4.1 Arrhenius Acids and Bases
4.1 Arrhenius Acids and Bases Thursday, September 05, 2013 12:01 AM
Arrhenius definition: • An acid is a substance that dissolves in water to produce H+ ions • A base is a substance that dissolves in water to produce OH- ions This was established back then, but we now know that these definitions aren't accurate to refer to an acid and base. Today's definition: An H+ ion immediately combines with a water molecule to give a hydronium ion H3O+ • Hydration of the hydronium ion itself gives the ion H3O2+ • The monohydrated and dihydrated forms (H3O+ and H3O2+) are the MAJOR hydrated forms present in aqueous solution ○ Present in equal concentrations •
Arrhenius is modified to take into account these interactions of H+ with water molecules. Arrhenius definitions are still valid as long as it's about AQUEOUS SOLUTIONS
Chapter 4 Page 1
4.2 Bonsted-Lowry Acids and Bases Thursday, September 26, 2013 9:14 AM
1923, Danish chemist Johannes Bronsted and English chemist Thomas Lowry independently proposed the following definitions Bronsted-Lowry definition: • An acid is a proton donor • A base is a proton acceptor • And acid-base reaction is a proton-transfer reaction
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Conjugate Acid-Base Pairs Differ by a Proton Thursday, September 26, 2013 9:16 AM
Conjugate acid-base pair - any pair of molecules or ions that can be interconverted by transfer of a proton • Conjugate base - when an acid transfers a proton to a base, the acid is converted to this • Conjugate acid - when a base accepts a proton, it's converted to this
Curved arrows can also be used to show the transfer of a proton:
**the arrows can't point backwards! The Bronsted-Lowry definitions DON'T REQUIRE WATER as a reactant Ex:
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Example 4.1 Thursday, September 26, 2013 9:20 AM
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Example 4.2 Thursday, September 26, 2013 9:20 AM
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Bronsted-Lowry Bases with Two or More Receptor Sites Thursday, September 26, 2013 9:21 AM
Many organic compounds have two or more receptor sites! The more stable charged species is the one in which the charge is more delocalized --- relative charge delocalization can be understand by resonance. Ex:
• We now examine each cation and determine which is more stable (lower in energy) ○ Write the contributing structures
A-1 and A-3 make the greater contributions ○ Two resonance contributing structure for protonation on the hydroxyl oxygen giving cation B
B-2 at best makes a minor contribution because of the adjacent positive charges
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Example 4.3 Thursday, September 26, 2013 9:27 AM
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π Electrons as Bronsted-Lowry Bases Thursday, September 26, 2013 9:28 AM
So far, we considered proton transfer to atoms having a nonbonding pair of electrons ----- also occur with compounds having π electrons Ex:
Carbocation - a species in which one of its carbons has only 6 electrons in its valence shell and carries a charge of +1
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Example 4.4 Thursday, September 26, 2013 9:36 AM
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4.3 Acid Dissociation Constants, pK a, and the Relative Strengths of Acids and Bases Thursday, September 26, 2013 9:37 AM
Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. • Strength of an acid is expressed by an equilibrium constant • Ex: ○
○ ○ Water is the solvent for this reaction Acid dissociation constant - Ka
• • • •
The larger the pKa, the weaker the acid The smaller the pKa, the stronger the acid The weaker the acid, the stronger its conjugate base The stronger the acid, the weaker its conjugate base
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Example 4.5 Thursday, September 26, 2013 9:43 AM
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4.4 The Position of Equilibrium in Acid-Base Reactions Thursday, September 26, 2013 9:44 AM
Negative pKa value ---- majority of molecules of the acid are dissociated in water • Ex: HCl has pKa = -7 ○ Positive pKa value ----most acid molecules remain undissociated in water • Ex: acetic acid has pKa of 4.76 ○ Example of the acid-base reaction of acetic acid and ammonia to from acetate ion and ammonium ion:
THE MOST IMPORTANT CONCEPT IS AS FOLLOWS: in an acid-base reaction, the position of equilibrium always favors reaction of the stronger acid and stronger base to form the weaker acid and weaker base.
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How to Calculate the Equilibrium Constants for Acid-Base Reactions Thursday, September 26, 2013 9:56 AM
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Example 4.6 Thursday, September 26, 2013 9:58 AM
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4.5 Thermochemistry and Mechanisms of Acid-Base Reactions Thursday, September 26, 2013 9:58 AM
Reaction mechanism - step-by-step description of how a chemical reaction occurs • Describes which bonds are broken and which bonds are formed Thermochemistry - the study of the energy of the entire system at every instant of a reaction
Chapter 4 Page 15
Thermal Reactions and Transition States Thursday, September 26, 2013 10:00 AM
Most chemical reactions occur via COLLISIONS! Thermal reactions - reactions that result by virtue of the kinetic energy put into a reaction vessel due to temperature • During collisions, kinetic energy of the reactants is converted into potential energy ○ The potential energy is stored in the chemical structures in the form of the structural strains Energy is released as the molecules again adopt their optimal geometries Transition state - commonly called activated complex; the highest energy point on a reaction coordinate diagram
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Reaction Coordinate Diagrams and Thermochemistry Thursday, September 26, 2013 10:07 AM
Reaction coordinate diagrams - a graph showing the energy charges that occur during a chemical reaction • Vertical axis = energy • Horizontal axis = reaction progress (reaction coordinate)
Thermodynamics - study of the relative energies between any two entities on a reaction coordinate diagram that are shown in wells, such as the energy between the reactants and products Change in Gibbs free energy - ∆G° or ∆G°rxn
• • R = 8.31 J/K mol • T = temperature in Kelvins Free energy of activation - the Gibbs free energy difference between the reactants and the transition state • Controls the rate of the reaction Kinetics - study of the rates of chemical reactions Exergonic reaction - negative ∆G; FAVORABLE • The more negative this value, the greater the driving force for the reaction to occur • ∆G° is less than 0 ------- Keq is greater than 1 Endergonic reaction - positive ∆G; UNFAVORABLE • ∆G° is greater than 0 ------- Keq is less than 1 • ∆G° is equal to 0 ------- Keq is equal to 1 Gibbs-Helmholtz equation: • Enthalpy (H°) - energy contained in chemical bonds and solvation • Heat of reaction - difference in enthalpy between reactants and products • Exothermic - bonds formed in the products are stronger than the bonds in the reactants; HEAT WILL BE RELEASED ○ ∆H° is negative ○ Warms up • Endothermic - bonds formed in the products are weaker; HEAT WILL BE ABSORBED ○ ∆H° is positive ○ Cools down • Entropy (S) - measures disorder (which is favorable) All systems become more stable as the number of freely moving particles increases and the chaotic movement of the particles increase!
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Example 4.7 Thursday, September 26, 2013 10:14 AM
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Mechanism and Thermochemistry of Acid-Base Reactions Thursday, September 26, 2013 1:06 PM
For an acid to transfer its proton to a base, the two entities must COLLIDE! • But they collide with a specific geometry Example:
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4.6 Molecular Structure and Acidity Thursday, September 26, 2013 1:08 PM
Ways to stabilize A-: • Having the negative charge on a more electronegative atom • Having the negative charge on a larger atom • Delocalizing the negative charge as described by resonance contributing structures • Spreading the negative charge onto electron-withdrawing groups by the inductive effect ○ Polarization of sigma bonds • Having the negative charge in an orbital with more s character A- needs to be stabilized resulting from loss of a proton --- the more stable the anion, the greater the acidity of the acid HA
Chapter 4 Page 20
Electronegativity of the Atom Bearing the Negative Charge Thursday, September 26, 2013 1:22 PM
Anions of atoms within the same period have approx. the same size, and their energies of solvation are approx. the same! Ex: carbon, the least electronegative, forms the least stable anion Because methanol forms the most stable anion, it's the strongest acid in the above series
Chapter 4 Page 21
Size of the Atom Bearing the Negative Charge Thursday, September 26, 2013 1:24 PM
Example:
Any set of hydrogen acids within a group of the periodic table is related to the size of the atom bearing the negative charge… Size increases from top to bottom within a group! • They are farther from the nucleus • They occupy a larger volume of space The bigger the size, the MORE STABLE!
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Delocalization of Charge in the Anion Thursday, September 26, 2013 1:28 PM
Carboxylic acids are weak acids. -- fall within the range 4 to 5 For most alcohols that contain an -OH group fall within the range of 15 to 18
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Inductive Effect and Electrostatic Stabilization of the Anion Thursday, September 26, 2013 6:08 PM
Inductive effect - the polarization of the electron density of a covalent bond caused by the electronegativity of a nearby atom • Seen in alcohols ○ An electronegative substituent adjacent to the carbon bearing the -OH group INCREASES the acidity of the alcohol
○
The negative charge from the oxygen atom of the anion has been delocalized onto the electronegative fluorine atoms ---> stabilizing the charged species through delocalization of the charge!
Stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atoms from the site of the negative charge! Ex:
Ex: also seen in the acidity of halogen-substituted carboxylic acids
Chloroacetate anion ----- negative charge is stabilized by electrostatic interaction between the partial negative charges on the oxygens and the partial positive charge on the carbon bearing the chlorine atom
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Hybridization and the Percent s Character of the Atom Bearing the Negative Charge Thursday, September 26, 2013 6:14 PM
A hydrogen bonded to a triply bonded carbon atom (alkynes) is sufficiently acidic that it can be removed by a strong base, su ch as sodium amide! Electrons in an s orbital are lower in energy than those in a p orbital --- they are held more tightly to the nucleus • The more s character in a hybrid orbital, the more electronegative the atom ○ The more acidic a hydrogen bonded to it The more stable the anion ALKYNES are the most stable of the series --- also strongest acid of the series!
Chapter 4 Page 25
4.7 Lewis Acids and Bases Thursday, September 26, 2013 10:50 PM
Lewis acid - any molecule or ion that can form a new covalent bond by accepting a pair of electrons Lewis base - any molecule or ion that can form a new covalent bond by donating a pair of electrons
Example: sex-butyl cation reacts with bromide ion to form 2-bromobutane
The Lewis concept of acids and bases includes proton-transfer reactions ---- all Bronsted-Lowry bases/acids are also Lewis bases/acids **Lewis model is more general! Protic acids - an acid that is a proton donor in an acid-base reaction Aprotic acid - an acid that is not a proton donor; an acid that's an electron pair acceptor in a Lewis acidbase reaction Nucleophile - electron-rich species; seeking a region of low electron density = Lewis bases Electrophile - low-electron-density species; seeking a high electron density = Lewis acids
Chapter 4 Page 26
Example 4.8 Thursday, September 26, 2013 10:56 PM
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Arrhenius definition: • An acid is a substance that dissolves in water to produce H+ ions • A base is a substance that dissolves in water to produce OH- ions This was established back then, but we now know that these definitions aren't accurate to refer to an acid and base. Today's definition: An H+ ion immediately combines with a water molecule to give a hydronium ion H3O+ • Hydration of the hydronium ion itself gives the ion H3O2+ • The monohydrated and dihydrated forms (H3O+ and H3O2+) are the MAJOR hydrated forms present in aqueous solution ○ Present in equal concentrations •
Arrhenius is modified to take into account these interactions of H+ with water molecules. Arrhenius definitions are still valid as long as it's about AQUEOUS SOLUTIONS
Chapter 4 Page 1
4.2 Bonsted-Lowry Acids and Bases Thursday, September 26, 2013 9:14 AM
1923, Danish chemist Johannes Bronsted and English chemist Thomas Lowry independently proposed the following definitions Bronsted-Lowry definition: • An acid is a proton donor • A base is a proton acceptor • And acid-base reaction is a proton-transfer reaction
Chapter 4 Page 2
Conjugate Acid-Base Pairs Differ by a Proton Thursday, September 26, 2013 9:16 AM
Conjugate acid-base pair - any pair of molecules or ions that can be interconverted by transfer of a proton • Conjugate base - when an acid transfers a proton to a base, the acid is converted to this • Conjugate acid - when a base accepts a proton, it's converted to this
Curved arrows can also be used to show the transfer of a proton:
**the arrows can't point backwards! The Bronsted-Lowry definitions DON'T REQUIRE WATER as a reactant Ex:
Chapter 4 Page 3
Example 4.1 Thursday, September 26, 2013 9:20 AM
Chapter 4 Page 4
Example 4.2 Thursday, September 26, 2013 9:20 AM
Chapter 4 Page 5
Bronsted-Lowry Bases with Two or More Receptor Sites Thursday, September 26, 2013 9:21 AM
Many organic compounds have two or more receptor sites! The more stable charged species is the one in which the charge is more delocalized --- relative charge delocalization can be understand by resonance. Ex:
• We now examine each cation and determine which is more stable (lower in energy) ○ Write the contributing structures
A-1 and A-3 make the greater contributions ○ Two resonance contributing structure for protonation on the hydroxyl oxygen giving cation B
B-2 at best makes a minor contribution because of the adjacent positive charges
Chapter 4 Page 6
Example 4.3 Thursday, September 26, 2013 9:27 AM
Chapter 4 Page 7
π Electrons as Bronsted-Lowry Bases Thursday, September 26, 2013 9:28 AM
So far, we considered proton transfer to atoms having a nonbonding pair of electrons ----- also occur with compounds having π electrons Ex:
Carbocation - a species in which one of its carbons has only 6 electrons in its valence shell and carries a charge of +1
Chapter 4 Page 8
Example 4.4 Thursday, September 26, 2013 9:36 AM
Chapter 4 Page 9
4.3 Acid Dissociation Constants, pK a, and the Relative Strengths of Acids and Bases Thursday, September 26, 2013 9:37 AM
Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. • Strength of an acid is expressed by an equilibrium constant • Ex: ○
○ ○ Water is the solvent for this reaction Acid dissociation constant - Ka
• • • •
The larger the pKa, the weaker the acid The smaller the pKa, the stronger the acid The weaker the acid, the stronger its conjugate base The stronger the acid, the weaker its conjugate base
Chapter 4 Page 10
Example 4.5 Thursday, September 26, 2013 9:43 AM
Chapter 4 Page 11
4.4 The Position of Equilibrium in Acid-Base Reactions Thursday, September 26, 2013 9:44 AM
Negative pKa value ---- majority of molecules of the acid are dissociated in water • Ex: HCl has pKa = -7 ○ Positive pKa value ----most acid molecules remain undissociated in water • Ex: acetic acid has pKa of 4.76 ○ Example of the acid-base reaction of acetic acid and ammonia to from acetate ion and ammonium ion:
THE MOST IMPORTANT CONCEPT IS AS FOLLOWS: in an acid-base reaction, the position of equilibrium always favors reaction of the stronger acid and stronger base to form the weaker acid and weaker base.
Chapter 4 Page 12
How to Calculate the Equilibrium Constants for Acid-Base Reactions Thursday, September 26, 2013 9:56 AM
Chapter 4 Page 13
Example 4.6 Thursday, September 26, 2013 9:58 AM
Chapter 4 Page 14
4.5 Thermochemistry and Mechanisms of Acid-Base Reactions Thursday, September 26, 2013 9:58 AM
Reaction mechanism - step-by-step description of how a chemical reaction occurs • Describes which bonds are broken and which bonds are formed Thermochemistry - the study of the energy of the entire system at every instant of a reaction
Chapter 4 Page 15
Thermal Reactions and Transition States Thursday, September 26, 2013 10:00 AM
Most chemical reactions occur via COLLISIONS! Thermal reactions - reactions that result by virtue of the kinetic energy put into a reaction vessel due to temperature • During collisions, kinetic energy of the reactants is converted into potential energy ○ The potential energy is stored in the chemical structures in the form of the structural strains Energy is released as the molecules again adopt their optimal geometries Transition state - commonly called activated complex; the highest energy point on a reaction coordinate diagram
Chapter 4 Page 16
Reaction Coordinate Diagrams and Thermochemistry Thursday, September 26, 2013 10:07 AM
Reaction coordinate diagrams - a graph showing the energy charges that occur during a chemical reaction • Vertical axis = energy • Horizontal axis = reaction progress (reaction coordinate)
Thermodynamics - study of the relative energies between any two entities on a reaction coordinate diagram that are shown in wells, such as the energy between the reactants and products Change in Gibbs free energy - ∆G° or ∆G°rxn
• • R = 8.31 J/K mol • T = temperature in Kelvins Free energy of activation - the Gibbs free energy difference between the reactants and the transition state • Controls the rate of the reaction Kinetics - study of the rates of chemical reactions Exergonic reaction - negative ∆G; FAVORABLE • The more negative this value, the greater the driving force for the reaction to occur • ∆G° is less than 0 ------- Keq is greater than 1 Endergonic reaction - positive ∆G; UNFAVORABLE • ∆G° is greater than 0 ------- Keq is less than 1 • ∆G° is equal to 0 ------- Keq is equal to 1 Gibbs-Helmholtz equation: • Enthalpy (H°) - energy contained in chemical bonds and solvation • Heat of reaction - difference in enthalpy between reactants and products • Exothermic - bonds formed in the products are stronger than the bonds in the reactants; HEAT WILL BE RELEASED ○ ∆H° is negative ○ Warms up • Endothermic - bonds formed in the products are weaker; HEAT WILL BE ABSORBED ○ ∆H° is positive ○ Cools down • Entropy (S) - measures disorder (which is favorable) All systems become more stable as the number of freely moving particles increases and the chaotic movement of the particles increase!
Chapter 4 Page 17
Example 4.7 Thursday, September 26, 2013 10:14 AM
Chapter 4 Page 18
Mechanism and Thermochemistry of Acid-Base Reactions Thursday, September 26, 2013 1:06 PM
For an acid to transfer its proton to a base, the two entities must COLLIDE! • But they collide with a specific geometry Example:
Chapter 4 Page 19
4.6 Molecular Structure and Acidity Thursday, September 26, 2013 1:08 PM
Ways to stabilize A-: • Having the negative charge on a more electronegative atom • Having the negative charge on a larger atom • Delocalizing the negative charge as described by resonance contributing structures • Spreading the negative charge onto electron-withdrawing groups by the inductive effect ○ Polarization of sigma bonds • Having the negative charge in an orbital with more s character A- needs to be stabilized resulting from loss of a proton --- the more stable the anion, the greater the acidity of the acid HA
Chapter 4 Page 20
Electronegativity of the Atom Bearing the Negative Charge Thursday, September 26, 2013 1:22 PM
Anions of atoms within the same period have approx. the same size, and their energies of solvation are approx. the same! Ex: carbon, the least electronegative, forms the least stable anion Because methanol forms the most stable anion, it's the strongest acid in the above series
Chapter 4 Page 21
Size of the Atom Bearing the Negative Charge Thursday, September 26, 2013 1:24 PM
Example:
Any set of hydrogen acids within a group of the periodic table is related to the size of the atom bearing the negative charge… Size increases from top to bottom within a group! • They are farther from the nucleus • They occupy a larger volume of space The bigger the size, the MORE STABLE!
Chapter 4 Page 22
Delocalization of Charge in the Anion Thursday, September 26, 2013 1:28 PM
Carboxylic acids are weak acids. -- fall within the range 4 to 5 For most alcohols that contain an -OH group fall within the range of 15 to 18
Chapter 4 Page 23
Inductive Effect and Electrostatic Stabilization of the Anion Thursday, September 26, 2013 6:08 PM
Inductive effect - the polarization of the electron density of a covalent bond caused by the electronegativity of a nearby atom • Seen in alcohols ○ An electronegative substituent adjacent to the carbon bearing the -OH group INCREASES the acidity of the alcohol
○
The negative charge from the oxygen atom of the anion has been delocalized onto the electronegative fluorine atoms ---> stabilizing the charged species through delocalization of the charge!
Stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atoms from the site of the negative charge! Ex:
Ex: also seen in the acidity of halogen-substituted carboxylic acids
Chloroacetate anion ----- negative charge is stabilized by electrostatic interaction between the partial negative charges on the oxygens and the partial positive charge on the carbon bearing the chlorine atom
Chapter 4 Page 24
Hybridization and the Percent s Character of the Atom Bearing the Negative Charge Thursday, September 26, 2013 6:14 PM
A hydrogen bonded to a triply bonded carbon atom (alkynes) is sufficiently acidic that it can be removed by a strong base, su ch as sodium amide! Electrons in an s orbital are lower in energy than those in a p orbital --- they are held more tightly to the nucleus • The more s character in a hybrid orbital, the more electronegative the atom ○ The more acidic a hydrogen bonded to it The more stable the anion ALKYNES are the most stable of the series --- also strongest acid of the series!
Chapter 4 Page 25
4.7 Lewis Acids and Bases Thursday, September 26, 2013 10:50 PM
Lewis acid - any molecule or ion that can form a new covalent bond by accepting a pair of electrons Lewis base - any molecule or ion that can form a new covalent bond by donating a pair of electrons
Example: sex-butyl cation reacts with bromide ion to form 2-bromobutane
The Lewis concept of acids and bases includes proton-transfer reactions ---- all Bronsted-Lowry bases/acids are also Lewis bases/acids **Lewis model is more general! Protic acids - an acid that is a proton donor in an acid-base reaction Aprotic acid - an acid that is not a proton donor; an acid that's an electron pair acceptor in a Lewis acidbase reaction Nucleophile - electron-rich species; seeking a region of low electron density = Lewis bases Electrophile - low-electron-density species; seeking a high electron density = Lewis acids
Chapter 4 Page 26
Example 4.8 Thursday, September 26, 2013 10:56 PM
Chapter 4 Page 27
