Chapter 1 Lecture Introduction and Review Organic Chemistry, 8 th Edition L. G. Wade, Jr.

Lecture 5: Acids and Bases, A General Discussion Acids & Bases Conjugate Acids & Bases Acid and Base Strength Evaluating Equilibrium Reactions 2

Chapter 93 What is an Acid or Base? Arrhenius Acids (H + ) and Bases (OH - ) HCl HBr HNO 3 H 2 SO 4 HC 2 H 3 O 2 H in formulaH + Chapter 93

4 An Acid is a “Proton Donor” Bronsted-Lowry Acid Arrhenius Acids (H + ) and Bases (OH - ) H + = “a proton” Why? ◦ Atomic Number = 1 ◦ 1 Proton and 1 Electron when neutral ◦ H that has lost an electron results in “a proton” Chapter 94

5 A Base is a “Proton Acceptor” Bronsted-Lowry Base Arrhenius Acids (H + ) and Bases (OH - ) NaOH KOH Ba(OH) 2 Metal Hydroxides OH - = hydroxide ion Chapter 95

Bronsted-Lowry Bases are More than Metal Hydroxides NH 3 = Ammonia N(CH 3 ) 3 = an Amine Ammonia and amines are bases because they can accept protons. How? 6

Bronsted-Lowry Acids Bronsted-Lowry Bases Key Point: Acids? ◦ Do they have protons? Bases? ◦ Is there a lone pair of electrons? 7

Chapter 98 What Happens When an Acid Dissolves in Water? Water acts as a Bronsted-Lowry base and abstracts a proton (H + ) from the acid. As a result, the conjugate base of the acid and a hydronium ion are formed. Chapter 98

Conjugate Acids and Bases The term conjugate comes from the Latin word “conjugare,” meaning “to join together.” Reactions between acids and bases always yield their conjugate bases and acids. 9

Bronsted-Lowry Acid-Base Theory: “Follow that Proton” 10

Conjugate Acids and Bases Conjugate acid – when base accepts a proton. Conjugate base – when an acid donates a proton. 11

Chapter 912 Acid and Base Strength Strong acids are completely dissociated in water. ◦ Their conjugate bases are quite weak. Weak acids only dissociate partially in water. ◦ Their conjugate bases are weak bases. Chapter 912

Chapter 913 Acid and Base Strength Substances with negligible acidity do not dissociate in water. ◦ Their conjugate bases are exceedingly strong. Chapter 913

Quantifying Acidity K a values range from to Will strong acids have low or high K a values? Will strong acids have low or high pK a values? Chapter 914

pK a ‘s for Several Compounds 15

Predicting the Strength of Bases The stronger the acid, the weaker its conjugate base The larger the pK a of the conjugate acid, the stronger the base. Cl - <CH 3 CO 2 - <OH - Very Weak BaseWeak BaseStrong Base (HCl) (CH 3 CO 2 H) (H 2 O) pK a = -7 pK a = 4.75 pK a =

How to Predict Outcome of Acid-Base Reactions Acid-base reactions always favor the formation of the weaker acid and the weaker base Acid-base reactions are under equilibrium control Reactions under equilibrium control always favor the formation of the most stable (lowest potential energy) species 17

Using pKa ‘s to Analyze Equilibria With the relevant pKa values given, which direction an acid-base equilibrium will favor becomes very simple with practice. pK a = 3.76 pK a = 9.22 Why is the equilibrium arrow bigger on top than on the bottom? 18

In the next class? We will discuss how to determine the relative strength of bases by simply looking at the structure of the base! From there we can determine the relative strength of acids! 19