CHEM 109A Organic Chemistry

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1 CHEM 109A Organic Chemistry Chapter 2 Acids and Bases Central to Understanding Organic Chemistry Draw the conjugate acid of each of the following: a. CH 3 CH 2 CH 2 OH b. CH 3 CH 2 O c. O O d. CH 3 NH 2 e. HCl 1

2 Which is the stronger base? (by comparing pk a s) a. CH 3 CH 2 O b. O O Approximate pk a Values p Ka ~ 35 RNH 2 NH 3 estimate the pk a values of the following compounds CH 3 CH 2 CH 2 NH 2, CH 3 CH 2 OH, CH 3 CH 2 CH 2 NH 3 + 2

3 The Acid Dissociation Constant (K a ) is a Measure of the Extent of Dissociation of an Acid Put the constants on the same side of the equation. General representation is of acid-base reaction simple formalization moving H + A H reversible + B A + H B charge charge changes changes by -1 by +1 conj. base conj. acid 3

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.

4 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 An Alcohol Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. 4

5 A Carboxylic Acid Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. An Amine Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor. 5

6 Approximate pk a Values Which Reactant is the Acid? The stronger acid behaves as the acid. Water is the base. Water is the acid. 6

7 The Position of Equilibrium The Position of Equilibrium 7

8 The Position of Equilibrium The equilibrium 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. 8

9 Electronegativity Affects pk a Values When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom. Why are Alcohols Stronger Acids than Amines? Oxygen is more electronegative than nitrogen. 9

10 Why are Protonated Alcohols Stronger Acids than Protonated Amines? Oxygen is more electronegative than nitrogen. Hybridization Affects Electronegativity The strongest acid has the most stable (weakest) conjugate base. 10

11 Size Affects pk a Values When atoms differ in size, the strongest acid has its hydrogen bonded to the largest atom. Some pk a Values 11

12 Substituents Affect the Strength of an Acid! INDUCTIVE electron withdrawal (Inductive Effect)! DELOCALIZED electrons (Resonance Effect) Substituents Affect the Strength of an Acid inductive electron withdrawal 12

13 A Substituent s Effect on pk a Depends on Distance Why is a Carboxylic Acid a Stronger Acid than an Alcohol? 1. Inductive electron withdrawal 13

14 Why is a Carboxylic Acid a Stronger Acid than an Alcohol? 2. Delocalized electrons Summary of Factors That Affect Acid Strength electronegativity and size 14

15 Summary of Factors That Affect Acid Strength hybridization Summary of Factors That Affect Acid Strength inductive electron withdrawal 15

16 Summary of Factors That Affect Acid Strength electron delocalization 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). 16

17 A Compound with a pk a = 5.2 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. 17

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. A Buffer Solution A buffer solution contains an acid and its conjugate base. A buffer solution maintains nearly constant ph. 18

Lewis Acids and Bases Broader definition than proton acids (Brønsted acids) Lewis Acids and Bases Lewis acid: a species that accepts a share in an

19 Lewis Acids and Bases Broader definition than proton acids (Brønsted acids) 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. 19

20 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. An Acid Loses a Proton A Base Gains a Proton 20

21 Most Acid Base Reactions Are Reversible 21

ACIDS AND BASES. Note: For most of the acid-base reactions, we will be using the Bronsted-Lowry definitions.

ACIDS AND BASES. Note: For most of the acid-base reactions, we will be using the Bronsted-Lowry definitions. DEFINITIONS: ACIDS AND BASES Arrhenius Definition An acid in aqueous solution produces H + ions. A base in aqueous solution produces OH - ions. Bronsted Lowry Theory An acid is a proton donor A base is