© 2014 Pearson Education, Inc. Acids and Bases: Central to Understanding Organic Chemistry Chapter 2 Paula Yurkanis Bruice University of California, Santa Barbara

© 2014 Pearson Education, Inc. An Acid Loses a Proton A Base Gains a Proton

© 2014 Pearson Education, Inc. Most Acid–Base Reactions are Reversible

© 2014 Pearson Education, Inc. An Acid and its Conjugate Base A Base and its Conjugate Acid When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid.

© 2014 Pearson Education, Inc. When an acid loses a proton, it forms its conjugate base. When a base gains a proton, it forms its conjugate acid. The stronger the acid, the weaker its conjugate base. An Acid and its Conjugate Base A Base and its Conjugate Acid

© 2014 Pearson Education, Inc. Acids Have Different Strengths

© 2014 Pearson Education, Inc. 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.

© 2014 Pearson Education, Inc. The stronger the acid, the larger the K a. The stronger the acid, the smaller the pK a. Acid Strength

© 2014 Pearson Education, Inc. pH The pH indicates the concentration of protons is a solution.

© 2014 Pearson Education, Inc. Carboxylic Acids are the Most Common Organic Acids

© 2014 Pearson Education, Inc. Alcohols

© 2014 Pearson Education, Inc. Amines

© 2014 Pearson Education, Inc. Protonated Amines

© 2014 Pearson Education, Inc. Protonated Alcohols and Protonated Carboxylic Acids

© 2014 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.

© 2014 Pearson Education, Inc. A Carboxylic Acid Can Behave as an Acid and as a Base A curved arrow points from the electron donor to the electron acceptor.

© 2014 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.

© 2014 Pearson Education, Inc.

Which Reactant is the Acid? Water is the base.Water is the acid. The stronger acid behaves as the acid.

© 2014 Pearson Education, Inc. The Position of Equilibrium The equilibrium favors formation of the weaker acid.

© 2014 Pearson Education, Inc. Electronegativity Affects pK a Values When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom.

© 2014 Pearson Education, Inc. The Stronger the Acid, the Weaker Its Conjugate Base stable bases are weak bases

© 2014 Pearson Education, Inc. Why are Alcohols Stronger Acids Than Amines? Oxygen is more electronegative than nitrogen.

© 2014 Pearson Education, Inc. Why Are Protonated Alcohols Stronger Acids Than Protonated Amines? Oxygen is more electronegative than nitrogen.

© 2014 Pearson Education, Inc. Hybridization Affects Electronegativity The weakest acid has the strongest conjugate base.

© 2014 Pearson Education, Inc. Size Affects pK a Values When atoms differ in size, the strongest acid has its hydrogen boded to the largest atom.

© 2014 Pearson Education, Inc. Some pK a Values

© 2014 Pearson Education, Inc. Substituents Affect the Strength of the Acid inductive electron withdrawal

© 2014 Pearson Education, Inc. A Substituent’s Effect on pK a Depends on Distance

© 2014 Pearson Education, Inc. Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 1. inductive electron withdrawal

© 2014 Pearson Education, Inc. Why is a Carboxylic Acid a Stronger Acid Than an Alcohol? 2. delocalized electrons (resonance)

© 2014 Pearson Education, Inc. electronegativity and size Summary of Factors That Affect Acid Strength

© 2014 Pearson Education, Inc. Summary of Factors That Affect Acid Strength hybridization

© 2014 Pearson Education, Inc. Summary of Factors That Affect Acid Strength inductive electron withdrawal

© 2014 Pearson Education, Inc. electron delocalization Summary of Factors That Affect Acid Strength

© 2014 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 is basic form (without its proton).

© 2014 Pearson Education, Inc. A Compound with pK a = 5.2 blue = acidic formgreen = basic form

© 2014 Pearson Education, Inc. Water and Diethyl Ether Form Two Layers Diethyl ether is less dense than water, so diethyl ether is on top.

© 2014 Pearson Education, Inc. Is a Compound Charged or Neutral? A carboxylic acid 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.

© 2014 Pearson Education, Inc. A Buffer Solution A buffer solution contains an acid and its conjugate base.

© 2014 Pearson Education, Inc. Lewis Acids and Bases Lewis definitions: acid: a species that accepts a share in an electron pair base: a species that donates a share in an electron pair All Brønsted acids are Lewis acids. All Brønsted bases are Lewis bases.

© 2014 Pearson Education, Inc. Lewis Acids and Bases

© 2014 Pearson Education, Inc. How Chemists Use the Terms “acid” = proton-donating acid “Lewis acid” = non-proton-donating acid “base” = a compound that shares its electrons with a proton “nucleophile” = a compound that shares its electrons with an atom other than a proton