20. 11/04 Acid-Base Chemistry [MT 6]

Chem 104A, UC, Berkeley
 Molecular Orbitals of Ethene
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Molecular Orbital Analysis of Ethene Dimerisation
the reaction is said to be a "symmetry forbidden" –
interestingly, this reaction is rare and very slow !
Chem 104A, UC, Berkeley
 Molecular Orbitals of 1,3-Butadiene
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
Molecular Orbital Analysis of Diels-Alder reaction
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Chem 104A, UC, Berkeley
Acid-Base Chemistry
Reading : MT 6
Chem 104A, UC, Berkeley
The Acid Base Theory of Brønsted and Lowry
In 1923, within several months of each other, Johannes Nicolaus
Brønsted (Denmark) and Thomas Martin Lowry (England) published
essentially the same theory about how acids and bases behave.
•An acid is a "proton donor."
•A base is a "proton acceptor."
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Chem 104A, UC, Berkeley
HCl + H2O <===> H3O+ + Cl¯
HCl - this is an acid, because it has a proton available to be transfered.
H2O - this is a base, since it gets the proton that the acid lost.
Now, here comes an interesting idea:
H3O+ - this is an acid, because it can give a proton.
Cl¯ - this is a base, since it has the capacity to receive a proton.
Notice that each pair (HCl and Cl¯ as well as H2O and H3O+ differ by
one proton (symbol = H+). These pairs are called conjugate pairs.
Chem 104A, UC, Berkeley
HCl
+
stronger
acid
NH4+
stronger
acid
NaOH
--->
OHstronger
base
+
Weaker
conjugate
acid
stronger
base
+
H2O
--->
H2O
weaker
conjugate
acid
NaCl
weaker
conjugate
base
+
NH3
weaker
conjugate
base
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Chem 104A, UC, Berkeley
H n A  H 2O  H n 1 A  H 3O 
[ H n 1 A ][ H 3O  ]
pK a1   log
[ H n A]
pK a  0  strong
pK a  0  weak
Chem 104A, UC, Berkeley
Amphoteric Compound
H n A  H 2O  H n 1 A  H 3O 
NH 3  H 2O  NH 4  OH 
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Chem 104A, UC, Berkeley
Oxyacids AOp(OH)q
A=Si, N, P, As, S, Se, Te, Cl, Br, I
p=# of nonhydrogenated oxygen atom
+
H
A ----O
As atom A is rendered more positive, it becomes easier to break the O-H bond
Because of enhanced bond polarization.
The greater the formal charge on A. the stronger the acid should be.
Chem 104A, UC, Berkeley
Oxyacids AOp(OH)q
p=# of nonhydrogenated oxygen atom
p=0:
p=1:
p=2:
p=3:
PKa
PKa
PKa
PKa
8-9 very weak
1-2 weak
-2 ~ -3 strong
-7 very strong
A=Si, N, P, As, S, Se, Te, Cl, Br, I
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Chem 104A, UC, Berkeley
O
p=3
2
1
0
Empirical Rule (Pauling)
PK a  8  5 p
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
As successive protons are removed, the PKa increases by ~4
Or 5 each time.
H 3 PO4     H 2 PO4     HPO4
( 2.15)      (7.20)    (12.37)
H 2 SO4     HSO4
( 3.0)      (2.0)
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Chem 104A, UC, Berkeley
Hn X
0.1 M HCl, H2S,H2O, H3PO4,H2PO4-,HPO42-,PO43-
Chem 104A, UC, Berkeley
Acidity
H2Se > H2S > H2O
Conjugated bases SeH-, SH-, OH- of larger molecules
 lower charge density
 weaker attraction for H+
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Chem 104A, UC, Berkeley
Acidity
NH3 < H2O < HF
Conjugated bases NH2-, OH- , FNH2-  -1/2 on each lone pair
HO-  -1/3 on each lone pair
F-  -1/4 on each lone pair
Strongest attraction for proton
Strongest conjugated base
Weakest acid NH3
Chem 104A, UC, Berkeley
Lewis Concept
A base: an electron-pair donor
Species with lone pair type orbitals
An acid: an electron-pair acceptor
Species with empty non-bonding type
orbitals
MO theory
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Chem 104A, UC, Berkeley
2 u*
2 g*
Lewis acid
BeL2, MgL2
1 u
1 ub
1 gb

 x*
Lewis acid
BL3, AlL3
GaL3
InL3
* 104A, UC, Berkeley
Chem
y
'
 s*
2pz

 yb
b
x
2s
 sb
2e1
2a1'
 znb
2py
2px
Ligand Group Orbitals
LGOs
1a
''
2
LGO1
1e1'
1a1'
B
LGO3
LGO2
3H
BH3
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Chem 104A, UC, Berkeley
Lewis base
NL3, PL3, AsL3, SbL3, BiL3

NH3 (C3v: E, 2C3, 3v)
z
3H
N
NH3
H(3)
H(1)
H(2)
2e
-13.5 eV
4a1
y
x
- s2 + s3
e
-15.5 eV
The symmetry of 3H’s group orbitals:
e (2px, 2py)
a1
2s1 - s2 - s3
r = A1 + E
a1 (2pz)
C3v
E 2C3
3v
A1
1
1
1
A2
1
1
-1
E
2
-1
0
3a1
s1 + s2 + s3
-17.0 eV
x2+y2, z2
z
1e
a1 (2s)
-25.6 eV
(x,y)
-31.0 eV
2a1
Chem 104A, UC, Berkeley
x
z
O
H
H
y
2pz
1s
-15.9 eV
2py
H
H
Other:
OL-,
SL-, SeLFClBrI-
2px
LGO
2s
Lewis Base
OL2, SL2, SeL2, TeL2
-32.4 eV
O
O
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Chem 104A, UC, Berkeley
Type of non-protic acid-base reactions:
1. Adduct formation: acid-base react to form a bond
and produce a single molecule.
2. Displacement: one base (or acid) displace another
3. Double displacement (Metathesis):
interchange of acids and bases
Chem 104A, UC, Berkeley
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
 x*
* 104A, UC, Berkeley
Chem
y
'

2e1
*
s
2a1'
 znb LUMO
2py
2px
2pz

 yb
b
x
1a
LGO3
LGO2
''
2
LGO1
1e1'
2s
 sb
1a1'
B
3H
BH3
Chem 104A, UC, Berkeley

NH3 (C3v: E, 2C3, 3v)
z
3H
N
NH3
H(3)
H(1)
H(2)
2e
-13.5 eV
4a1
y
x
e
- s2 + s3
-15.5 eV
The symmetry of 3H’s group orbitals:
e (2px, 2py)
HOMO
a1
2s1 - s2 - s3
r = A1 + E
a1 (2pz)
C3v
E 2C3
3v
A1
1
1
1
A2
1
1
-1
E
2
-1
0
3a1
s1 + s2 + s3
-17.0 eV
z
x2+y2, z2
1e
a1 (2s)
-25.6 eV
(x,y)
-31.0 eV
2a1
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Chem 104A, UC, Berkeley
Adduct formation
AlCl3  Cl   AlCl4
BH 3  CO  H 3 B  C  O
SO 3  Et 2O  O3 S  OEt2
Chem 104A, UC, Berkeley
Displacement: one base (or acid) displace another
A  B  B'  A  B' B
F3 B  SEt2  Et 2O  F3 B  OEt 2  SEt 2
Double displacement (Metathesis):
interchange of acids and bases
A  B  A' B'  A  B' A' B
Me3 Si  Cl  F3 B  F  Me3 Si  F  F3 B  Cl
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Chem 104A, UC, Berkeley
Relative strength of acid/base
Reference acid
H+,Mg2+, Sc3+
Hg2+
Base strength
F   Cl   Br   I 
F   Cl   Br   I 
Chem 104A, UC, Berkeley
Hard and Soft Acids and Bases (HSAB)
Hard (class a):
having contracted (tightly held) frontier orbitals
Not readily polarized
Soft (Class b):
Having diffuse frontier orbitals
Readily polarized
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Chem 104A, UC, Berkeley
Have vacant orbitals held in tight
Have vacant orbitals, but larger radius
Chem 104A, UC, Berkeley
Have lone pairs in tightly held orbitals
Have lone pairs in larger orbitals
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Chem 104A, UC, Berkeley
Table 1 Hard and Soft Acids and Bases
Hard Bases
H2O OH- FAcO- SO42- Cl-
Soft Bases
R2S RSH RSIR3P (RO)3P
Borderline Bases
ArNH2 C5H5N
N3- Br-
CO32- NO3
ROH
CN-
NO2-
RO- R2O
RNH2
NH3
RCN
CO
C2H4 C6H6
H- R-
Hard Acids
Soft Acids
H+
Cu+
Li+
Na+
Borderline Acids
Pd2+
Fe2+ Co2+ Cu2+
K+ Mg2+ Ca2+
Pt2+ Hg2+ BH3
Zn2+ Sn2+ Sb3+
Al3+
GaCl3 I2 Br2
CH2 carbenes
Bi3+ BMe3 SO2
R3C+ NO+ GaH3
C6H5+
Cr2+
Fe3+
BF3 B(OR)3 AlMe3
AlCl3 AlH3 SO3
RCO+ CO2
HX (hydrogen-bonding
molecules)
Ag+
Chem 104A, UC, Berkeley
Hard Bases
(nucleophiles)
Donor atoms have high electronegativity (low HOMO) and low
polarizability and are hard to oxidize. They hold their valence
electrons tightly.
Soft Bases
(nucleophiles)
Donor atoms have low electronegativity (high HOMO) and
high polarizability and are easy to oxidize. They hold their
valence electrons loosely.
Hard Acids
(electrophiles)
Possess small acceptor atoms, have high positive charge and
do not contain unshared electron pairs in their valence shells.
They have low polarizability and high electronegativity (high
LUMO).
Soft Acids
(electrophiles)
Possess large acceptor atoms, have low positive charge and
contain unshared pairs of electrons (p or d) in their valence
shells. They have high polarizability and low electronegativity
(low LUMO)
n.b “The HSAB principle is not a theory but a statement of experimental facts.”
Pearson, R.G, Songstad, J.Amer.Chem.Soc., 1967, 89, 1827
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Chem 104A, UC, Berkeley
Pearson’s Principle:
Hard acids prefer to bind to hard bases
And soft acids prefer to bind to soft bases.
Chem 104A, UC, Berkeley
Keq
CH3Hg
CH3HgB
BH
H
The simplest hard acid is the proton and methyl mercury cation is the simplest soft acid.
Keq small  B hard base
Keq large B soft base
I   Br   Cl   S 2O3
2
 S 2  PPh3  CN 
 SCN   SO3  NH 3  F   OH 
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Chem 104A, UC, Berkeley
Li-I +Ag-F  H-F +Ag-I
H=-17kcal/mol
HgF2 +BeI2  HgI2 +BeF2
H=-95kcal/mol
Chem 104A, UC, Berkeley
Absolute Hardness

IE  EA
 ( ELUMO  EHOMO )
2
Hard: large HOMO-LUMO gap
Soft: small HOMO-LUMO gap,
ease of mixing ground/excited states, electron
Density redistributed (polarized).
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Chem 104A, UC, Berkeley
Strong polar bond!
Strong covalent bond!
Chem 104A, UC, Berkeley
Empirical approach to determine enthalpy
of adduct formation
A + B  AB
 H  E A EB  C AC B
E: susceptibility to undergo electrostatic interaction
C: tendency to form covalent bonds
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Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
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Chem 104A, UC, Berkeley
SO2  Me32 N  Me3 N  SO2
H  (0.56 1.21  1.52  5.61)  9.2kcal / mol
Experimental value: -9.6 kcal/mol
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