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04/18/23
Acid-Base Strength
The strength of an acid-base interactiondepends on the nature of the reacting species
and solvent effects.
Solvent effects can be eliminated by consideringgas phase reactions.
The strength of an acid is a measure of its abilityto dissociate and protonate another compound.
Non-aqueous solvents can be used todistinguish between strong acids.
04/18/23
Gas Phase BasicityProton Affinity
An energy cycle can be used to determinethe strength of a base
in the absence of solvent effects.
B(g) + H(g)-HA,BH
BH(g)
+e- -HIE,H -e- HIE,BH
B(g) + H+(g)H = PA
BH+(g)
04/18/23
Gas Phase BasicitySubstituted Amines
N3-
NH2-
NH2-
N(CH3)3 974basi
city
3084 kJ·mol-1
2565
1689
NH2CH3 919
NH3 872
NF3 604
NH(CH3)2 954
protonaffinityvalues
Solution Phase BasicitySubstituted Amines
NR3 The basicity of amines depends oninductive effects of substituentssteric effects
pKb
solvent effects
NF3
no basiccharacter
604
N(CH3)3
4.28
974
NH2CH3
3.36
919
NH3
4.74
872
NH(CH3)2
3.29
954PA
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Gas Phase AcidityProton Affinity
The conjugate base of stronger acidshave lower proton affinities.
HF(g)H = PA
H+(g) + F-(g)
CH3COO- 1459 kJ·mol-1
CF3COO- 1351 kJ·mol-1
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Gas Phase AcidityProton Affinity
I-
Br-
Cl-
F-
CH3COO-
OH-
1554
1459
1635
Con
jugate
Aci
d A
cid
ity
1315 kJ·mol-1
1354
1395
HI
HBr
HCl
HF
CH3COOH
H2O
Solution Phase AcidityAcid Strength in Solution
In water, HCl, HBr and HI are all consideredstrong acids.
Why are they all considered equally strongin water?
How can the strongest acid be identified?
Solution Phase AcidityAcid Strength in Solution
In which solvent would HCl ionize less?
CH3COOH
H2O
NH3
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
Comparison of many acid-base reactionsindicates that both ionic and covalent factors
contribute to the enthalpy of reaction.
Each acid and base can be assignedan electrostatic (E) factor and
a covalent (C) factor.
-ΔHrx = EAEB + CACB
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
OH
H
SH
H
K+
I - I
Which acid-base pair reactions are more exothermic?
EB = 2.28CB = 0.10
EB = 0.04CB = 1.56
EA = 0.50CA = 2.00
EA = 3.78CA = 0.10
-8.6
-3.1
-1.3 -0.3 kcal/mol
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
-ΔHrx = EAEB + CACB
These factors provide ∙ an estimate of the enthalpy of reaction ∙ insight into the bonding nature of each species
I - IEA = 0.50CA = 2.00
K+ EA = 3.78CA = 0.10
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
-ΔHrx = EAEB + CACB + RATB
More precise estimates can include acorrection factor for the transference of charge
from the base to the receptor acid.
K+ EA = 3.78CA = 0.10
RA = 20.79
OH
HEB = 2.28CB = 0.10TB = 0.43
(more important for ions)
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
OH
H
K+
EB = 2.28CB = 0.10TB = 0.43
EA = 3.78CA = 0.10
RA = 20.79-17.6 kcal/mol
-ΔHrx = EAEB + CACB + RATB
04/18/23
Acid-Base ReactionsIonic and Covalent Contributions
E, C, R & T values represent gas phase reactions.
F- has stronger ionic and covalent terms than Cl-, Br- or I-
F- 9.73 4.28 37.40
Cl- 7.50 3.76 12.30
Br- 6.74 3.21 5.86
I- 5.48 2.97 6.26
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Acid-Base ReactionsBond Energies
Three contributing factors to bond energy
covalent energy
Madelung (Coulombic) energy
electronegativity energy
-ΔHrx = EAEB + CACB + RATB
04/18/23
Hard and Soft Acids and Bases
Hardnon-polarizable
high charge density
Softpolarizable
large, low charge
cations
anions
H+, Groups I & II,Ti4+, Cr3+, Fe3+, Co3+
Cu+, Ag+, Hg22+,
Hg2+, Pd2+, Pt2+
(class a) (class b)
N>>P>As>SbO>>S>Se>Te
F>Cl>Br>I
N<<P>As>SbO<<S<SeTe
F<Cl<Br<I
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Hard and Soft Acids and Bases
Dithiooxalate ioncan chelate on two sides.C
C
O
- O
S -
S
C
C- O
O
S
S -
C
CO
O
S
S
- -
prefers hardercations
prefers softercations
04/18/23
Hard and Soft Acids and Bases
Dithiooxalate ioncan chelate on two sides.
C
CO
O
S
S
-
-
prefers hardercations
prefers softercations
C
CO
O
S
S
-
-
Fe3+ Cu+
04/18/23
Capable of protonating compoundsthat don’t want to be protonated.
Causing isomerization in the most unlikely candidates.
More acidic than concentrated sulfuric acid.
04/18/23
Acids which are stronger than pure sulfuric acid.
Super acids are measuredwith the Hammett acidityfunction (Ho) Ho = pKBH+ - log
[BH+]
[B]
B is an indicator base:e.g., nitroaniline, benzene, picramide
04/18/23
Superacid Acidity
100% sulfuric acid H2SO4 -11.9
hydrofluoric acid HF -11.0
perchloric acid HClO4 -13.0
fluorosulfuric acid HSO3F -15.6
triflic acid HSO3CF3 -14.6 trifluoromethanesulfonic acid
magic acidTM HSO3F-SbF5 -21 to -25
fluoroantimonic acidHF-SbF5 -21 to -28
Ho
non-aqueous systems
04/18/23
(C6H5)3COH + superacid → (C6H5)3C+ + H3O+
trivalent carbocation(carbenium ion)
alkyl-Cl + superacid → alkyl+ + HCl
hydrocarbon + superacid → hydrocarbon-H+
pentacoordinate carbocation(carbonium ion)
George Olah 1994 Nobel Prize
04/18/23
Formed by mixing
fluorine containingBronsted acidor metal oxide
fluorinated Lewis acid
HF, HSO3F,HSO3CF3
BF3, SbF5, TaF5
TiO2, SiO2
04/18/23
2 HF + 2 SbF5 H2F+ +
F
F
FF
FF Sb
F
F
FF
F
Sb
-
→
HSO3F + SbF5
fluorosulfonicacid
antimony pentafluoridecomplexing agent
F
F
FF
FH-O-S—F
O
O
Sb→
Why are superacids so acidic?
magicacidTM
fluoroantimonic acid
04/18/23
Hammett acidityis effectively the extension of pHbelow zero.
Ho = pKBH+ - log[BH+]
[B]
B is an indicator base:e.g., nitroaniline, benzene, picramide
BH+ B + H+ pKBH+
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Hammett acidityis determined
spectrophotometricallyby determining [BH+] and [B]
Ho = pKBH+ - log[BH+]
[B]
B is an indicator base:e.g., nitroaniline, benzene, picramide
04/18/23
Applications
Isomerization/polymeriztion
R+ + C C C C+R
C CR C C+
C C
Heterogeneous catalystssolid superacidsZrO2-SO4, TiO2-SO4
for cracking & isomerizing alkanes
04/18/23
Applications
Hydrocracking
heavy oilbitumen
H2
lower boiling distillate
also, coal liquifaction
04/18/23
Redox Reactions
F-S-O-O-S-F
O
O
O
O
peroxydisulfuryldifluoride
+ I2 I
FO2SO
OSO2F
OSO2F
I2+
I3+
+ SO3F -
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Acid-Base ReactionsIonic and Covalent Contributions
OH
H
SH
H
K+
I - I
Which acid-base pair reactions are more exothermic?
EB = 2.28CB = 0.10
EB = 0.04CB = 1.56
EA = 0.50CA = 2.00
EA = 3.78CA = 0.10
-8.6
-3.1
-1.3 -0.3 kcal/mol