Acid-Base Theory
Contents
I Background
1 Arrehnius Theory
2 Brønsted-Lowry Theory
II Acid-Base Theories
1 Lewis Acid-Base Theory
2 Lux-Flood Theory
3 Hard-Soft Acid-Base Theory
III Measurements of Acidity & Basicity
IV Descriptive Chemistry of Acids & Bases
I Background
Ø What is Arrhenius (Ostwald) Theory?
Ø What is Arrhenius acid and Arrhenius
base?
Acid - proton donor, Base – hydroxide
donor
Ø What is the core concept of Arrhenius
Theory?
Ø What is ionization theory?
Ø Why is water special to Arrhenius Theory?
Ø What is self-ionization?
Ø Are there other substances that can have
self ionization?
Ø What is the Brønsted-Lowry Theory?
Ø What is the difference between the
Arrhenius Theory and the Brønsted-Lowry
Theory?
Acid – proton donor,
acceptor
Ø
What’s the physical
Brønsted-Lowry Theory?
Base – proton
meaning
of
1 competition for protons rather than
proton & hydroxide donors
2 A-B conjugate
3 insight of the atom transfer processes
4 useful in non-aqueous systems
Substances
that
(self-ionization)
Aqueous system:
have
auto-dissociation
2 H2O
H3O+ + OH- Kw = 1.0x10-14 at 24.8 °C,
1 atm
Neutral point: (1/2)pKw = 7.0
leveling effect The acidity and basicity in the
aqueous solutions are limited by H+ and OH
－
. The acidity of any substance is stronger than
H+ or the basicity is stronger than OH－ will be
leveled off. This is called “leveling effect”.
2 EtOH
2 NH3
2 H2SO4
EtOH2+ + EtO- KAB = 10-20
NH4+ + NH2-
KAB = 10-29
H3SO4+ + HSO4-
2 OPCl3
transfer)
(phosphoryl halides)
OPCl4- + OPCl2+ (Cl－
Solvolysis
A process in which the solute
can combine with the ionic
component of the solvent or have
the solvent ionized.
1 Hydrolysis combination of H+ in water
NaOAc + H2O
HOAc + NaOH
OPCl3 + 3 H2O
OP(OH)3 + 3 HCl
2 Non-aqueous solvolysis
OPCl3 + PCl5
OPCl2+ + PCl6-
Alcoholysis
OPCl3 + 3 ROH
OP(OR)3 + 3 HCl
Gutman elaborates the reaction of Me4N+Cl －
and ferric chloride in phosphoryl chloride as
solvolysis via chloride-transfer pathway.
Me4N+Cl-
OPCl3
FeCl3 + OPCl3
OPCl2+ + Cl-
Me4N+ + ClFeCl4- + OPCl2+
OPCl3
As for the amphoteric substance, such as
Al(OH)3 is insoluble in water, but dissolves in
strong acid and strong base. Similarly,
Al2(SO3)3 is insoluble in liquid sulfur dioxide,
but dissolves by adding SOCl2 or CsSO3.
Question: Does the reaction of thionyl chloride
with sulfite undergo solvolysis in SO2?
n A clever experiment of isotope labeling shows
no exchange between 32SOCl2 and 35SO2.
n No oxygen exchange observed for the reverse
reaction either.
n However, the halide exchange between SOCl2
and SOBr2 was observed.
Lewis Acid-Base Theory
In water:
B(OH)3+H2O→H++B(OH)4–, so B(OH)3 is an
Arrehnius acid.
NH3+H2O→NH4++OH–, so NH3 is an Arrehnius
base.
Boric acid being an acid is not only because it is
ionized to give proton, but for it decreases
[OH–]：
B(OH)3 + OH– → B(OH)4–
Ammonia being a base is not only because it is
ionized to give proton, but for it decreases [H+]：
NH3 + H+ → NH4+
Lewis Base – electron pair donor,
Lewis Acid – electron pair acceptor
For the reactions in which adducts are formed.
Me3N(g) + BF3(g) → R3N-BF3(s) (m.p. > 200 °C)
Inductive effect
Base strength: Me3N>NH3>NF3
Acid
strength:
Me3B<BH3<BF3
(how
to
determine it)
BF3<BCl3<BBr3
resonance?)
Steric effect
(double
bond
N
N
N
BMe3
BMe3
BMe3
DHrxn = –71
–74
–42 kJ/mol
B(mesityl)3 does not react even with MeO
－
.
f-strain: front steric hindrance

B
b-strain: back steric hindrance
R
R
R
R
R
R
>
N
R
N
>
R
N
R
I (internal)-strain: oxygen basicity drops with
the decreasing ring size.
O
Me
O
Me
N
>
Me
N
>
O
N
Lewis A-B theory is important to coordination
chemistry (Chp 11, 12).
2 NH3 + Ag+ → Ag(NH3)2+
Acid – electron density deficiency (poorness)
Base – electron density abundance (richness)
1 The chemistry correlates the reactivity due to
the
change
of
electron-density
with
thermodynamics.
2 Energetically, HOMO are often low-lying and
LUMO are often high lying.
3 The donor atom that has less s-character or
more p-charcater has stronger basicity.
Lux-Flood Theory –— Reactions of Atom
Transfer
Base – oxide donor
Acid – oxide acceptor
Oxides
are
chemistry.
important
to
the
molten
But oxides do not exist in the
aqueous solutions.
O2－ + H2O → 2OH－
CaO + SiO2 → CaSiO3 (major content of slag)
(base) (acid)
SiO2 + H2O → H2SiO3 (important to sol gel
process)
Sol:A colloid of tiny solid particles dispersed
throughout a liquid.
Solid Emulsion: A colloid of tiny droplets of
liquid or solid particles dispersed through
out a solid.
Gel: A soft solid emulsion but holds its shape.
Hard-Soft Acid-Base Theory
Irving-William series
For a given ligand, the stability of complexes
with M2+ follow the following order: Ba2+ < Sr2+
< Ca2+ < Mg2+ < Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+
> Zn2+
Different Coordination Correlations
Class (a):
1 alkali metal ions
2 alkaline earth metal ions
3 light transition metal ions in high oxidation
states:
Ti4+, Cr3+, Fe3+, Co3+
4 Al3+, H+
Class (b): metal ions in low oxidation states
Cu+, Ag+, Hg+, Hg2+, Pd2+, Pt2+
Borderline:
Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Rh3+, Ir3+, Ru3+,Os2+,
Sn2+, Pb2+, carbonium ions, NO+
Donors having tendency to bind with Class (a)
metal ions
N >> P > As > Sb
O >> S > Se > Te
F > Cl > Br > I
Donors having tendency to bind with Class (b)
metal ions
N << P > As > Sb
O << S > Se ~ Te
F < Cl < Br < I
Ralph G. Pearson suggested the term “hard” for
the metal ions of Class (a) and their
corresponding bases; “soft” for the metal ions of
Class (b) and their corresponding bases.
Hard acids prefer to bind to hard bases; and
soft acids prefer to bind to soft bases.
MeHgF + HSO3－ → MeHgSO3－ + HF Keq ~
103
MeHgOH + HSO3－ → MeHgSO3－ + H2O Keq
> 107
Transmetallation used in the Synthesis of
Organometals
metathesis
MXn + n RLi → MRn + n LiX
MXn + n RMgX → MRn + n MgX2
Carbon or hydride nucleophiles: LiR, NaH,
Na(C5H5), NaBH4, KHB(SitBu)3, LiAlH4 (LAH),
AlR3, AlRn(OR’)3-n, GaMe3, R2Mg, R2Zn, RZnX,
LiCuR(CN), NaNH2,
radical donor: RHgX, R4Sn
Electrophiles: proton sauce, RX, R3SiX, CO2,
ROTs, ROTf, Ph3C+BF4-,
Anions: X–, BF4–, BArf4–, PF6–, ClO4–, NO3–
Coupling
Reactions:
Heck
reaction
(AX/RnSnX4-n),
reaction,
Suzuki
Stille
reaction
(ArX/RB(OH)2), etc.
R
R'
RX
0
Pd
R
R
Pd
Pd
X
R'
EX
R'E
Physical meanings behind HSAB
1 ionization energy & electronegativity
2 polarizability
3 HOMO-LUMO gap: large for hard species,
small for soft species
4 soft-soft interactions imply covalency; and
hard-hard
interactions
are
for
bonding
III Measurements of Acidity and Basicity
Aqueous basicity & acidity
ionic
pH = –log[H+];
pH + pOH = 14.0 at 24.8 °C, 1
atm
Ka,
DG˚ = -RTlnK; DS = klnΩ
Either the logarithm of an equilibrium constant
or of a measurement for the arrangement of the
states means a thermodynamic function.
Gase-Phase basicity & acidity
Proton affinity of a base B is defined as the
negative
value
of
the
enthalpy
for
the
reaction: B(g) + H+(g) → BH+(g)
Using Born-Haber cycle, PA may be determined
by measuring ionization energies of H and BH
as well as bond enthalpy of BH.
B(g) + H(g)
Bond enthalpy
-HA(BH)
-e- HIE(BH)
-HIE(H) +eB(g) +
BH(g)
H+(g)
-PA
BH+(g)
proton affinity
Some determined PA (kJ/mol):
For N3–, 3084; NH2–, 2565; NH2–, 1689 (charge
effect)
NH3, 872; NF3, 604; Me3N, 974 (inductive effect)
Difference between NH3 and NF3 is 268 kJ/mol;
between PH3 and PF3 is only 106, but DEN is
larger for P—F. This is attributed to the
resonance effect, thus the charge in PF3H+ is
substantially delocalized.
Gase-phase acidity for a conjugate acid of B will
be the enthalpy of dissociation of BH+.
Gase-phase acidity for the Lewis acids of metal
cations parallels the electron affinities.
Acidity of Oxides
An
acidity
scale
for
the
oxides
is
the
measurement of the reaction enthalpy, and
make aB–aA = (DHrxn)1/2.
For instance
CaO + SiO2 → CaSiO3 DH = –86 kJ/mol aB–aA
= 9.5
The “a parameter” is calibrated with awater =
0.0. Therefore, the negative values for the “a
parameter” indicates high basicity, e.g. –15.2 for
Cs2O; and the positive values for the nonmetal
oxides, e.g. 11.5 for Cl2O7, an anhydride of
orthochloric acid.
Drago-Wayland Equation for the HSAB
–DH = EAEB + CACB for the reaction: A + B →
AB
wherein E stands for electroststic and C for
covalent.
IV Descriptive Chemistry of Acids & Bases
1 general guidelines
Ø Basicity of metal oxides increases while
going down the periodic table. BeO is
amphoteric.
With increasing covalency,
oxides become more acidic or less basic.
Ø The acidity of nonmetal oxides increases
when the values of (aB-aA) increases.
Ø The acidity of the hydrated metal ions of
high positive charge density increases when
the value of z2/r increases. Hydration and
hydrolysis are more feasible with the same
trend. The metal ions of high charge to size
density are more acidic, e.g. Fe3+ pKa 2.19;
Fe2+ pKa 10.1.
Ø The EN of the central atom is directly
influence the acidity of oxyacids. Acidity:
HClO4 ~ HNO3 > H2SO4 >> H3PO4 > H2CO3
>> H3BO3
Ø The more oxygens around the central atom,
the larger acidity is.
Acidity: HClO4 > HClO3 > HClO2 > HClO
Ø
Change
the
substituents
basicity.
inductive
of
amines,
effect
of
change
the
the
NH2OH < NH2NH2 < NH3 <
MeNH2; NF3 is not a base;
2 sulfuric acid
O
H
H
S
O
O
O
O
H
S
O
2-
O
O
O
S
O
O
O
hydrogen sulfate ion sulfate ion
sulfuric acid
water
b.p.
300 ˚C
100 ˚C
m.p.
10.371 ˚C
0 ˚C
1.83 g cm-3 (25 ˚C)
1.00 g cm-3 (4 ˚C)
110 e0 (20 ˚C)
81.7 e0 (18 ˚C)
1.04x10-2 W-1cm-1 (25
4x10-8 W-1cm-1
d
dielectric constant
specific conductance
viscosity
ion product constant
˚C)
24.54 g cm-1s-1 (20 ˚C)
2.7x10-4 (25 ˚C)
1.01 g cm-1s-1 (20 ˚C)
1.008x10-14 (25 ˚C)
3 ammonia
ammonia
water
b.p.
-33.38 ˚C
100 ˚C
m.p.
-77.7 ˚C
0 ˚C
0.725 g cm-3 (-70
1.00 g cm-3 (4 ˚C)
d
˚C)
dielectric
constant
26.7 e0 (-60 ˚C)
specific
1x10-11 W-1cm-1
conductance
4x10-8 W-1cm-1
1.01 g cm-1s-1 (20
˚C)
0.254 g cm-1s-1 (-33
˚C)
viscosity
ion
81.7 e0 (18 ˚C)
product
1.008x10-14 (25 ˚C)
5.1x10-27
constant
Ammonia is a poor solvent for highly charged
ions, including carbonates, sulfates, phosphates,
etc.,
but
nonpolar
molecules,
or
large
polarizable ions such as iodide, thiocyanate are
soluble in ammonia.
Sovolysis takes place in the presence of chlorine
or phosphoryl chloride. The former reaction
may also be considered as disproportionation.
4 oxoacids
Oxoacids
HmXO(m+n).
are
customarily
The
functional
written
as
formula
is
XOn(OH)m. Hydrated oxometals such as uranyl
UO22+, vanadyl VO2+.
Acidity:
HClO4 > HClO3 > HClO2 > HClO > HBrO >
HIO
HClO4 > H2SO4 > H3PO4 or OP(OH)3 > H2CO3
> H3BO3
CrO3 > Cr2O3 > CrO (basic)
CrO3 + H2O → H2CrO4 (chromic acid)
5 binary oxides
Oxides of the metals of Groups 1 and 2 are
generally
basic,
except
BeO.
Al2O3
is
amphoteric. Nonmetal oxides are generally
acidic.
Acidity: Cl2O7 > SO3 > P4O10 > SiO2
5 solid acids:
Clays and Zeolites are porous solids. The
clays have layer structures, and the zeolites
are aluminosilicates with channels.
have
replaceable
cations
and
They
may
be
exchanged with H+, thereby forming solid
acids.
The open channels of zeolites vary in size, thus
may
serve
selectivity.
as
catalysts
For
for
instance,
specific
ZSM-5
(Na3(AlSi31O64)3·16H2O) is used to produce
gasoline from methanol.
6 superacids:
The protonated solvent molecules of the already
strong acids.
Examples: SbF5 produces extremely strong
proton donor in HF or HSO3F.
SbF5 + 2 HF → H2F+ + SbF6－
SbF5 + 2 HSO3F → H2SO3F+ +
FSO3SbF5－