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Chapter 2 Acids and Bases: Central to Understanding Organic Chemistry Paula Yurkanis Bruice University of California, Santa Barbara 2.1 An Introduction to Acids and Bases An Acid Loses a Proton A Base Gains a Proton Most Acid–Base Reactions are Reversible An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, its conjugate base is formed When a base gains a proton, its conjugate acid is formed An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, its conjugate base is formed When a base gains a proton, its conjugate acid is formed The stronger the acid, the weaker is its conjugate base Brønsted–Lowry Acids and Bases Acidity: a measure of the tendency of a compound to give up a proton. The stronger the acid, the more readily it gives up a proton. Basicity: a measure of a compound’s affinity for a proton. The stronger the acid, the weaker is its conjugate base. 2.2 pKa and pH Acids Have Different Strengths stronger acid weaker acid 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. The Acid Dissociation Constant, Ka The Ka implies the concentrations of the acid and the ions The larger the Ka, the stronger the acid. Ka > 1 Ionized products greater than intact acid. Ka < 1 Ionized products less than intact acid. Ka >> 1 Ionization goes to completion (strong acid). (e.g., > 103) Ka << 1 (e.g., < 10–3) Ionization does not occur to an appreciable amount. The Acid Dissociation Constant, Ka Since the Ka values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use pKa = – log (Ka) pKa = – log Ka for Ka = 10–20, pKa = 20 for Ka = 10–5, pKa = 5 for Ka = 107, pKa = –7 Compare pKa and Ka Values pKa Ka strong acids -2 102 0 weak acids 2 10-2 4 6 10-6 8 10 10-10 12 14 10-14 The smaller the value of the pKa, the stronger the acid. pKa is a Covenient Scale for Acid Strength The stronger the acid, the larger the Ka value The stronger the acid, the smaller the pKa value pH The concentration of hydrogen ions is used as a measure of acidity This concentration is expressed as pH pH = – log[H+] The higher the concentration, the more acidic the solution and the lower the pH pH Indicates the Acidity of a Solution H2O + H2O Neutral water: pH < 7.00 pH = 7.00 pH > 7.00 OH + H3O [H3O+] = 1.0 × 10–7 M pH = – log[H3O+] = 7 Acidic solution Neutral solution Basic solution pH vs. pKa • Do not confuse pH and pKa! • The pH scale is used to describe the acidity of a solution. • The pKa is characteristic of a particular compound, indicating the tendency of the compound to give up its proton. 2.3 Organic Acids and Bases Carboxylic acids Most common organic acids! pKa: 3~5 Weak acids! Alcohols Compounds having an OH group! pKa~ 16 Much weaker acids! Amines Compounds with NH2 groups! pKa~ 40 Extremely weak, rarely behave as acids. In fact, amines are the most common organic bases! Protonated Amines Protonated Alcohols and Protonated Carboxylic Acids An Alcohol Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor 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 2.4 How to Predict the Outcome of an Acid-Base Reaction • To determine which of the two reactants of the reaction will be the acid, we need to compare their pKa values! • The stronger acid will donate a proton to the weaker acid. Here water is behaving as a base. Here water is behaving as an acid. 2.5 How to Determine the Position of Equilibrium • Compare the pKa value of the acid on the left and right of the arrow! • The equilibrium favors: Strong reacts to form weak! 2.6 How the Structure of an acid Affects its pKa • The stronger the acid, the weaker is its conjugate base! • The stronger the acid, the more stable is its conjugate base! Factors that influence stability of the conjugate base include: • Resonance • Electronegativity • Atomic Size • Hybridization • Inductive Effects Electronegativity Elements in the same row are all about the same size, but they have very different electronegativity! When atoms are similar in size, the strongest acid will have its hydrogen attached to the most electronegative atom! Why are Alcohols Stronger Acids Than Amines? Oxygen is more electronegative than nitrogen. Why are Protonated Alcohols Stronger Acids Than Protonated Amines? Oxygen is more electronegative than nitrogen. Electronegativity increasing electronegativity pKa Values CH4 ~60 NH3 36 H2O 15.7 HF H H C 3.2 H H N O RCH3 45 RNH2 35 ROH 18 R C CH3 20 O R C NH2 15 H O H O F Consider the conjugate bases R C OH 5 Hybridization Affects Electronegativity The weakest acid has the strongest conjugate base. Atomic Size When atoms are very different in size, the strongest acid will have its hydrogen attached to the largest atom! Atomic Size increasing size pKa Values O HF 3.5 F– 1.36 Å H2O 16 HCl –7 Cl– 1.81 Å H2S 7 H2Se 4 H2Te 3 HBr HI –9 –10 Br– 1.95 Å I– Consider the ionic radii 2.16 Å R C OH O R C SH S R C SH Some pKa Values 2.7 How substituents affect the strength of an acid inductive electron withdrawal A Substituent’s Effect on pKa Depends on Distance Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 1. inductive electron withdrawal Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 2. delocalized electrons (resonance) Summary of Factors That Affect Acid Strength electronegativity and size Summary of Factors That Affect Acid Strength hybridization Summary of Factors That Affect Acid Strength inductive electron withdrawal Summary of Factors That Affect Acid Strength electron delocalization 2.10 How pH Affects the Structure of an Organic Compound • A compound will exist primarily in its acidic form if the pH of the solution is less than its pKa. • A compound will exist primarily in its basic form if the pH of the solution is greater than its pKa. Is a Compound Charged or Neutral? 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. 2.11 Buffer Solutions • A solution of a weak acid (HA) and its conjugate base (A-) is called a buffer solution. • A buffer solution will maintain nearly constant pH when small amounts of acid or base are added to it!