Download Chapter 4 Acids and bases

Chapter 4
Acids and bases
Proton transfer equilibria in water
Arrhenius (1884) acids and bases:
HF(aq) + H2O (l)
H3O+(aq) + F-(aq)
NH3(aq) + H2O (l)
NH4+(aq) + OH-(aq)
Acids: compounds that produce hydrogen
ions (H+ or H3O+) in water
Bases: compounds that form OH- when they
dissolve in water
Conjugate acids and bases
Brønsted – Lowry (1923) acids and bases
The strength of Brønsted acids
A Brønsted acid is a proton donor
A Brønsted base is a proton aceptor
HX(aq) + H2O (l)
When a species donates a proton, it becomes the conjugate base;
when a species gains a proton, it becomes the conjugate acid.
Conjugate acids and bases are in equilibrium.
H3O+(aq) + X-(aq) Ka =
[H3O+] [X-]
Ka: acidity constant or acid ionization constant
HF(g) + H2O (l) H3O+ + F-(aq)
BH+(aq) + OH-(aq)
B(aq) + H2O (l)
NH3(aq) + H2O (l) NH4+(aq) + OH-(aq)
the hydronium ion
Chapter 4
Kb: basicity constant
1
[HX]
[BH+] [OH-]
Kb =
[B]
Chapter 4
2
Autoprotolysis
2 H2O (l)
H3O+(aq) + OH-(aq)
Kw = [H3O+] [OH-]
Kw: autoprotolysis constant or ionic product, 1.00 × 10-14 at 25oC
HX
X-
acid
conjugate acid of X-
conjugate base of HX
base
Ka
Kb
Strong acids
Weak acids
Ka Kb = Kw
For a weak acid
pH = – log[H3O+]
Ka =
pKa + pKb = pKw
pK = – log K
Strong and weak acids and bases – measured by pKa or pKb
Chapter 4
[H3O+] [A-]
[HA]
HA(aq) + H2O (l)
3
H3O+(aq) + A-(aq)
Chapter 4
4
1
Chapter 4
5
Chapter 4
6
Polyprotic acids
A polyprotic acid loses protons in succession, and successive
deprotonations are progressively less favorable.
H2S
H2S + H2O HS- + H3O+
HS- + H2O S-2 + H3O+
H3PO4, a triprotic phosphorus oxyacid
H3PO4 H+ + H2PO4-
Chapter 4
7
Ka1 =
H2PO4- H+ + HPO42-
Ka2 =
HPO42- H+ + PO43-
Ka3 =
[H+] [H2PO4-]
[H3PO4]
[H+] [HPO42-]
H2PO4-, HPO32-, PO43-
[H2PO4-]
[H+] [PO43-]
[HPO42-]
Chapter 4
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2
Solvent leveling
HI (l) + H2O(l) H3O+(aq) + I-(aq)
HBr (l) + H2O(l) H3O+(aq) + Br-(aq)
Because HI(l) and HBr(l) are strong acids, both transfer their protons
essentially completely to give H3O+. In effect, solutions of HI and HBr
behave as though they are solutions of H3O+ regardless HI is
intrinsincally stronger than HBr. Water is therefore said to have a
leveling effect that brings all strong acids down to the acidity of H3O+.
To distinguish the acidity strengths of HBr and HI, one has to use a
less basic solvent.
Similar situation exists for strong bases. In water, strong bases behave
as solutions of OH-. The strengths of strong bases can only be
distinguished using a less acidic solvent.
Chapter 4
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Chapter 4
10
Characteristics of Brønsted acids
There are three classes of acids to be considered.
(1) Aqua acids, water solvated metal ions
[Fe(H2O)6]+3 + H2O <=> [Fe(H2O)5OH]+2 + H3O+
discussion
on general
trends
The acid-base discrimination window for a variety of solvents. The width of each
window is proportional to the autoprotolysis constant of the solvent.
Chapter 4
11
Chapter 4
12
3
(2) Hydroxoacids
B(OH)3, Si(OH)4, Te(OH)6
(3) Oxoacids
H2SO4, HNO3, etc
Pauling’s rules
(a) For the oxoacids OpE(OH)q, pKa ≈ 8-5p
(b) The successive pKa values of polyprotic acids (q>1),
increase by 5 units for each successive proton
transfer.
OH
Si
HO
O
O
Si
OH
OH
HO
+ H+
P
OH
OH
HO
number of
oxo- ligands
0
OH
OH
P
HO
+ H+
OH
O-
1
?
acidity
?
O
Chapter 4
13
Chapter 4
Relationship between the three classes of Brønsted acids
4+
OH2
H3N
2 H+
NH3
2+
OH
NH3
H3N
Ru
H+
H3N
C
OH
OH
OH2
NH3
H3N
OH
OH
BaO + H2O Ba(OH)2
Acidic and basic oxides
Anhydrous (無水的
無水的)
無水的 oxides
O
O
Ru
NH3
H3N
OH2
C
NH3
Ru
NH3
H3N
O
CO2 + H2O H2CO3
+
O
14
Ba
O
Ba
OH
OH
OH2
CO2 + H2O C(O)(OH)2 acid
SO3 + H2O S(O)2(OH)2 acid
Hydration process of
an anhydrous oxide
H2O
A
BaO + H2O Ba(OH)2
A
O
O-
A
O
O S
O
H
H
A = alkali metals, alkaline earth metals
O
H
H+ + HSO3-
Na
O
H
Na+ + OH-
HO
A: metal or non-metal atoms
Chapter 4
O
A = non metals
H
H
H
+O
O
A
base
15
Chapter 4
16
4
Amphoterism (amphoteric 兩性的(酸性的或鹼性的))
The frontier between metals and
nonmetals in the periodic table is
characterized by the formation of
amphoteric oxides; amphoterism
also varies with the oxidation
state of the element.
Al2O3(s) + 6 H3O+(aq) + 3 H2O(l) 2 [Al(OH2)6]3+(aq)
Al2O3(s) + 2 OH-(aq) + 3 H2O(l) 2 [Al(OH)4]-(aq)
Na2O MgO
Al2O3
SiO2 P 4O10 SO3 Cl2O7
SB WB Amphoteric WA
A
SA
VSA
Chapter 4
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Chapter 4
H
H2O
HO
H
O
HO
Al
B
HO
B
HO
OH
B(OH) 4- + H+
OH
HO
HO
+ OH-
Al(OH) 4-
acid
Al
HO
H
H2 O
Al
HO
OH
+ 3 H+ + 3 H2O
Al(OH2)63+
base
H
O
HO
OH
OH
HO
boric acid H3BO3
Al
18
OH
Al(OH)4 - + H+
HO
A
H+
OH
HO
A = metal
HO
Al
OH
+ 6 OH2
Al(OH2)63+ + 3 OH-
H 2O
A = main group nonmetal
Al3+
?
dative bonds
HO
Chapter 4
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Chapter 4
20
5
Lewis acids and bases
Lewis acids: electron pair acceptors
Lewis bases: electron pair donors
Some interesting examples of Lewis acids and bases
Ag+
(1) Metal complexes
Co2+ + 6 H2O [Co(H2O)6]2+
(2) A molecule with an incomplete octet can complete its octet
by accepting an electron pair.
Chapter 4
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(3) Valence shell rearrangement
Chapter 4
22
Group characteristics of Lewis acids
(1) s-block elements
CsF + SF4 Cs+[SF5]n BeCl2 (BeCl2)n
BeCl2 + 2 Cl- [BeCl4]2-
(4) Expand the octet
SiF4 + 2F- [SiF6]2-
(2) Group 13 Lewis acids
(5) Charge transfer
I2 + I- I3-
Chapter 4
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Chapter 4
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6
(3) Group 14 Lewis acids
“AlCl3” is a dimer in the gas phase
Expand the octet
SiF4 + 2F- [SiF6]2-
AlCl3 is widely used as a Lewis acid catalyst for organic reactions
(4) Group 15 Lewis acids – again heavy elements
SbF5 + F- [SbF6]Super acids: SbF5(l) + 2 HF(l) H2F+ (sol) + [SbF6]- (sol)
The catalytic cycle for the Friedel-Craft alklation reaction
Chapter 4
SbF5(l) + 2 HSO3F(l) H2(SO3F)+ (sol) + [SbF5(SO3F]- (sol)
25
Reactions and properties of Lewis acids and bases
(5) Group 16 Lewis acids
The fundamental types of reaction
O
O
S
+
Magic acid, able to dissolve candle wax 26
Chapter 4
N
S
N
S
N
O
O
O
O
S
O +
N
O
O
O
O
O
S
O + O S
S
O
S
O
HO
OH
O
O
O
OH
OH
H2S 2O 7
O
(6) Lewis acids of the halogens
I2 + I- I3I
I
I
I
Chapter 4
I
I
27
Chapter 4
28
7
Ligand group orbitals
More examples showing the orbital interactions
NH3 HOMO
BH3 LUMO
Three bonding MOs
are filled, accounting
for the three B-H σ
bonds
Chapter 4
29
Chapter 4
30
31
Chapter 4
32
H
y
N
H
H
x
3a1
2a1
1a1
Chapter 4
8
σ p*
πp*
πp
σp
σs*
σs
Chapter 4
R
C
Br2 Chapter 4
33
LUMO
O
R
R
π∗
R
R
C
34
C
C
O
R
O
Br
Br
LUMO
R
R
R
C
HOMO
N
R
HOMO
R
non-bonding
C
n
C
O
R
R
C
R
O
R
C
R
NH3
R
O
Br2
O
R
R
C
R
R
C
O
π
2p
R
C
R
O
R
R
R
σ p*
C
O
R
C
O
R
NH3
O
R
R
O
Br
Br
R
sp2 carbon
Chapter 4
C
R
C
35
OH
Chapter 4
36
NH2
9
(a) Displacement reactions – depending on the relative acidity or basicity
BF3 +
Me2O
NH4Cl +
N
BF3
N
H3N
BF3 + OMe2
BF3
+ HCl
(b) Metathesis (置換) reactions
Me3Si–I + AgBr(s) Me3Si–Br + AgI(s)
R
C
O
R
Chapter 4
37
Hard and soft
acids and bases
Chapter 4
38
Classification
,arenes
C5H5N
Some general observations:
(1) Hardness increases across a period and decreases down a group
(2) Hard bases are associated with O, N and F
(3) Hard acids are those with higher charges
Mo(C6H6)(CO)3
(4) Many transition metals are soft acids
Chapter 4
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Chapter 4
40
10
HSAB principle
Klopman proposed that:
Hard acids tend to bind to hard bases.
Soft acids tend to bind to soft bases.
Hard [Lewis] acids bind to hard [Lewis] bases to give charge-controlled (ionic)
complexes. Such interactions are dominated by the +/– charges on the Lewis
acid and Lewis base species.
Why?
Hard acid-base interactions are predominantly electrostatic; soft acidbase interactions are predominantly covalent.
and
Soft [Lewis] acids bind to soft [Lewis] bases to give orbital-controlled (covalent)
complexes. These interactions are dominated by the energies of the
participating frontier molecular orbitals (FMO), the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
Chemical consequences of hardness
The tendency of soft acids to bond to soft bases and of hard acids to
bond to hard bases explains certain aspects of the Goldschmidt
classification of the elements into 4 types. (2 of the classes are the
lithophile elements and the chalcophile elements.)
Lithophile elements: Li, Mg, Ti, Al, Cr (hard cations which are found in
association to the hard base O2-)
Chalcophile elements: Cd, Pb, Bi, Ag (soft cations, which are found in
associaion with the soft bases S2-, Se2-)
Chapter 4
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Chapter 4
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Thermodynamic acidity parameters
A + B A–B
A: acid
B: base
Exercises
Lewis acids:
Ru2+ versus Os2+
Na+ versus Cu+
Os2+ versus Os3+
Mg2+ versus Ca2+
Lewis bases:
Cl- versus BrCO versus NH3
NH3 versus PH3
∆Ho
More examples
(1) W(CO)6 is air stable but W(NH3)6 is unknown
(2) ZnO + 2 LiMe ZnMe2 + Li2O
K > 1 or < 1 ?
(3) MeHgI + HCl MeHgCl + HI
K > 1 or < 1 ?
(4) AgCl2- + 2 CN- Ag(CN)2- + 2 Cl-
K > 1 or < 1 ?
Drago-Wayland equation
∆Ho = EAEB + CACB (kJ/mol)
Chapter 4
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Chapter 4
44
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