Organic Chemistry, 5th ed. Marc Loudon Chapter 3 Acids and Bases. The Curved-Arrow Notation Eric J. Kantorows ki California Polytechnic State University San 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 3.1 Lewis Acid-Base Association Reactions 3
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) 3.1 Lewis Acid-Base Association Reactions 4
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 3.2 Electron-Pair Displacement Reactions 5
Curved-Arrow Notation for Displacement Displacement reactions require two arrows Watch for conservation of total charge! 3.2 Electron-Pair Displacement Reactions 6
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) 3.2 Electron-Pair Displacement Reactions 7
The Wrong Way Curved-arrows show the movement of electron pairs not nuclei Electrons are responsible for chemistry! 3.2 Electron-Pair Displacement Reactions 8
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) 3.3 Review of the Curved-Arrow Notation 9
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] 3.3 Review of the Curved-Arrow Notation 10
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 electronpair displacement on a proton 3.4 BrØnsted-Lowry Acids and Bases 11
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 3.4 BrØnsted-Lowry Acids and Bases 12
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 3.4 BrØnsted-Lowry Acids and Bases 13
Organic Reactions The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry For example: looks similar to: 3.4 BrØnsted-Lowry Acids and Bases 14
Nucleophiles and Electrophiles Nucleophile = Lewis base ( nucleus loving ) 3.4 BrØnsted-Lowry Acids and Bases 15
Nucleophiles and Electrophiles Electrophile = Lewis acid ( electron loving ) The atom that receives the electron pair 3.4 BrØnsted-Lowry Acids and Bases 16
Leaving Groups The group or atom that receives electrons from the breaking bond is a leaving group 3.4 BrØnsted-Lowry Acids and Bases 17
Leaving Groups Can also be applied to Lewis acid-base dissociation reactions 3.4 BrØnsted-Lowry Acids and Bases 18
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: 3.4 BrØnsted-Lowry Acids and Bases 19
The Dissociation Constant As [H 2 O] effectively remains constant: Each acid has its own dissociation constant A larger K a indicates more H + s are transferred 3.4 BrØnsted-Lowry Acids and Bases 20
The pk a Scale and ph pk a values are more manageable than K a values Stronger acids have smaller pk a values ph is a measure of [H + ], a property of a solution pk a is a measure of acid strength, a fixed property 3.4 BrØnsted-Lowry Acids and Bases 21
Relative Strengths of Some Acids and Bases 3.4 BrØnsted-Lowry Acids and Bases 22
Strengths of BrØnsted Bases Directly related to pk a of the conjugate acid Example: The base strength of chloride is indicated by the pk a of HCl If a base is weak, its conjugate acid is strong If a base is strong, its conjugate acid is weak 3.4 BrØnsted-Lowry Acids and Bases 23
Equilibria in Acid-Base Reactions Determine by comparing pk a of both acids The side with the weaker acid and weaker base is favored 3.4 BrØnsted-Lowry Acids and Bases 24
Equilibria in Acid-Base Reactions To estimate the equilibrium constant (K eq ): 3.4 BrØnsted-Lowry Acids and Bases 25
Standard Free Energy ( G ) K a is related to the standard free-energy difference between products and reactants Or, more generally, 3.5 Free Energy and Chemical Equilibrium 26
Chemical Equilibrium Rearrangement of the previous relationship: K eq is exponentially dependent on G Small changes in G large changes in K eq If G < 0 then K eq > 1 If G > 0 then K eq < 1 3.5 Free Energy and Chemical Equilibrium 27
Relationship Between G and K eq at 25 C 3.5 Free Energy and Chemical Equilibrium 28
The Element Effect Evaluate the atom attached to the proton 3.6 Relationship of Structure to Acidity 29
The Charge Effect Positively charged compounds attract electrons better than neutral ones pk a of H 3 O + = -1.7 vs pk a of H 2 O = 15.7 3.6 Relationship of Structure to Acidity 30
The Polar Effect Carboxylic acids illustrate the effect The conjugate base is resonance-stabilized Consider the following series: 3.6 Relationship of Structure to Acidity 31
The Polar Effect Electrostatic interactions can be stabilizing or destabilizing Electronegative substituents increase the acidity of carboxylic acids (inductive effect) 3.6 Relationship of Structure to Acidity 32
The Polar Effect G a and pk a are directly proportional: Lowering the standard free-energy of a conjugate base makes the conjugate acid more acidic 3.6 Relationship of Structure to Acidity 33
The Polar Effect 3.6 Relationship of Structure to Acidity 34
The Polar Effect Halogens and other electronegative groups exert an electron-withdrawing polar effect This lowers the pk a of carboxylic acids Other groups can exert an electron-donating polar effect This raises the pk a of carboxylic acids 3.6 Relationship of Structure to Acidity 35