Chapter 3: Acid-Base Chemistry Reaction Classification: Substitution: 3 C Cl + Na 3 C + NaCl Addition: Br 2 Br Br Elimination: Br -Br Rearrangement: We ll deal with these later
Bond Cleavage eterolytic Bond Cleavage (Polarized Bonds) Generate Ionic Species (Cation and Anion) omolytic Bond Cleavage (Generate Radicals) Radicals: Species Containing Unpaired Electrons A B A + B A B A + B
Acid-Base Chemistry Fundamental Chemical Reaction Very Fast Reactions (ET Faster) Chemical Equilibria Acids/Bases Classified in a Number of Ways Arrhenius (ydrogen and ydroxide Ions) Brønsted Lowry (+ Donors and Acceptors) Lewis (Lone Pair Donors and Acceptors)
Brønsted Lowry Acids and Bases Brønsted Acid: Proton ( + ) Donor Brønsted Base: Proton ( + ) Acceptor Reaction Mechanism Note: Electron Source to Electron Sink + Br + Br Base Acid Conjugate Acid C. Base
Common Strong (Inorganic) Acids Cl (ydorchloric) Br (ydrobromic) N 3 Cl 4 (Nitric) (Perchloric) I (ydriodic) 2 S 4 (Sulfuric) All Classified as Brønsted Acids ( + Available to Donate) Note: nly the First Proton Dissociation in 2 S 4 Quantitative
Lewis Acids and Bases Lewis Acid: Lone Pair Acceptor Lewis Base: Lone Pair Donor Reaction Mechanism Note: Electron Source to Electron Sink I + N 3 N 4 + I
Lewis Acids and Bases ther Lewis Acids: ZnCl 2 FeBr 3 ave Available Acceptor rbital ther Lewis Bases: R- Br 2 ave Lone Pair to Donate Lewis Acid/Base Reactions Essentially Electrostatic (pposite Charges Attract)
eterolysis of C Z Bonds eterolysis of C Z Bonds Generates Ionic Species Carbocation: Postively Charged C Atom Carbocations Are Lewis Acids δ C δ Z C + Z Carbanion: Negatively Charged C Atom Carbanions Are Lewis Bases δ C δ Z C + Z
Nucleophiles and Electrophiles Carbocations: Electrophiles Seek Electrons in Reaction to Fill/Stabilize Valence Carbanions: Nucleophiles Seek Proton or Some ther Positive Center Nucleo From Nucleus (Where Positive Charge Resides)
More Reaction Mechanisms + 2 2 + + 2
Acid/Base Reactions & Equilibrium We have viewed Acid/Base reactions as forward reactions; they are actually Chemical Equilibria + [ C C ][ + ] K = 3 2 3 eq [ C C ][ ] 3 2 2
Acid/Base Reactions & Equilibrium (2) A + 2 A - + 3 + Acid Dissociation Constant (K a ): K a = K eq [ A ][ [ ] = 3 2 [ A] + ] pk a = -log(k a ) pk a analagous to p (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 igher pk a Values Weaker Acids C 3 C 2 versus C 3 C 2 pk a : 16 4.75 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 C 3 C 2 versus C 3 C 2 pk a : 16 4.75 C 3 C 2 - versus C 3 C 2 - Acetic Acid is the Stronger Acid; Ethoxide is the Stronger Base
Predicting Acid/Base Reaction utcomes Acid/Base Reactions Favor Formation of Weaker Acid/Base Use pk a Values to elp 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. X Bond Strength Weaker Bonds Stronger Acids Consider alogen Acid Series Acid: F Cl Br I pk a : 3.2-7 -9-10 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,, F Acids Acid: C 4 N 3 2 F pk a : 48 38 15.7 3.2 Look at the Polarization of the Bonds: C least polarized; F most polarized
Structural Factors Influencing Acidity 3. ybridization More s character in the orbital more stable anion Consider Alkanes, Alkenes, Alkynes Acid: CC 2 CC 2 3 CC 3 pk a : 25 44 50 ybrid. sp sp 2 sp 3 % s: 50 33 25 s rbital 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: 3 CC 3 3 C C 2 F 3 C C 2 C 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 Cl F 3 C pka = 4.75 2.86 0.18 Greater alogen Substitution α to Carbonyl Greater Anion (Carboxylate) Stability Stronger Carboxylic Acid
Reaction Mechanisms: Sequential A/B Rxns - 2 Cl Cl Each Reaction an Acid/Base Reaction: Lewis or Brønsted?
Non-Aqueous Acid/Base Reactions If Base is Stronger than ydroxide; Water Can t be Solvent 2 + - N 2 - + N 3 pk a = 15.7 38 N 2 + N 3 N 3 pk a = 25 pk a = 38 C 3 C 2 + - C 3 C 2 - + 2 pk a = 16 35 Same Rules: Reaction to Weaker Acid/Base Pair
Acid/Base Chemistry: Summary Equilibra (Procede in Weak Acid/Base Direction) Lewis Acidity/Basicity of rganics pk a Ranges of Common rganic Compounds Anion Stability (CB) Acid Strength Relationship Know Factors Affecting Anion Stability Resonance, Inductive Effects, etc.