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Chapter 8:Organic Acids and Bases
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Acid/Base Reactions
• Some organic chemicals have exchangeable protons (acids) or lone pair electrons which can accept hydrogen (bases)
• Ionized form of these compounds acts very differently from the neutral form (different HLC, Kow, etc)
• Proton transfer reactions are usually very fast and reversible, so we can treat them as an equilibrium process
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Acidity Constant
Organic acids:HA + H2O H3O+ + A-
choosing pure water as a reference state:
H+ + H2O H3O+
by convention, G = 0, K = 1Thus, HA H+ + A- ; lnKa = -(G /RT)
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ln ln' '
'K
H A
HA
G
RTa
H A
HA
o
At equilibrium:
Ka = acid dissociation constant
typically measure activity of H+, and conc of HA, A- (mixed acidity constant)
at low ionic strength, 1
ln lnKH A
HAa
log logA
HAK pH pH pKa a
when pH = pKa, [A-] = [HA]
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For organic bases, treatment is similar:B + H2O BH+ + OH-
K
OH BH
Bb
OH BH
B
' '
'
written as acidity constant:BH+ B + H+
BH
BHK
BH
BHa '
''
pKa + pKb = pKw = 14 at 25C
Ka * Kb = Kw = 10-14 at 25C
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acids
bases
carboxylic acids
phenols
amines
heterocycles with N
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O
Important functional groups:Acids:
CH3-OH alcohols (pKa > 14)
Carboxylic acids (pKa = 4.75)CH3-C-OH
Phenols (pKa = 9.82)
benzoic acids (pKa = 4.19)
Bases:
NH3 ammonia (pKa = 9.25)
CH3NH2 primary amine (pKa = 10.66)
(CH3)2NH secondary amine (pKa = 10.73)
(CH3)3N tertiary amine (pKa = 9.81)
anilines (pKa = 4.63)
pyridines (pKa = 5.42)N
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Temperature effect on pKa
21
0
1
2 11ln
TTR
H
K
K r
Tia
Tia
recall that the effect of temperature on any equilibrium constant:
for strong acids, rHo is very small
rHo increases as pKa increases (weaker acids have higher temperature dependence) (Why?)
hmmm… what is the rS of a proton transfer reaction?
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Speciation in natural watersQ: Does the presence of an organic acid affect the pH of the water?
A: Probably not. Why?
Natural waters are usually buffered by carbonate (among other things).
If carbonate is present at 10-3 M and the pH is neutral, then addition of acid at 10-5 M (a factor of 100 less than the buffer) will have virtually no effect on pH.
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Speciation in natural waters
)(101
1
][][
1
1apKpH
HAAAHA
HA
fraction of acid in the neutral form:
fraction of base in the neutral form:
1
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Chemical structure and pKa
We are mostly concerned with compounds for which 3 < pKa <11
aliphatic and aromatic carboxyl groupsaromatic hydroxyl groups (phenols)aliphatic and aromatic amino groupsN heterocyclesaliphatic or aromatic thiols
These classes of compounds have pKa’s which vary widelyWhy?
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Substituent effects are of three types:
Inductive effectspositive (electron donating) for O-, NH-, alkylnegative (electron withdrawing) for NO2, halogen, ether, phenyl, etc.
Delocalization effects (resonance)positive for halogen, NH2, OH, ORnegative for NO2, others
Proximity effects intramolecular hydrogen bondingsteric effects
Substituents can have a dramatic effect on the pKa of the compound
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Inductive effects
pKa
acetic acid 4.75
propanoic acid 4.87
butyric acid 4.85
4-chlorobutyric acid 4.52
3-chlorobutyric acid 4.05
2-chlorobutyric acid 2.86
alkyl groups are weakly electron donating
chlorines are strongly electron withdrawing
proximity is crucial
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Delocalization effects (resonance)positive for halogen, NH2, OH, ORnegative for NO2, others
Example:chlorinated phenols:
phenol 9.922-chlorophenol 8.443-chlorophenol 8.984-chlorophenol 9.292.4-dichlorophenol 7.852,4,5-trichlorophenol 6.912,4,6-trichlorophenol 6.192,3,4,5-tetrachlorophenol 6.352,3,4,6-tetrachlorophenol 5.40pentachlorophenol 4.83
general reduction in pKa due to chlorine substitution is caused by inductive (electron withdrawing effect)
specific reduction in pKa (dependent on chlorine position) is caused by resonance effect
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Resonance effect of hydroxyl and amino groups
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Resonance effects are heavily influenced by position
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Proximity effects
highly specific interactions due to proximity of substituents to the functional group:
often difficult to quantify
intramolecular hydrogen bondingsteric effects
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examples
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Predicting acidity constant
For some specific aromatic structures, acidity constant can be estimated via:
Hammett Correlationeffects of substituents are quantified via values
pK pKa aH ii
the pKa of the unknown = the pKa of the unsubstituted parent structure minus the susceptibility factor times the sum of all the Hammett constants
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sigma
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rho
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Due to promixity (steric) effects, influence of ortho substituents is hard to quantify.
The same substituent in the ortho position may have a different effect on pKa for different acids.
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Hammett constants can be used to predict properties other than pKa
Specifically, rate constants for hydrolysis (chapter 13)
Also, redox potential?
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-150
-130
-110
-90
-70
-50
0 1 2 3 4 5S
H2
Chlorobenzenes: Hammett constants
Cl (meta) = 0.37
Cl (para) = 0.23
Exptl without trichlorobenzenes:
Cl (ortho) = 1.53
R2 = 0.985
Cl (ortho) R2
exptl 2.01 0.63bond 1.23 0.90comp 1.17 0.99
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Taft correlationsCHaa EpKpK **
3
Similar to Hammett correlation, but applicable to aliphatic systems.
Reference compound has methyl group at the position of the substituent.
Influence of substituent on pKa is divided into polar (electronic) and steric effects.
* = polar substitutent constant* = susceptibility of backbone to polar effects
Es = steric substituent constant = susceptibility of backbone to steric effects
also used to predict reactivity . . . see Chapter 13
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Partitioning Behavior of Organic Acids and BasespH dependence of solubility: speciation
HA H+ + A-
HA (pure liquid or solid)
Solubility is the equilibrium partitioning of a compound between the pure liquid phase and water.
The solubility and activity coefficient of HA (the neutral form) depends on its size, polarity, and H-bonding ability.
The intrinisic solubility of HA is not affected by acid-base reactions.
However, the apparent solubility is highly dependent on pH due to protonation. the charged species has a much higher solubility than the neutral form.
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HA
HA A
1
1 10pH pKa
CC
w totsat w HA
sat
,,
1,
,
satBwsat
totw
CC
similarly, for B:
represents the fraction of the total amount of the compound that is in the neutral form.
To determine the total solubility of an ionizable compound, first determine the solubility of the neutral form, then determine at the given pH.
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Assume that the ionized form cannot volatilize (no ionized gases allowed!)Only the neutral species is avialable for air/water exchange
D HA A KK
RTaw HH, '
D B BH KK
RTaw HH, ' 1 1
For base:
Henry's Law (air-water partitioning):
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Octanol-water partitioning:
ionized form can partition into octanol by itself or as an ion pair
observations suggest Kow (HA) 100 Kow (A-)
so, by analogy to KH:
K HA A K HAow ow,
K B BH K Bow ow, 1
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Problem 8-1Represent graphically the speciation of 4-methyl-2,5-dinitrophenol and 3,4,5-trimethylaniline, and 3,4-dihydroxybenzoic acid as a function of pH (2-12). Estimate (if necessary) the pKa values of the compounds.
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Problem 8.2
Represent graphically the approximate fraction of (a) total 2,3,4,6-tetrachlorophenol and (b) total aniline present in the water phase of a dense fog (air-water volume ratio ~105) as a function of pH (pH range 2 to 7) at 5 and 25C. Neglect adsorption to the surface of the fog droplet. Assume and awHi value of about 70 kJ/mol for TCP and 50 kJ/mol for aniline. All other data can be found in Appendix C.