CHAPTER 3 ACIDS AND BASES IN ORGANIC CHEMISTRY 3.1 INRODUCTION The important of acid-base reaction 1. is a simple, fundamental reaction 2. enable you to see the mechanism of the reaction 3. illustrate the process of bond breaking and bond making examine important ideas about the relationship between the structure of molecules and their reactivity 5. illustrate the important role solvents play in chemical reactions 6. find something familiar in General chemistry
CHAPTER 3 ACIDS AND BASES IN ORGANIC CHEMISTRY 3.1 INRODUCTION The important of acid-base reaction: 1. is a simple, fundamental reaction. 2. enable you to see the mechanism of the reaction 3. illustrate the process of bond breaking and bond making 4. examine important ideas about the relationship between the structure of molecules and their reactivity. 5. illustrate the important role solvents play in chemical reactions 6. find something familiar in General chemistry
3.1A THE BRNSTED-LOWRY DEFINATION OF ACIDS AND BASES According to the brnsted-Lowry theory, an acid is a substance that can donate a proton, and a base is a substance that can accept a proton. For example H-0:+H H-O-H + CI H Base Acid Conjugate Conjugate (proton (proton acid of base of acceptor) sonor) HCI bydrogen chloride, a very strong acid, transfers its proton to water Water acts as a base and accepts the proton
3.1A THE BRNSTED-LOWRY DEFINATION OF ACIDS AND BASES According to the brnsted-Lowry theory, an acid is a substance that can donate a proton, and a base is a substance that can accept a proton. For example: H O H H Cl H O + H H Cl - + + Base (proton acceptor) Acid (proton sonor) Conjugate acid of H2 O Conjugate base of HCl Hydrogen chloride, a very strong acid, transfers its proton to water. Water acts as a base and accepts the proton
The molecule or ion forms when an acid loses its proton is called the conjugate base of that acid Such as the chloride ion The molecule or ion that forms when a base accepts a proton is called the conjugate acid of that base. Such as the hydronium ion Other strong acids that completely transfer a proton when dissolved in water are hydrogen iodide hydrogen bromide and sulfuric acid HI ho H3O +I HBr+H2O→H3O+Br H2SO4+H2O—-H3O+HSO HSO+H2O之一H2O++SO4 The proton transfer is stepwise in sulfuric acid, the first proton Transfer completely, the second only to the extent of 10%
The molecule or ion forms when an acid loses its proton is called the conjugate base of that acid. Such as the chloride ion. The molecule or ion that forms when a base accepts a proton is called the conjugate acid of that base. Such as the hydronium ion. Other strong acids that completely transfer a proton when dissolved in water are hydrogen iodide, hydrogen bromide, and sulfuric acid. HI + H2 O HBr + H2 O H2 SO4 + H2 O HSO4 - + H2 O H3O + + I - H3O + +Br - H3 O + +HSO4 - H3 O + + SO4 2- The proton transfer is stepwise in sulfuric acid, the first proton Transfer completely, the second only to the extent of ～10%
When an aqueous solution of sodium hydroxide is mixed with an aqueous solution of hydrogen chloride, the reaction that occurs is between hydronium and hydroxide ions Total lonic reaction: H-0+-H+:cr:+Na++H-0:--2H-0+Nat+:Cr H Spectator ions Net reaction. H-OH+H-o H H The net reaction is simply HO OH 2 HbO
When an aqueous solution of sodium hydroxide is mixed with an aqueous solution of hydrogen chloride, the reaction that occurs is between hydronium and hydroxide ions. H O - H O+ H H Cl - + + Na + + H O H + Na + + Cl - 2 Spectator ions Total Ionic Reaction: Net Reaction: H O + H H + H O - H O H 2 The net reaction is simply: H3 O + + OH - 2 H2 O
3.1B THE LEWS DEFINITION OF ACIDS AND BASES Lewis proposed that acids be defined as electron-pair acceptors and bases be defined as electro-pair donors For example H NH H-NH Lewis acid Lewis base (electron-pair(electron-pair acceptor) donor) Cl-Al+: NH- CI-Al-NH3 C Lewis acid ewis base (electron-pair(electron-pair acceptor) donor
3.1B THE LEWS DEFINITION OF ACIDS AND BASES Lewis proposed that acids be defined as electron-pair acceptors and bases be defined as electro-pair donors. For example: H + + NH3 NH3 + H Lewis acid (electron-pair acceptor) Lewis base (electron-pair donor) + NH3 Lewis acid (electron-pair acceptor) Lewis base (electron-pair donor) Cl Al Cl Cl Cl Al - Cl Cl NH3 +
F-B +NH3→FB-NH Lewis acid ewis base (electron-pair (electron-pair acceptor donor) The lewis theory by virtue of its broader definition of acids, allows acid-base theory to include all of the brnsted-Lowry reactions agnd as we shall see, a great many others Any electron-deficient atom can act as a Lewis acid. many compounds containing group iiia elements such as aluminium are Lewis acids because group Iiia atoms have only a sextet of electrons in their outer shell. Many other compounds that have atoms with orbitals also act as Lewis acids. For example
+ NH3 Lewis acid (electron-pair acceptor) Lewis base (electron-pair donor) F B F F F B - F F NH3 + The Lewis theory , by virtue of its broader definition of acids, allows acid-base theory to include all of the Brnsted-Lowry reactions aqnd , as we shall see, a great many others. Any electron-deficient atom can act as a Lewis acid. Many compounds containing group IIIA elements such as aluminium are Lewis acids because group IIIA atoms have only a sextet of electrons in their outer shell. Many other compounds that have atoms with orbitals also act as Lewis acids. For example:
R-O-H ZnCl R-O-ZnCI2 Lewis base Lewis acid H (electron-pair (electron-pair donor) acceptor) Br一Br:+FeBr3 Br一Br-FeBr3 Lewis base Lewis acid (electron-pair (electron-pair donor) acceptor) 3.2 THE USE OF CURVED ARROWS IN ILLUSTRATING REACTIONS the direction of electron flow in a reaction. Besides it is a usely ow The curved arrows is commonly used by organic chemists to sh Method for indicating which bonds form and which bonds break
R O H R O+ Lewis acid H (electron-pair acceptor) Lewis base (electron-pair donor) ZnCl 2 ZnCl 2 - + Br Br + FeBr 3 Br Br + FeBr 3 - Lewis base (electron-pair donor) Lewis acid (electron-pair acceptor) 3.2 THE USE OF CURVED ARROWS IN ILLUSTRATING REACTIONS The curved arrows is commonly used by organic chemists to show the direction of electron flow in a reaction. Besides it is a useful Method for indicating which bonds form and which bonds break
Now. lets us illustrate some of the basic ideas of the curved-arrow notation with simple lewis acid-base reactions H-O?+ H-Cl H—O-H+:Cl H H this bond breaks this bond is formed The following acid-base reactions gives other examples of the use of the curved -arrow notation H-。H+0-H--2H-0 H H Acid B ase
Now, lets us illustrate some of the basic ideas of the curved-arrow notation with simple Lewis acid-base reactions: H O H H Cl H O + H H Cl + - + this bond breaks this bond is formed The following acid-base reactions gives other examples of the use of the curved-arrow notation: H O + H H + O H - H O H 2 Acid Base
H3C-C-0-H+H-0:÷H2C-C-0:+H-O+-H H Acid Base H3C-C-0-H +:O-H H3C=C-O:+H—0-H Acid Base 3.3 THE STRENGTH OF ACIDS AND BASES: KaAND pKa When acetic acid dissolves in water, the following reaction dose not proceed to completion H3C-C-0-H HO H3C-C-0 H3o
H O + H + H O H H Acid Base H3 C C O O H H3 C C O O + H O H Acid Base H3 C C O O H H3 C C O O + - + - O H 3.3 THE STRENGTH OF ACIDS AND BASES: Ka AND pKa When acetic acid dissolves in water, the following reaction dose not proceed to completion: H3 C C O O H H3 C C O + H O 2 O + H3 O +
3.3A THE ACIDITY CONATANT Ka Experiments show that in a 0. IM solution of acetic acid at 25C only about 1% of the acetic acid molecules ionize by transferring their protons to water. The reaction is a equilibrium, we can describe it with an expression for the equilibrium constant H3O] [CH3CO21 Kcq=TCH3CO2田2O For dilute aqueous solutions, the concentration of water is essentially constant, so the equilibrium constant can be expressed with the acidity constant(Ka) Ka= Kea hoi=i H3O][CH3CO CH3CO2H
Experiments show that in a 0.1M solution of acetic acid at 25℃ only about 1% of the acetic acid molecules ionize by transferring their protons to water. The reaction is a equilibrium, we can describe it with an expression for the equilibrium constant. For dilute aqueous solutions, the concentration of water is essentially constant, so the equilibrium constant can be expressed with the acidity constant (Ka). Keq [H2 O] = [H3 O + ] [CH3 CO2 - ] [CH3 CO2 H] Ka = Keq = [H3 O + ] [CH3 CO2 - ] [CH3 CO2 H] [H2 O] 3.3A THE ACIDITY CONATANT Ka