18 Electronegativity Affects pKa Values When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom. © 2017 Pearson Education, Inc. Why are Alcohols Stronger Acids than Amines? Oxygen is more electronegative than nitrogen. © 2017 Pearson Education, Inc. 9 1/26/18 Why are Protonated Alcohols Stronger Acids than Protonated Amines? Oxygen is more electronegative than nitrogen. © 2017 Pearson Education, Inc. Hybridization Affects Electronegativity The strongest acid has the most stable (weakest) conjugate base. © 2017 Pearson Education, Inc. 10 1/26/18 Size Affects pKa Values When atoms differ in size, the strongest acid has its hydrogen bonded to the largest atom. © 2017 Pearson Education, Inc. Some pKa Values © 2017 Pearson Education, Inc. 11 1/26/18 Substituents Affect the Strength of an Acid ! INDUCTIVE electron withdrawal (Inductive Effect) ! DELOCALIZED electrons (Resonance Effect) © 2017 Pearson Education, Inc. Substituents Affect the Strength of an Acid inductive electron withdrawal © 2017 Pearson Education, Inc. 12 1/26/18 A Substituent’s Effect on pKa Depends on Distance © 2017 Pearson Education, Inc. Why is a Carboxylic Acid a Stronger Acid than an Alcohol? 1. Inductive electron withdrawal © 2017 Pearson Education, Inc. 13 1/26/18 Why is a Carboxylic Acid a Stronger Acid than an Alcohol? 2. Delocalized electrons © 2017 Pearson Education, Inc. Summary of Factors That Affect Acid Strength electronegativity and size © 2017 Pearson Education, Inc. 14 1/26/18 Summary of Factors That Affect Acid Strength hybridization © 2017 Pearson Education, Inc. Summary of Factors That Affect Acid Strength inductive electron withdrawal © 2017 Pearson Education, Inc. 15 1/26/18 Summary of Factors That Affect Acid Strength electron delocalization © 2017 Pearson Education, Inc. The Henderson–Hasselbalch Equation The Henderson–Hasselbalch equation tells us whether (at a given pH) a compound will be in its acidic form (with its proton) or in its basic form (without its proton). © 2017 Pearson Education, Inc. 16 1/26/18 A Compound with a pKa = 5.2 © 2017 Pearson Education, Inc. Is a Compound Neutral or Charged? A carboxylic acid is neutral in its acidic form and charged in its basic form. An alcohol is neutral in its acidic form and charged in its basic form. An amine is charged in its acidic form and neutral in its basic form. © 2017 Pearson Education, Inc. 17 1/26/18 Water and Diethyl Ether Form Two Layers Diethyl ether is less dense than water, so diethyl ether is on top. Neutral compounds dissolve in diethyl ether. Charged compounds dissolve in water. © 2017 Pearson Education, Inc. A Buffer Solution A buffer solution contains an acid and its conjugate base. A buffer solution maintains nearly constant pH. © 2017 Pearson Education, Inc. 18 1/26/18 Lewis Acids and Bases Broader definition than proton acids (Brønsted acids) © 2017 Pearson Education, Inc. Lewis Acids and Bases Lewis acid: a species that accepts a share in an electron pair Lewis base: a species that donates a share in an electron pair All Brønsted acids are Lewis acids. All Brønsted bases are Lewis bases. © 2017 Pearson Education, Inc. 19 1/26/18 How Chemists Use the Terms “acid” = a proton-donating acid “Lewis acid” = a non-proton-donating acid All bases are Lewis bases because they have a pair of electrons they can share. © 2017 Pearson Education, Inc. An Acid Loses a Proton A Base Gains a Proton © 2017 Pearson Education, Inc. 20 1/26/18 Most Acid–Base Reactions Are Reversible © 2017 Pearson Education, Inc. 21 .