Transcript Chapter 3: Acid-Base Chemistry

Chapter 3: Acid-Base Chemistry
Reaction Classification:
• Substitution:
H3C
Cl
+
NaO H
H3C
OH
Br
Br2
• Addition:
Br
• Elimination:
Br
-HBr
• Rearrangement: We’ll deal with these later…
+
NaC l
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 ]
Keq 
[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
F3C
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.