Chapter 8. Organic acids and Bases: acidity constants and partitioning behavior
Many organic environmental chemicals are involved in proton transfer reactions that result in charged species
Is the partitioning behavior of charged species different from neutral species??
A proton transfer only occurs if an acid (HA) called here a proton donor in the classical sense (Bronstead acid, 1924) reacts with a base (proton acceptor)
HA A- + H+
H++ B BH+
HA++ B BH+ + A-
A- is called the conjugate base of HA and BH+ is the conjugate acid of B
The reactions is usually very fast and reversible and hence can be treated as an equilibrium process.
For organic acids (HA)
HA + H2O H3O+ + A-
For the solvent water
H+ + H2O H3O+
the equilibrium constant K is
K= [H3O+ ]/{[H+] [H2O]}
Since water is the reference state
K is set to one and DGO = O
Because -RT ln K = DGO
So
HA + H2O H3O+ + A-
becomes
HA H+ + A-
The equilibrium for
HA H+ + A- is
The prime on the “g’s” indicates infinite dilution
Kia is commonly referred to as the acidity constant
Typically when analytical measurements are made the hydrogen activity is measured
pH = -log {g’H+ [H+]} while [HA] and [A-], are usually in concentrations are in molar units,
this gives a mixed acidity constant K*ia that is usually defined for a given aqueous medium
(ie. 0.05 - 0.01 M salt solution)
since
for low concentrations of [HA] and [A] in an aqueous media K*ia = Kia
in treating low concentrations the convention is
pX = -log [X]
pH = -log {g’H+ [H+]}
log {[A-]/[HA]} = log Kia - log {g’H+ [H+]}
log {[A-]/[HA]} = pH – pK*ia (Henderson-Hasselbach equation)
at what point does pH = pKia?
when [A-] = [HA]
Take the case where the pKia of an organic acid is low , say the pKia = - 0.3
pKia = -log [Kia]
If pKa is negative, log Ka is positive and this makes H+ high in the definition of Ka
Examples of strong acids are trichloroacetic acid and 2,4,6 trinitrophenol.
At ambient pH values (4-10) low pKa compounds will be present in natural waters in their ionized (dissociated form) as their anions.
Conversely, weaker acids are ones with relatively high pKa values, and very weak acids (pKa = 9 - 12) will exist in natural waters in their undissociated form.
Many important organic acids have pKia(s) between 4 and 10;
Given this treatment it is useful to describe the amount present in the acid (unionized or dissociated) form and this will be denoted by aa.
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log {[A-]/[HA]} = pH - pKa (Henderson-Hasselbach equation)
for example, 2-nitrophenol has a pKia of 7.17 at a pH of 7. What is the fraction in the undissociated acid form?
To Review, what we have said is that from
log {[A-]/[HA]} = pH - pKa
and pH and pKa of an acid we can estimate
the [A-]/[HA] ratio, and the fraction in the acid form, from aa (Table 8.1 page 180, old book; see p 250 new book)
Example: Calculate the fraction of pentachlorophenol (PCP) in the neutral form in a rain drop at pH 4 and in lake water at pH 8.
From Table 8.1 (new and old book), PCP has a pKa value of 4.75
arain= 1/(1+10-0.75) = 0.849
alake= 1/(1+103.25) = 0.00056
in the rain drop, only 15% of the PCP is present as the phenolate anion, whereas in the lake water, virtually all of the PCP is ionized.
Organic Bases
By analogy with acids, the basic content (basicity) of an organic base in water can be described as:
B + H2O OH- + BH+ (BH+ is the conjugate acid of B)
The reaction of a neutral base [B ] with water forms the cation, BH+
Instead of working with a Kib dissociation equilibrium constant, by convention, the acidity constant is defined in terms of the conjugate acid of B, or BH+.
BH+ H+ + B
If we multiply Kia and Kib
(BH+ is the conjugate acid of B)
Kia Kib = Kw = g’H+ [H+] x g’OH- [OH-] = 1.01 x10-14
So for a given compound in water
pKia = pKw - pKib
Low pKia means a strong acid and the pKia of its conjugate base is high, which means the conjugate base it not in its ionized from or is weak.
The stronger the base (low pKib means the negative log of a high Kb, which means more ionized base) the weaker its conjugated acid (high pKa of the conjugate acid)
Hence a neutral base like methylamine with a conjugate acid pKia of 10.66 will thus have a pKib of:
CH3–NH2 + H20à CH3-NH3++ OH-
pKa = pKw – pKb; 10.66 = 14 - pKb
and will exist in natural waters in its dissociated (ionized) or cation state
We will be interested in compounds which have pKia values in the 3-11 range
The above structures suggest compounds with phenolic, carboxylic, amino , nitrogen in the rings groups, and aromatic thiol groups should be considered.
For example depending on the structure pKa can differ by an order of magnitude. So understanding how the structure influences pKia is important.
It is also possible to have diportic acidic compounds and di-basic compounds.
From measurements it is possible to see how different species of a parent molecule vary with pH, depending on their pKa
Speciation in Natural Waters
Given the pKia of a compound, to what extent does it distribute in natural waters.
The pH in natural water is determined largely by inorganic acids and bases (H2CO3 Û HCO3- Û CO3-2). These species act as H+ buffers; ie. a small amt. of acid or base will not effect the overall distribution.
To illustrate, let’s assume that for a given acid-base pair with a pKia of 7, is at equal concentrations in water at 10-3moles/L
log {[A-]/[HA]} = pH – pKia (Henderson-Hasselbach equation)
so, pH = pKa + log {10-3/10-3} = 7
If we add 10-5 moles of additional strong acid the total acid will be: HA + 10-5 new acid = 1.01x10-3;
A-, however reacts with the additional acid, to give HA and its concentration goes from
1x10-3 to 0.99x10-3
hence: pH = pKia + log {0.99x10-3/1.01x10-3} = 6.991;
so, the addition of small amts. of a strong acid will not overly affect the pH of the natural water.
Chemical Structure and Acidity Constants
Since Kia is a partitioning constant we can directly write:
ln Kia = -DrGo/RT
the question is what is DrGo ???
Is it not the chemical potential between the products and initial reactants ??
for
HA àß H+ + A-
-DrGo = moHA- moA-B-
pKa = (moHA- moA-B-)/{RT 2.303}
so we must ask the question, what will change the standard state chemical potential, so that it alters its tendency to react in the case of the acid HA to is conjugate base A-.
The engineers… would say its chemical structure???
If there are substituents that makes it easier for HA to loose a H+, this would influence moHA
Look at the following structures and their pKia values,
where HA à A- + H+
CH3CH2 CH2-COOH CH2CH2 CH2-COOH
Cl
pKia = 4.81 pKia = 4.52
CH3 CH CH2-COOH CH3CH2 CH-COOH
Cl Cl
PKia = 4.05 pKia = 2.86
chlorine is very electronegative and has a high electron with drawing potential. The effect is called a negative inductive effect. The resulting A- anion from the above acids are stabilized by Cl withdrawing the negative charge into the structure. Notice as the chlorine gets closer to the carboxylic group, the effect increases.
Delocalization electron effects: Compounds that have alternate double bonds (p electrons) can have inductive effects transmitted over longer distances than just single bonded carbon molecules
Conjugated structure Resonance structure
H3C- C=C-C=CH2 H3C-C-C-C-CH2
H H H H H H
We would also expect resonance to be important with aromatic ring structures (Figure 8.3 p 168, new book, p 258)
Effects of position of a nitro substituent (Figure 8.4 p 169, p 259 new book)
Proximity effects; (top) hydrogen bonding and (bottom) steric interactions(Figure 8.5 p 170)
The Hammett Correlation
In 1940 Hammett recognized for substituted benzoic acids the effects of substituent groups on the dissociation of the acid group
DrGo= DrGoH + S DrGoi
since DrGo = -RTln Ka
-RTln Ka = -RTln KaH - RTln Ka,i
Effect on the free energy change from dissociation could be represented as the sum of the free energy change by the unsubstituted benzoic acid and the contributions from the various R groups.
Hammett then says that the effects of the substituent group i, on the free energy can be represented as: DGoi = -2.303 RTsi; where si =ln Ka,i
DGoi = -2.303 RTsi; where si = ln Ka,i
going back to:
-RTln Ka = -RTln Ka,H - RTln Ka,i
log Ka= log KaH +Ssi
so, log (Ka / KaH )= SsI
and pKa = pKaH - SsI
Table 8.4 Hammet Constants
Hammet also observes that when considering other compound classes, like phenyl acetic acids the s values developed for benzoic acid can be used
log (Ka / KaH) = r Ssi
Exceptions: As a function of direct resonance, para phenol substituents decrease the pKa more than would be predicted by just the para s values. Apparent s values for phenols have been determined as per Table 8.4
The air-water equilibrium
The distribution coefficient between the air and water of an organic acid can be described as:
Daw (HA,A-) = [HA]air/{[HA]water+ [A-]water}
Multiplying by [HA]water/[HA]water
Daw (HA,A-) = aa Kiaw
Go through example on page 269 of new book
Conversion of Nicotine in tobacco smoke to its volatile and Available Free-Base Form through the Action of Gaseous Ammonia (Jim Pankow, et al, ES&T, 32, 2428-2433, 1997)
pka1 =3.12
pKa2 = 8.02
For pKa1
Nicotine-H+ à nicotine + H+
At a pH of 5.5 for normal tobacco smoke
Pankow shows that the fraction afb in the free base form =
at a pH of 5.5, afb= 0.002
at a pH of 8, afb= 0.5
So increasing the pH increases the amount of free base nicotine
The amount of nicotine that is partitioning if it were all in the free base form would be:
Kp = nicotinepart/(nicotinegasxTSP)
To account for acid-base equilibrium Kp is multiplied by afb
Pankow writes: Nicotine in the particle phase that is not in the free base form is not available for gas-particle partitioning. By raising the pH with ammonium diphosphate as a “flavor enhancer” more nicotine can come out of the particle phase and be available for gas phase transfer to the lungs.
He also suggests that nicotine on high pH particles deposited in the lungs will also diffuse off the particles directly to the alveolar lung lining.
He then goes on to say: “the role of ammonia in tobacco smoke is analogous to what happens when alkaloid cocaine is “free based” or used in the “crack” form. Free-based cocaine and crack are typically prepared by reacting “street” cocaine (usually the HCL salt of cocaine) with aqueous ammonia and ammonium bicarbonate” the basified cocaine is more lipid soluble and gives a “high” when smoked that is comparable to intravenous injection.
Influence of organic acid-base equilibrium on partitioning.
The water solubility of an ionized organic acid or salt, is generally several orders of magnitude higher then the neutral species, HA, (Csatw,HA).
The sum of the ionized and HA species is Ctot, and it has a saturated concentration of (Csatw,tot).
Csatw,totx aia = Csatw,HA; so Csatw,tot = Csatw,HA/aia
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For octanol water partitioning things are not as straight forward because the organic and the ionized organic can partition into both the water and the octanol phase.
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