Chapter 3: Acid-Base Chemistry Reaction Classification: Substitution: Addition: Elimination:...
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Transcript of Chapter 3: Acid-Base Chemistry Reaction Classification: Substitution: Addition: Elimination:...
![Page 1: Chapter 3: Acid-Base Chemistry Reaction Classification: Substitution: Addition: Elimination: Rearrangement: We’ll deal with these later…](https://reader036.fdocuments.us/reader036/viewer/2022081419/56649cc15503460f94988bf6/html5/thumbnails/1.jpg)
Chapter 3: Acid-Base Chemistry
Reaction Classification:
• Substitution:
• Addition:
• Elimination:
• Rearrangement: We’ll deal with these later…
+ NaOH + NaClH3C Cl H3C OH
Br2
Br
Br
Br-HBr
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Bond Cleavage
A B A B+
A B A B+
• Heterolytic Bond Cleavage (Polarized Bonds)
Generate Ionic Species (Cation and Anion)
• Homolytic Bond Cleavage (Generate Radicals)
Radicals: Species Containing Unpaired Electrons
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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)
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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
H Br+H O H
H
+ Br
Base Acid Conjugate Acid C. Base
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Common Strong (Inorganic) Acids
• HCl (Hydorchloric)
• HBr (Hydrobromic)
• HI (Hydriodic)
• HNO3 (Nitric)
• HClO4 (Perchloric)
• H2SO4 (Sulfuric)
Note: Only the First Proton Dissociation in H2SO4 Quantitative
All Classified as Brønsted Acids (H+ Available to Donate)
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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
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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)
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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
C Z
C + Z
C Z
C + Z
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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)
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More Reaction Mechanisms
H O H
H
+ OH 2 H2O
O
O
H O H
HO
O
+H O H
H
O
O
H
O
O
+OH H2O
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Acid/Base Reactions & Equilibrium
• We have viewed Acid/Base reactions as forward reactions;they are actually Chemical Equilibria
O
O
H O H
HO
O
+H O H
H
]][[
]][[
223
323
OHHCOCH
OHCOCHKeq
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Acid/Base Reactions & Equilibrium (2)
HA + H2O A- + H3O+
][
]][[][ 3
2 HA
OHAOHKK eqa
Acid Dissociation Constant (Ka):
pKa = -log(Ka)
pKa analagous to pH (logarithmic)
Table 3.1 Contains pKa Values You Should be Familiar With
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pKa Values
• Provide Information About Acid Strength
Lower pKa Values Stronger Acids
Higher pKa Values Weaker Acids
CH3CH2OH versus CH3CO2H
pKa: 16 4.75
pKa Gives Information About Conjugate Base Strength as Well
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pKa and Base Strength
• Stronger Acid has Weak Conjugate Base
• Weaker Acid has Strong Conjugate Base
CH3CH2OH versus CH3CO2H
pKa: 16 4.75
CH3CH2O- versus CH3CO2-
Acetic Acid is the Stronger Acid; Ethoxide is the Stronger Base
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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
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Structural Factors Influencing Acidity
1. H—X Bond Strength
Weaker Bonds Stronger Acids
Consider Halogen Acid Series
Acid: H—F H—Cl H—Br H—I
pKa: 3.2 -7 -9 -10
Stronger Bonds Weaker Bonds
Also Think About the Stability of the Ion (Conjugate Base)
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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
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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 25s Orbital Stability from Proximity to Nucleus
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Structural Factors Influencing Acidity
4. Inductive Effects
Polarized Bonds (Electronegative Atoms) AffectNeighboring 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
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Acidity of Carboxylic Acids: Resonance
O
O
H O H
HO
O
+H O H
H
• Conjugate Base of a Carboxylic Acid is Resonance Stabilized
O
O O
O
• Also can be explained in terms of an inductive effect
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Inductive Effects and Carboxylic Acids
O
OH
O
F3C OH
O
OHCl
pKa = 4.75 2.86 0.18
Greater Halogen Substitution to Carbonyl
Greater Anion (Carboxylate) Stability
Stronger Carboxylic Acid
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Reaction Mechanisms: Sequential A/B Rxns
OH HOH
H
O H
H
-H2O
ClCl
Each Reaction an Acid/Base Reaction: Lewis or Brønsted?
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Non-Aqueous Acid/Base Reactions
H HNH2
H + NH3
pKa = 25 pKa = 38
NH3
• If Base is Stronger than Hydroxide; Water Can’t be Solvent
H2O + -NH2 HO- + NH3
CH3CH2OH + H- CH3CH2O- + H2
pKa= 15.7 38
pKa= 16 35
Same Rules: Reaction to Weaker Acid/Base Pair
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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.