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Chapter 6. Acid-Base and Donor-Acceptor Chemistry
Definitions
HOMO (fairly high lying)
LUMO (relatively low energy)
Modern
e- pair donore- pair acceptorLewis
H+ acceptorH+ donorBrrnsted-Lowry
OH- producerH+ producerArrheniusBaseAcid
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Acid-Base ReactionsDefinitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
Developmentof concept
Defined only in aqueous solutions
acid base
HCl(aq) + NaOH(aq) NaCl(aq) + H2O
salt water
Acid-Base ReactionsDefinitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
Developmentof concept
Defined in both aqueous and non-aqueous solutions
acidbase baseacid
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Acid-Base ReactionsDefinitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
Developmentof concept
Defined in both aqueous and non-aqueous solutions
acid base acid and base
NH4+ + NH2
- 2NH3
A Brønsted acid must contain at least one ionizable proton!
Acid-Base ReactionsDefinitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
Developmentof concept
Defined in both aqueous and non-aqueous solutions
acid base acid and base
NH4+ + NH2
- 2NH3
How about?
SbF5 + BrF3 BrF2+ + SbF6
-
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Acid-Base ReactionsDefinitions
Acid Base
Solvent System cation anion
(from solvent dissociation)
or solute increaing the cation conc. decreasing anion conc.
How about?
SbF5 + BrF3 BrF2+ + SbF6
-
Acid-Base ReactionsDefinitions
Acid Base
Solvent System cation anion
(from solvent dissociation)
or solute increaing the cation conc. decreasing anion conc.
2BrF3 BrF2+ + BrF4
-
solvent acid base
SbF5 + BrF3 BrF2+ + SbF6
-
acid
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Acid-Base ReactionsDefinitions
Acid Base
Solvent System cation anion
(from solvent dissociation)
or solute increaing the cation conc. decreasing anion conc.
2BrF3 BrF2+ + BrF4
-
solvent acid base
F- + BrF3 BrF4-
base
Acid-Base ReactionsDefinitions
Acid Base
Solvent System cation anion
(from solvent dissociation)
or solute increasing the cation conc. increasing anion conc.
2H2O H3O+ + OH-
solvent acid base
H2SO4 + H2O H3O+ + HSO4
-
acid Acid-base reactions in the solvent system concept are the
reverse of autodissociation.
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Acid-Base ReactionsDefinitions
Acid Base
Solvent System cation anion
(from solvent dissociation)
or solute increaing the cation conc. decreasing anion conc.
How about?
NH3+ BF3 NH3BF3
Acid-Base ReactionsDefinitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
acidbase baseacid
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Acid-Base Reactions
How about?
NH3+ BF3 NH3BF3
Definitions
Acid Base
Arrhenius H+ producer OH- producer
Brønsted-Lowry H+ donor H+ acceptor
Lewis e- pair acceptor e- pair donor
base acid adduct
A2' E'
MOs of Polyatomic MoleculesBF3 D3h
A1' E'
A1' E'
A2'' E''
3F(2px)
3F(2py)
3F(2pxz
3F(2s)
SALCs
B
2pz A2''
2s A1'
E'
2py
2px
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MOs of Polyatomic MoleculesBF3 D3h
bonding
non-bondingalmost non-bonding
antibonding
Lewis ?
VBT?
? σ and ? π
same for SO3, NO3-, CO3
2-
MOs of Polyatomic MoleculesNH3 C3v
pz characternon-bonding
2s
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How about?
NH3+ BF3 NH3BF3
acid base adduct
Lewis Acid-Base
coordination compoundwhen involving metal
stabilized
How about?
NH3+ BF3 NH3BF3
acid base adduct
Frontier Orbitals and Acid-Base Reactions
coordination compoundwhen involving metal
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Frontier Orbitals and Acid-Base ReactionsH+ + NH3 NH4
+
2s
NH3
Td
stabilized
Frontier Orbitals and Acid-Base ReactionsH+ + NH3 NH4
+
2s
NH3
Td
a HOMO-LUMO combination new HOMO-LUMO of the product
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Frontier Orbitals and Acid-Base ReactionsH+ + NH3 NH4
+
2s
NH3
Td
Need shape (symmetry) and energy match !!
Frontier Orbitals and Acid-Base Reactions
E
A + B No adductInstead, an electron transfer can occur (Redox).
H2O + Ca 2H2O- + Ca2+ (?)H2O + Ca 2OH- + H2 + Ca2+
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Frontier Orbitals and Acid-Base Reactions
E
A + C AdductnH2O + Cl- [Cl(H2O)n]-
A + D Adduct6H2O + Mg2+ [Mg(H2O)6]-
Frontier Orbitals and Acid-Base Reactions
E
A + E No Adduct
2H2O + 4F2 4H+ + O2 + 4F-
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A + E No Adduct
2H2O + 4F2 4H+ + O2 + 4F-
A + C AdductnH2O + Cl- [Cl(H2O)n]-
A + D Adduct6H2O + Mg2+ [Mg(H2O)6]-
A + B No adductH2O + Ca 2OH- + H2 + Ca2+
H2O
Oxiant
Acid
Base
Reductant
Hydrogen Bonding
Dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N)
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Hydrogen Bonding
Dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N)
Why is the difference so big for H2O?
Hydrogen Bonding
Ice Water
b.p ↑m.p ↑ d↓
forming clathrates
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Hydrogen Bonding
α-helix
β-sheet
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C
147152155120van der Waals radius (pm)
71737532Covalent radius (pm)
FONHelement
~2.7 Å
What happens?
Remind....FHF- D∞h D2h
F(2px)+F(2px)2 -2 0 0 0 0 2 -2
F(2py)+F(2py)2 -2 0 0 0 0 -2 2
F(2pz)+F(2pz)2 2 0 0 0 0 2 2
F(2s)+F(2s)2 2 0 0 0 0 2 -2
B3u + B2g
B2u + B3g
Ag + B1u
Ag + B1u
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Remind....FHF-
F---F: SALCs H: 1s orbital
Agcan combine to form MOs
Remind....FHF-
(-18.7 eV)
(-40.2 eV)
(-13.6 eV) H 1s
H 1s will strongly interact with F2Pzs (Ag).
Don't forget if F2s contributes, 3 MOs are formed.
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Remind....FHF-
bonding
antibonding
non-bonding* there are slight long-range interactions.
Remind....FHF-
F H F
Lewis structure
MO3-center 2-electron bond
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Other way to think ...FHF-
2pz
H1S ± F2pz
MOs for H-bonding in FHF-
Hydrogen Bonding
FHF-
MOs for H-bonding in FHF-
2pz
MOs for asymmetricH-bonding
H-bonding can be describedby HOMO(Base)-LUMO(Acid) interaction !!not just dipole-dipole interaction
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When Hydrogen Bonding ?
poor matchno reaction
ex) CH4 in H2O
good matchforming H-bonding
ex)acetic acid, water,acetic acid in water
very poor matchH+ transfer
ex) HCl in H2O
HCl + H2OH3O
+ + Cl-
Evidences ofAcid-Base Adduct Formation (I2)
π* σ*
σ σ*
purple
purple
red-violet
brown
actually, hasfine structures
CT
hv I2
.Donor [I2]-.Donor+
yellow-brown (water)colorless(I3
-)
adduct formation with water (minor) and I- (major)
weak donor
strongdonor
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Hard-Soft Acid-Base (HSAB)
AgF(s) + H2O Ag+(aq) + F-(aq) Ksp= 205
AgCl(s) + H2O Ag+(aq) + Cl-(aq) Ksp= 1.8 x 10-10
AgBr(s) + H2O Ag+(aq) + Br-(aq) Ksp= 5.2 x 10-13
AgI(s) + H2O Ag+(aq) + I-(aq) Ksp= 8.3 x 10-17
Hard-Soft Acid-Base (HSAB)
Acids and bases can be divided up into groups based on their hardness or softness.
“Hard” means the acid or base has a high charge density:
- it has a small radius and a large charge (less polarizable)
“Soft” means the acid or base has a low charge density:
- it has a large radius and a small charge (more polarizable)
The important predictive ability of HSAB theory is that hard
acids prefer to react with hard bases, and soft acids prefer to
form adducts with soft bases.
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Hard-Soft Acid-Base (HSAB)
HSAB theory is based on polarizability.
Polarizability is “The ease of distortion of the electron cloud
of a [molecule] by an electric field (such as that due to the
proximity of a charged reagent).”
By this definition, hard acids and bases have tightly bound
electron clouds that are difficult to polarize. Soft acids and
bases have looser electron clouds that are easier to distort.
Hard-Soft Acid-Base (HSAB)
Polarizabilityincreases down the groupincreases with increasing negative charge (base)increases with decreasing positive charge (acids)
hardnessreverse
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Hard-Soft Acid-Base (HSAB)
AgF(s) + H2O Ag+(aq) + F-(aq) Ksp= 205
AgCl(s) + H2O Ag+(aq) + Cl-(aq) Ksp= 1.8 x 10-10
AgBr(s) + H2O Ag+(aq) + Br-(aq) Ksp= 5.2 x 10-13
AgI(s) + H2O Ag+(aq) + I-(aq) Ksp= 8.3 x 10-17
Hardness
F- > Cl- > Br- > I-Ag+ : soft cation
Hard-Soft Acid-Base (HSAB)
O2- O22- O2
- O3-
hardness ??
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Hard-Soft Acid-Base (HSAB)
O2- O22- O2
- O3-
hardness ??
LiNaKRbCs
+ O2
Hard-Soft Acid-Base (HSAB)
염기
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Hard-Soft Acid-Base (HSAB)Quiz
Some of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.
a. Will Cu2+ react more strongly OH- or NH3? With O2- or S2-?b. Will Fe3+ react more strongly OH- or NH3? With O2- or S2-?c. Will Ag+ react more strongly with NH3 or PH3?d. Will Fe, Fe2+, or Fe3+ react more strongly with CO?
Hard-Soft Acid-Base (HSAB)Quiz
Some of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.
a. Will Cu2+ react more strongly OH- or NH3? With O2- or S2-?b. Will Fe3+ react more strongly OH- or NH3? With O2- or S2-?c. Will Ag+ react more strongly with NH3 or PH3?d. Will Fe, Fe2+, or Fe3+ react more strongly with CO?
AnswerSome of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.
a. Will Cu2+ react more strongly OH- or NH3? With O2- or S2-?b. Will Fe3+ react more strongly OH- or NH3? With O2- or S2-?c. Will Ag+ react more strongly with NH3 or PH3?d. Will Fe, Fe2+, or Fe3+ react more strongly with CO?
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Hard-Soft Acid-Base (HSAB)
HSAB is in general working but not always.
Hard-Soft Acid-Base (HSAB)Ambidentate ligand : ex) SCN-
soft borderline
hard soft
borderline
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Hard-Soft Acid-Base (HSAB)
Quantitative HSABAbsolute hardness (η), Absolute softness (σ)
(EHOMO = -I, ELUMO = -A)
Mulliken’s electronegativity (χ)
A for anion is approximated fromthe corresponding atom.
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Quantitative HSABAbsolute hardness (η)
* Absolute hardness is working in gas phase.
Quantitative ABDrago-Wayland Equation
-∆H = EAEB + CACB for A + B AB( in gas phase or inert solvent)
E: capacity for electrostatic (ionic) interactionC: measure of the tendency to form covalent bonds
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Quantitative AB
most acids
CA < 1EA > 1
most bases
CB > 7.40EA < 1.32
Drago-Wayland Equation
Drago's AB and Peasrson's HSAB
Drago Pearson
electrostatic and covalent covalent
Both are useful but neither covers all the cases.When E and C are available, use it !!When not, predict with HSAB.
Don't forget solvation.
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How to measure?• bp/mp: predict the presence of adduct• calorimetric (thermodynamic)• formation of protonated species (gas phase)• IR: ex) Ni(CO)4 ν(C-O)• NMR• UV/VIS
Acid-Base Interaction
Acid-Base Strengths
Acidity constant (Acid dissociation constant): KAcidity constant (Acid dissociation constant): Kaa
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
Ka = [H3O+][A-] = [H+][A-]
[HA] [HA]
pKa = -log[Ka]
ln Ka = -∆H/RT + ∆S/R
* pKa ranges between –10 (very strong acids) to 50 (very weak acids).
*The stronger an acid is, the lower (more negative) its pKa is.
Calorimetric
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Acid-Base StrengthsHA(aq) + H2O(l) ⇌ H3O
+(aq) + A-(aq)
Ka = [H3O+][A-] = [H+][A-]
[HA] [HA]
pKa = -log[Ka]
Acid-Base Strengths
BasicityBasicity constant : Kconstant : Kbb
NH3 (aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Kb = [NH4+][OH-]
[NH3]
•The higher Kb is, the stronger basicity is.
• Strong base is virtually fully protonated.
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Acid-Base StrengthsProton Affinity (gas phase)
BH+(g) B(g) + H+(g) proton affinity = ∆H
Mass spec., Ion cyclotron resonance spec.
∆H ↑ : Basicity of B ↑, acidity of BH+ ↓
Basicity: LiOH < NaOH < KOH < CsOH in gas phaseLiOH ~ NaOH ~ KOH ~ CsOH in aqueous soln
Basicity: > NH3 in gas phase
< NH3 in aqeous soln
Useful for sorting out the factors of acid-base behavior
AH(g) A-(g) + H+(g) ∆H: enthalpy of dissociation:
acidity
EN
why?
Acid-Base StrengthsBinary Hydrogen Compounds
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Acid-Base Strengths
AH(g) A-(g) + H+(g) ∆H: enthalpy of dissociation:
acidity
ENwhy? Do your homework.
acidity
EN
Binary Hydrogen Compounds
Acid-Base StrengthsInductive Effect
P
H HH
P
F FF
Basicity? >
NMe3 NHMe2 NH2Me NH3> > >
BF3 BCl3 BBr3Acidity? > > ?
BX
XX
π-bonding strength
electron density on B
BF3 BCl3 BBr3< <
acidity
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Acid-Base StrengthsOxoacids
pKa (exp)
pKa (Pauling)= 9-7npKa (modified)= 8-5nn (# of non-hydrated oxygen)
0 1 2 3
7.2 2 -1 (-10)
9 2 -5 -12
8 3 -2 -7
Acid-Base StrengthsOxoacids
n (# of nonhydrated oxygen) ↑positive charge on central atom (A)↑pull hydrated oxygen toward A weaken O-H bond
or n (# of nonhydrated oxygen) ↑
spread e- density of the conjugate base in wider rangestability ↑ basicity of the conjugate base ↓
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Acid-Base StrengthsAqua acids
Fe(H2O)63+ + H2O(l) ⇌ H3O
+(aq) + Fe(H2O)5 (OH)2+
pKa ∝ (Electrostatic parameter)-1
Working well for s- and p-blocksDeviation: due to covalency
Q/r
Positive cations in aqueous soln
Acid-Base StrengthsSteric Effect
N N N N
Basicity toward hydrogen ion
NN NN
Basicity toward BF3 or BMe3
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Acid-Base StrengthsSteric Effect
Acid-Base StrengthsLeveling Effect
2H2O H3O+ + OH-
Strongest acid in aq soln Strongest base in aq soln
Kw = 10-14
CH3COOH + H2O H3O+ + CH3COO-
HCl + H2O H3O+ + Cl- 100%
Any stronger acid forms hydronium ion immediately.
than H3O+
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Acid-Base StrengthsLeveling Effect
2H2O H3O+ + OH-
Strongest acid in aq soln Strongest base in aq soln
Kw = 10-14
NH3 + H2O NH4+ + OH-
Na2O + H2O 2Na+ + 2OH- 100%
Any stronger base forms hydroxide ion immediately.
than OH-
Acid-Base StrengthsLeveling Effect
H2SO4 + CH3COOH CH3COOH2+ + HSO4
-
Strongest base in pure acetic acid
NH3 + CH3COOH NH4+ + CH3COO- 100%
Any stronger base forms acetate ion immediately.
than acetate ion
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Acid-Base StrengthsLeveling Effect
Superacids
A medium having a high acidity, generally greater than that of 100 wt.% sulfuric acid.”…“By analogy, a compound having a very high basicity, such as lithium diisopropylamide, is called a ‘superbase’
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