chapter 3 3 acids and bases. the curved-arrow notation organic chemistry, 5th ed. marc loudon eric...
TRANSCRIPT
Chapter 3Acids and Bases. The Curved-Arrow Notation
Organic Chemistry, 5th ed.Marc Loudon
Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA
Chapter 3 Overview
• 3.1 Lewis Acid-Base Association Reactions
• 3.2 Electron-Pair Displacement Reactions
• 3.3 Review of the Curved-Arrow Notation
• 3.4 BrØnsted-Lowry Acids and Bases
• 3.5 Free Energy and Chemical Equilibrium
• 3.6 Relationship of Structure to Acidity
2
Electron-Deficient Compounds
• Atoms that have less
than an octet
• They act as Lewis acids in order to fulfill their valence-shell octet
33.1 Lewis Acid-Base Association Reactions
Curved-Arrow Notation
• A tool for tracking electrons in a chemical reaction
• Electrons flow from the electron donor (Lewis base) to the electron acceptor (Lewis acid)
43.1 Lewis Acid-Base Association Reactions
Other Electron Donation Reactions
• Not all acceptors are electron-deficient
• An electron pair must depart from the atom receiving an electron pair
• This preserves the octet rule
53.2 Electron-Pair Displacement Reactions
Curved-Arrow Notation for Displacement
• Displacement reactions require two arrows
• Watch for conservation of total charge!
63.2 Electron-Pair Displacement Reactions
Curved-Arrow Notation for Displacement
• Donated electron pairs can also originate from a bond
• Imagine the bond as “hinged” to the transferred atom (the H of the B-H bond)
73.2 Electron-Pair Displacement Reactions
The Wrong Way
• Curved-arrows show the movement of electron pairs not nuclei
• Electrons are responsible for chemistry!
83.2 Electron-Pair Displacement Reactions
Two Reactions Represented by Curved Arrows
• Lewis base + an electron-deficient compound
• Electron-pair displacement reactions
• Every reaction involving electron pairs fits into one of these two categories (or combinations)
93.3 Review of the Curved-Arrow Notation
Curved-Arrow Notation for Resonance
• Resonance structures differ only by movement of electrons (and usually electron pairs)
• Curved-arrow notation is ideal to help derive resonance contributors
[Structures p95 solution (a) and (b) of questions]
103.3 Review of the Curved-Arrow Notation
BrØnsted Acids and Bases
• BrØnsted Acid: A species that donates a H+
• BrØnsted Bases: A species that accepts a H+
• A BrØnsted acid-base reaction is an electron-pair displacement on a proton
113.4 BrØnsted-Lowry Acids and Bases
Conjugate Acids and Bases
• When a BrØnsted acid loses a proton, its conjugate base is formed
• When a BrØnsted base gains a proton, its conjugate acid is formed
123.4 BrØnsted-Lowry Acids and Bases
Amphoteric Compounds
• Compounds that can act as either an acid or a base are called amphoteric
• Observe the behavior of a compound in a reaction to classify it as an acid or base
• Water is amphoteric
133.4 BrØnsted-Lowry Acids and Bases
Organic Reactions
• The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry
• For example:
• …looks similar to:
143.4 BrØnsted-Lowry Acids and Bases
Nucleophiles and Electrophiles
• Nucleophile = Lewis base (“nucleus loving”)
153.4 BrØnsted-Lowry Acids and Bases
Nucleophiles and Electrophiles
• Electrophile = Lewis acid (“electron loving”)
• The atom that receives the electron pair
163.4 BrØnsted-Lowry Acids and Bases
Leaving Groups
• The group or atom that receives electrons from the breaking bond is a leaving group
173.4 BrØnsted-Lowry Acids and Bases
Leaving Groups
• Can also be applied to Lewis acid-base dissociation reactions
183.4 BrØnsted-Lowry Acids and Bases
Strengths of BrØnsted Acids
• A measure of the extent of proton transfer to a BrØnsted base
• The standard base traditionally used is water
• The equilibrium constant is:
193.4 BrØnsted-Lowry Acids and Bases
The Dissociation Constant
• As [H2O] effectively remains constant:
• Each acid has its own dissociation constant
• A larger Ka indicates more H+’s are transferred
203.4 BrØnsted-Lowry Acids and Bases
The pKa Scale and pH
• pKa values are more manageable than Ka
values
• Stronger acids have smaller pKa values
• pH is a measure of [H+], a property of a solution
• pKa is a measure of acid strength, a fixed property
213.4 BrØnsted-Lowry Acids and Bases
Relative Strengths of Some Acids and Bases
223.4 BrØnsted-Lowry Acids and Bases
Strengths of BrØnsted Bases
• Directly related to pKa of the conjugate acid
• Example: The base strength of chloride is indicated by the pKa of HCl
• If a base is weak, its conjugate acid is strong
• If a base is strong, its conjugate acid is weak
233.4 BrØnsted-Lowry Acids and Bases
Equilibria in Acid-Base Reactions
• Determine by comparing pKa of both acids
• The side with the weaker acid and weaker base is favored
243.4 BrØnsted-Lowry Acids and Bases
Equilibria in Acid-Base Reactions
• To estimate the equilibrium constant (Keq):
253.4 BrØnsted-Lowry Acids and Bases
Standard Free Energy (∆G°)
• Ka is related to the standard free-energydifference between products and reactants
• Or, more generally,
263.5 Free Energy and Chemical Equilibrium
Chemical Equilibrium
• Rearrangement of the previous relationship:
• Keq is exponentially dependent on ∆G°
• Small changes in ∆G° → large changes in Keq
• If ∆G° < 0 then Keq > 1
• If ∆G° > 0 then Keq < 1
273.5 Free Energy and Chemical Equilibrium
Relationship Between ∆G° and Keq at 25 ˚C
283.5 Free Energy and Chemical Equilibrium
The Element Effect
• Evaluate the atom attached to the proton
293.6 Relationship of Structure to Acidity
The Charge Effect
• Positively charged compounds attract electrons better than neutral ones
• pKa of H3O+ = -1.7 vs pKa of H2O = 15.7
303.6 Relationship of Structure to Acidity
The Polar Effect
• Carboxylic acids illustrate the effect
• The conjugate base is resonance-stabilized
• Consider the following series:
313.6 Relationship of Structure to Acidity
The Polar Effect
• Electrostatic interactions can be stabilizing or destabilizing
• Electronegative substituents increase the acidity of carboxylic acids (inductive effect)
323.6 Relationship of Structure to Acidity
The Polar Effect
• ∆Ga° and pKa are directly proportional:
• Lowering the standard free-energy of a conjugate base makes the conjugate acid more acidic
333.6 Relationship of Structure to Acidity
The Polar Effect
343.6 Relationship of Structure to Acidity
The Polar Effect
• Halogens and other electronegative groups exert an electron-withdrawing polar effect
• This lowers the pKa of carboxylic acids
• Other groups can exert an electron-donating polar effect
• This raises the pKa of carboxylic acids
353.6 Relationship of Structure to Acidity