Chapter 3: Acid-Base Chemistry Reaction Classification: Substitution: Addition: Elimination: Rearrangement: We’ll deal with these later…

Bond Cleavage Heterolytic Bond Cleavage (Polarized Bonds)  Generate Ionic Species (Cation and Anion) Homolytic Bond Cleavage (Generate Radicals)  Radicals: Species Containing Unpaired Electrons

Acid-Base Chemistry Fundamental Chemical Reaction Very Fast Reactions (ET Faster) Chemical Equilibria Acids/Bases Classified in a Number of Ways  Arrhenius (Hydrogen and Hydroxide Ions)  Brønsted—Lowry (H+ Donors and Acceptors)  Lewis (Lone Pair Donors and Acceptors)

Brønsted—Lowry Acids and Bases Brønsted Acid: Proton (H + ) Donor Brønsted Base: Proton (H + ) Acceptor Reaction Mechanism  Note: Electron Source to Electron Sink BaseAcidConjugate AcidC. Base

Common Strong (Inorganic) Acids HCl(Hydorchloric) HBr(Hydrobromic) HI(Hydriodic) HNO 3 (Nitric) HClO 4 (Perchloric) H 2 SO 4 (Sulfuric) Note: Only the First Proton Dissociation in H 2 SO 4 Quantitative All Classified as Brønsted Acids (H + Available to Donate)

Lewis Acids and Bases Lewis Acid: Lone Pair Acceptor Lewis Base: Lone Pair Donor Reaction Mechanism  Note: Electron Source to Electron Sink

Lewis Acids and Bases Other Lewis Acids:ZnCl 2 FeBr 3  Have Available Acceptor Orbital Other Lewis Bases:R-OHBr 2  Have Lone Pair to Donate Lewis Acid/Base Reactions Essentially Electrostatic (Opposite Charges Attract)

Heterolysis of C—Z Bonds Heterolysis of C—Z Bonds Generates Ionic Species  Carbocation:Postively Charged C Atom  Carbocations Are Lewis Acids  Carbanion:Negatively Charged C Atom  Carbanions Are Lewis Bases

Nucleophiles and Electrophiles Carbocations:  Electrophiles  Seek Electrons in Reaction to Fill/Stabilize Valence Carbanions:  Nucleophiles  Seek Proton or Some Other Positive Center  “Nucleo” From Nucleus (Where Positive Charge Resides)

More Reaction Mechanisms

Acid/Base Reactions & Equilibrium We have viewed Acid/Base reactions as forward reactions; they are actually Chemical Equilibria

Acid/Base Reactions & Equilibrium (2) HA + H 2 O A - + H 3 O + Acid Dissociation Constant (K a ): pK a = -log(K a ) pK a analagous to pH (logarithmic) Table 3.1 Contains pK a Values You Should be Familiar With

pK a Values Provide Information About Acid Strength  Lower pK a Values  Stronger Acids  Higher pK a Values  Weaker Acids CH 3 CH 2 OH versus CH 3 CO 2 H pK a : pK a Gives Information About Conjugate Base Strength as Well

pK a and Base Strength Stronger Acid has Weak Conjugate Base Weaker Acid has Strong Conjugate Base CH 3 CH 2 OH versus CH 3 CO 2 H pK a : CH 3 CH 2 O - versus CH 3 CO 2 - Acetic Acid is the Stronger Acid; Ethoxide is the Stronger Base

Predicting Acid/Base Reaction Outcomes Acid/Base Reactions Favor Formation of Weaker Acid/Base  Use pK a Values to Help Determine Weaker Pair Reactions Under Equilibrium Control  Favor Most Stable, Lowest Potential Energy Species General Rule: If pK a Difference > 5; Goes to Completion

Structural Factors Influencing Acidity 1. H—X Bond Strength Weaker Bonds  Stronger Acids Consider Halogen Acid Series Acid:H—FH—ClH—BrH—I pK a : Stronger Bonds Weaker Bonds Also Think About the Stability of the Ion (Conjugate Base)

Structural Factors Influencing Acidity 2. Electronegativity For Same Row: > Electronegativity  Stonger Acid Consider Series of C, N, O, F Acids Acid: CH 4 NH 3 H 2 OH—F pK a : Look at the Polarization of the Bonds: C—H least polarized; H—F most polarized

Structural Factors Influencing Acidity 3. Hybridization More ‘s’ character in the orbital  more stable anion Consider Alkanes, Alkenes, Alkynes Acid: HCCHH 2 CCH 2 H 3 CCH 3 pK a : Hybrid. sp sp 2 sp 3 % s: s Orbital Stability from Proximity to Nucleus

Structural Factors Influencing Acidity 4. Inductive Effects Polarized Bonds (Electronegative Atoms) Affect Neighboring Atoms Magnitude of Effect Related to Proximity Also Called Electron Withdrawing Effect Acid: H 3 CCH 3 H 3 C—CH 2 —F H 3 C—CH 2 —CH 2 —F The Further Away the Atom; The Lesser the Inductive Effect

Acidity of Carboxylic Acids: Resonance Conjugate Base of a Carboxylic Acid is Resonance Stabilized Also can be explained in terms of an inductive effect

Inductive Effects and Carboxylic Acids Greater Halogen Substitution  to Carbonyl  Greater Anion (Carboxylate) Stability  Stronger Carboxylic Acid

Reaction Mechanisms: Sequential A/B Rxns Each Reaction an Acid/Base Reaction: Lewis or Brønsted?

Non-Aqueous Acid/Base Reactions If Base is Stronger than Hydroxide; Water Can’t be Solvent H 2 O + - NH 2  HO - + NH 3 CH 3 CH 2 OH + H -  CH 3 CH 2 O - + H 2 pK a = pK a = Same Rules: Reaction to Weaker Acid/Base Pair

Acid/Base Chemistry: Summary Equilibra (Procede in Weak Acid/Base Direction) Lewis Acidity/Basicity of Organics pK a Ranges of Common Organic Compounds Anion Stability (CB)  Acid Strength Relationship  Know Factors Affecting Anion Stability  Resonance, Inductive Effects, etc.