Chapter 8: Organic Acids and Bases

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Chapter 8Organic 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, DG
? 0, K 1 Thus, HA ? H A- lnKa
-(DG ?/RT)
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At equilibrium
Ka acid dissociation constant ? typically
measure activity of H, and conc of HA, A-
(mixed acidity constant) ? at low ionic
strength, ? ? 1
when pH pKa, A- HA
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For organic bases, treatment is similar B H2O
? BH OH-
written as acidity constant BH ? B H
pKa pKb pKw 14 at 25?C Ka Kb Kw
10-14 at 25?C
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amines
carboxylic acids
acids bases
phenols
heterocycles with N
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Important functional groups Acids
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)
CH3-OH alcohols (pKa gt 14)
O
Carboxylic acids (pKa 4.75)
anilines (pKa 4.63)
N
pyridines (pKa 5.42)
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Temperature effect on pKa
recall that the effect of temperature on any
equilibrium constant
for strong acids, DrHo is very small DrHo
increases as pKa increases (weaker acids have
higher temperature dependence) (Why?) hmmm
what is the DrS of a proton transfer reaction?
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Speciation in natural waters
Q 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
fraction of acid in the neutral form
fraction of base in the neutral form
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Chemical structure and pKa
We are mostly concerned with compounds for which
3 lt pKa lt11 aliphatic and aromatic carboxyl
groups aromatic hydroxyl groups
(phenols) aliphatic and aromatic amino groups N
heterocycles aliphatic or aromatic thiols These
classes of compounds have pKas which vary
widely Why?
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Substituents can have a dramatic effect on the
pKa of the compound
Substituent effects are of three
types Inductive effects positive (electron
donating) for O-, NH-, alkyl negative (electron
withdrawing) for NO2, halogen, ether, phenyl,
etc. Delocalization effects (resonance) positive
for halogen, NH2, OH, OR negative for NO2,
others Proximity effects intramolecular
hydrogen bonding steric effects
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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-chlorobu
tyric 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, OR negative for
NO2, others Example chlorinated
phenols phenol 9.92 2-chlorophenol 8.44 3-
chlorophenol 8.98 4-chlorophenol 9.29 2.4-dich
lorophenol 7.85 2,4,5-trichlorophenol 6.91 2,
4,6-trichlorophenol 6.19 2,3,4,5-tetrachlorophen
ol 6.35 2,3,4,6-tetrachlorophenol 5.40 pentachlo
rophenol 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
bonding steric 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
Correlation effects of substituents are
quantified via ? values
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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Chlorobenzenes Hammett constants
sCl (ortho) R2 exptl 2.01 0.63 bond 1.23
0.90 comp 1.17 0.99

• sCl (meta) 0.37
• sCl (para) 0.23
• Exptl without trichlorobenzenes
• sCl (ortho) 1.53
• R2 0.985

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Taft correlation
Similar to Hammett correlation, but applicable to
aliphatic systems. Reference compound has methyl
group at the position of the substituent. Influenc
e of substituent on pKa is divided into polar
(electronic) and steric effects. s polar
substitutent constant r susceptibility of
backbone to polar effects Es steric
substituent constant d susceptibility of
backbone to steric effects also used to predict
reactivity . . . see Chapter 13
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Partitioning Behavior of Organic Acids and
Bases pH dependence of solubility speciation
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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a 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 a at the given pH.
similarly, for B
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Henry's Law (air-water partitioning)
Assume that the ionized form cannot volatilize
(no ionized gases allowed!) Only the neutral
species is avialable for air/water exchange
For base
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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
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Problem 8-1
Represent graphically the speciation of
4-methyl-2,5-dinitrophenol and 3,4,5-trimethylanil
ine, 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 DawHi value of about 70 kJ/mol for TCP
and 50 kJ/mol for aniline. All other data can be
found in Appendix C.