Chapter 3 3 Acids and Bases. The Curved-Arrow Notation Organic Chemistry, 5th ed. Marc Loudon Eric...

Chapter 3Acids and Bases. The Curved-Arrow Notation

Organic Chemistry, 5th ed.Marc Loudon

Eric J. KantorowskiCalifornia Polytechnic State UniversitySan Luis Obispo, CA

http://www.pdfcomplete.com/cms/hppl/tabid/108/Default.aspx?r=q8b3uige22

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!



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)



The Wrong Way

• Curved-arrows show the movement of electron pairs not nuclei

• Electrons are responsible for chemistry!



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



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



Organic Reactions

• The BrØnsted-Lowry acid-base concept is central to many reactions in organic chemistry

• For example:

• …looks similar to:



Nucleophiles and Electrophiles

• Nucleophile = Lewis base (“nucleus loving”)



Nucleophiles and Electrophiles

• Electrophile = Lewis acid (“electron loving”)

• The atom that receives the electron pair



Leaving Groups

• The group or atom that receives electrons from the breaking bond is a leaving group



Leaving Groups

• Can also be applied to Lewis acid-base dissociation reactions



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:



The Dissociation Constant

• As [H2O] effectively remains constant:

• Each acid has its own dissociation constant

• A larger Ka indicates more H+’s are transferred



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



Relative Strengths of Some 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



Equilibria in Acid-Base Reactions

• Determine by comparing pKa of both acids

• The side with the weaker acid and weaker base is favored



Equilibria in Acid-Base Reactions

• To estimate the equilibrium constant (Keq):



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



Relationship Between ∆G° and Keq at 25 ˚C



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



The Polar Effect

• Carboxylic acids illustrate the effect

• The conjugate base is resonance-stabilized

• Consider the following series:



The Polar Effect

• Electrostatic interactions can be stabilizing or destabilizing

• Electronegative substituents increase the acidity of carboxylic acids (inductive effect)



The Polar Effect

• ∆Ga° and pKa are directly proportional:

• Lowering the standard free-energy of a conjugate base makes the conjugate acid more acidic



The Polar Effect



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



Chapter 3 3 Acids and Bases. The Curved-Arrow Notation Organic Chemistry, 5th ed. Marc Loudon Eric...

Documents

Transcript of Chapter 3 3 Acids and Bases. The Curved-Arrow Notation Organic Chemistry, 5th ed. Marc Loudon Eric...