Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chemical equilibrium wikipedia , lookup
Chemical bond wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Sulfuric acid wikipedia , lookup
Equilibrium chemistry wikipedia , lookup
Ene reaction wikipedia , lookup
Enzyme catalysis wikipedia , lookup
Chapter 3: Acid-Base Chemistry Reaction Classification: • Substitution: H3C Cl + NaOH H3C OH Br Br2 • Addition: Br • Elimination: Br -HBr • Rearrangement: We’ll deal with these later… + NaCl Bond Cleavage • Heterolytic Bond Cleavage (Polarized Bonds) Generate Ionic Species (Cation and Anion) • Homolytic 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 (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 O H + H Br H O H + Br H H Base Acid Conjugate Acid C. Base Common Strong (Inorganic) Acids • HCl (Hydorchloric) • HNO3 (Nitric) • HBr (Hydrobromic) • HClO4 (Perchloric) • HI (Hydriodic) • H2SO4 (Sulfuric) All Classified as Brønsted Acids (H+ Available to Donate) Note: Only the First Proton Dissociation in H2SO4 Quantitative Lewis Acids and Bases • Lewis Acid: Lone Pair Acceptor • Lewis Base: Lone Pair Donor • Reaction Mechanism Note: Electron Source to Electron Sink I H + NH3 NH4 + I Lewis Acids and Bases • Other Lewis Acids: ZnCl2 FeBr3 Have Available Acceptor Orbital • Other Lewis Bases: R-OH Br2 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 C Z C + Z Carbanion: Negatively Charged C Atom Carbanions Are Lewis Bases C Z C + Z 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 H O H + 2 H 2O OH H O O H O O H + H O H O O H O H O + OH O H 2O H Acid/Base Reactions & Equilibrium • We have viewed Acid/Base reactions as forward reactions; they are actually Chemical Equilibria O O H O O + H O H H O H [CH 3CO2 ][ H 3O ] K eq [CH 3CO2 H ][ H 2O] H Acid/Base Reactions & Equilibrium (2) A- + H3O+ HA + H2O Acid Dissociation Constant (Ka): [ A ][ H 3O ] K a K eq [ H 2O] [ HA] pKa = -log(Ka) pKa analagous to pH (logarithmic) Table 3.1 Contains pKa Values You Should be Familiar With pKa Values • Provide Information About Acid Strength Lower pKa Values Stronger Acids Higher pKa Values Weaker Acids CH3CH2OH pKa: 16 versus CH3CO2H 4.75 pKa Gives Information About Conjugate Base Strength as Well pKa and Base Strength • Stronger Acid has Weak Conjugate Base • Weaker Acid has Strong Conjugate Base CH3CH2OH pKa: 16 CH3CH2O- versus CH3CO2H 4.75 versus CH3CO2- 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 pKa Values to Help Determine Weaker Pair • Reactions Under Equilibrium Control Favor Most Stable, Lowest Potential Energy Species • General Rule: If pKa Difference > 5; Goes to Completion Structural Factors Influencing Acidity 1. H—X Bond Strength Weaker Bonds Stronger Acids Consider Halogen Acid Series Acid: H—F pKa: 3.2 Stronger Bonds H—Cl H—Br H—I -7 -9 -10 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: CH4 NH3 H2O H—F pKa: 48 38 15.7 3.2 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: HCCH H2CCH2 H3CCH3 pKa: 25 44 50 Hybrid. sp sp2 sp3 % s: 50 33 25 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: H3CCH3 H3C—CH2—F H3C—CH2 —CH2—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 O O H O O + H H O H O H O O O O • Also can be explained in terms of an inductive effect H Inductive Effects and Carboxylic Acids O O O Cl OH pKa = 4.75 OH F 3C 2.86 OH 0.18 Greater Halogen Substitution a to Carbonyl Greater Anion (Carboxylate) Stability Stronger Carboxylic Acid Reaction Mechanisms: Sequential A/B Rxns H H O H H O O H H -H2O Cl Cl 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 H2O + -NH2 HO- + NH3 pKa= 15.7 H H 38 NH2 NH3 + H pKa = 25 pKa = 38 CH3CH2OH + H- CH3CH2O- + H2 pKa= NH3 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 Organics • pKa Ranges of Common Organic Compounds • Anion Stability (CB) Acid Strength Relationship Know Factors Affecting Anion Stability Resonance, Inductive Effects, etc.