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

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

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

H Br+H O H

H

+ Br

Base Acid Conjugate Acid C. Base

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)

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

Carbanion: Negatively Charged C Atom Carbanions Are Lewis Bases

C Z

C + Z

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

H

+ OH 2 H2O

O

O

H O H

HO

O

+H O H

H

O

O

H

O

O

+OH H2O

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

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

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

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

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 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)

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 25s Orbital Stability from Proximity to Nucleus

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

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

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

Reaction Mechanisms: Sequential A/B Rxns

OH HOH

H

O H

H

-H2O

ClCl

Each Reaction an Acid/Base Reaction: Lewis or Brønsted?

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

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.