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Oxoacids
Simple oxoacids: the simplest oxoacids are the mononuclear acids that contain one atom of the parent element Ee.g. H2CO3, HNO3, H3PO4 and H2SO4
E(O)p(OH)q
p = 0 H-O-Cl
p = 1
O
PHO OH
OH
OH
SiHO OH
OH
OH
TeHO OH
OH
OH
HO HO BOHOH
O ClOH
OH SeO OH
OH
p = 2 O NOH
OS
O OHOH
O
p = 3 ClO O
OH
O
Cl+O O-
OH
O
Cl+
O O-OH Cl
O OOH
O-
I+HO OH
OH
OH
HOO
IHO OH
OH
OH
HO
107.2 7.8, 11.2 9.1
2.1, 7.4, 12.7 3.6 1.6, 7.0 2.6, 8.0
-1.4-2.0, 1.9 -1.0
-10
pKa
The first dissociation constant of oxoacids pKa(1)pKa(1) decreases as the number of “hydrogen free” oxygen atoms (p) increases
-1.02Cl(O)2(OH)HClO3
2.01Cl(O)(OH)HClO2
≈-83Cl(O)3(OH)HClO4
7.20Cl(OH)HOCl
≈-22S(O)2(OH)2H2SO4
1.821S(O)(OH)2H2SO3
-1.402N(O)2(OH)HNO3
3.371N(O)(OH)HNO2
pKa(1)nE(O)p(OH)qFormula of acid
•After the first dissociation, the number of “hydrogen free” O atoms increases to p+1
•The greater the number of the oxygen atom in the conjugated base[E(O)p+1(OH)q-1]-, the more the negative charge of the anion can be spread out.
e.g. the negative charge of nitrate ion is delocalized uniform over via the π-bonding system
O
O ON
-1/3
-1/3-1/3
Empirical rules devised by Linus Pauling
•For the oxoacids EOp(OH)q. pKa ≈ 8 – 5p
•The successive pKa values of polyproticacids (those with q > 1) increases by 5 units for each successive deprotonation
Aquated Cations as Brønsted Acids – Aqua Acids
•When a metal salt dissolves in H2O, the cation and anion are hydrated
e.g. Aqueous solution of NaCl
Na+
O
O
O
O
O
O
HHH
H
H
HH H
H
H
H
H
δ-
δ-
δ-
δ-
δ-
δ-
Ion-dipoleinteractions
First hydrationshell
For many transition metal salts, the cations Mn+ form covalent bonds with H2O
Mn+
O
O
O
O
O
O
HHH
H
H
HH H
H
H
H
H
Covalent bond
Coordination/dative/metal-ligand bond
Cu2+
O
O
O
O
O
O
HHH
H
H
HH H
H
H
H
H
Hexaaquacopper(II) ion
Distorted octahedralComplex/adduct
The metal-H2O complexes can act as BrønstedAcids by loss of H+ from a coordinated H2O molecule:
[M(H2O)6]n+(aq) + H2O (l)
[M(H2O)5(OH)](n-1)+(aq) + H3O+
(aq)
The position of the equilibrium depends on the degree of the O-H bonds are polarized
Hydrolysis/deprotonation
M
OH H
When H2O coordinates to M with n+ positivecharge, charge is withdrawn towards the metal center, leaving the H more δ+ than in bulk water
δ+ δ+
e- density
OH
H
Mn+
OH
OH
HH
+
n+(n-1)+
Hydroxide ion
13.30.030132Sr2+
11.40.04786Mg2+
12.80.035114Ca2+
13.50.027149Ba2+
13.60.01190Li+14.20.009116Na+
14.50.007152K+
pKa(a)RadiusIon
pKa Values of Metal Cations
(a) = Z2/r
a pKa
Harnessing the Acidity of Metal Cations in Biology
e.g. Carbonic anhydraseAn enzyme located in RBC that catalyzes conversion of CO2to bicarbonate ion
CO2 + H2O HCO3- + H+
Carbonic anhydrase
•In the absence of enzyme, this reaction is very slow: ~ 10-2 s-1
in the presence of enzyme: 106 s-1
•The active site of this enzyme is composed of a Zn2+ ion bonded to the backbone of the enzyme
bicarbonate
(aq) (aq) (aq)(l)
• One of the reasons for the slow rate constant is the poor nucleophlicity of H2O. Hydroxide is a strong nucleophile but its concentration at pH = 7 is only 10-7 M
HO
H
O
C
O
δ+
δ-
δ--H+ O
C
O-HO
Metal-OH has stronger nucleophilicity
Coordination of a water molecule to Zn2+ in the enzyme increases its acidity to 107 times!
Zn2+
O
HH
δ+δ+Coordinated water
pKa = 7H
ydrophobic region
Hydrophobic region – binding of CO2
Active site
Zn
O
HH
O
C
O Zn
OH
O
C
O
δ+δ+
O
HH
Zn
OH
O
C
O
HCO3-
H2O
H3O+δ+
δ-
δ-δ+
δ-
δ-
(b) Lewis Concept
•Lewis Acid – electron pair acceptor
•Lewis Base – electron pair donor
Lewis Acid + Lewis Base Acid – base adduct
N H Cl+ N H
+
Cl-+
AdductLewis base Adductacid base
Lewis base
H+ + OH- H2Oe--acceptor e--donor Adduct
N +4 Ni2+
N
Ni2+
N
NN
2+
Lewis base Lewis acid Adduct/complex
Metal ion
Examples of Lewis Acids and Bases
•Anions i.e. OH-, O2-
Halides, S2-, Se2-, NH2-,
S-R-, O-R-, alkyl & aryl ions
•pseduohalides,•Neutral molecules i.e.
H2O, amines, ammonia, phosphines, arsines,
ethers, thioethers, heterocyclics, (many of them contain group 15, 16 or 17 elements as
their donor atoms)
•Alkali metal ions•Alkaline earth metal ions
•Transition metal ions(e.g. Fe3+, Cu2+, Zn2+, Ni2+, Au+1,
Ti4+, Co3+)•Main group metal ions (e.g. Tl+,
Pb2+, Al3+) •Neutral molecules(e.g. BF3, BeCl2)
BaseAcid
Examples of Lewis Acids: Transition Metals
Different Oxidation States
e.g. Fe0, Fe+1, Fe+2, Fe+3, Fe+4
Different ionic radii
e.g. Fe0 > Fe+1> Fe+2> Fe+3> Fe+4
Ru+2 > Fe+2; Au+1> Au+3
Cu2+
O
O
O
O
O
O
HHH
H
H
HH H
H
H
H
HCu2+ + 6 H2O
Ag+ + 2 NH3 H3N Ag NH3
2+
+
Cu2+ + 4 Cl- CuCl42-
Lewis acid Lewis base Adduct/complex
Acid-Base Reactions Involving Metal Ions
H3N
Cl Cl
NH3
Pt S Au ClH3C
H3C
WCl
Cl Cl
Cl
Cl
Cl
WH3C
H3C CH3
CH3
CH3
CH3
Metal-carbon bond
HexamethyltungstenAn organometallic complex
Identify the acids and bases
Examples of Lewis Acids: Group 2
Cl Be Cl 4-e-
BeCl2 +
Beryllium dichloride
2 Cl-
CH2CH3
O
CH2CH32
Cl
BeCl Cl
Cl
2-
O
BeO Cl
ClEt
Et
Et Et
Examples of Lewis Acids: Group 13
X
X XB
6-e-X = halides, alkyl
X
X XAl
X
X XGa
X
X XIn
X
X XTl
F
F FB
CH3
CH3
CH3N
+
N
BF F
F
CH3
CH3H3C
+ CH2CH3
OCH2CH3
trimethylamine
diethyl ether
O
BF F
F
CH2CH3H3CH2C
Acid-Base Reactions Involving Group 13 Acids
Al
Cl
Al
ClCl
Cl
Cl
Cl+ 2 CN CH3 2
AlCl
Cl
Cl
NC
CH3
H
H HGa
+ NMe3
GaH
H
H
Me3N
+ 2 NMe3 H GaH
HNMe2
NMe3
Acid-Base Reactions Involving Group 13 Acids