What does pKa mean? The pKa of a neutral molecule or ion is the negative logarithm of the dissociation constant of a particular hydrogen atom under defined solvent conditions. Thus for a molecule A with a particular attached hydrogen H that we designate as A-H then Ka=[H+] [A- ] / [H-A] this is the hydrogen ion concentration in the solution multiplied by the concentration of the anion of A all divided by the concentration of the undeprotonated H-A. pKa is –log Ka = -log [H+] –log [A- ] + log [H-A].
pKa = pH –log [A- ] + log [H-A]
pKa is best understood as the pH at which equal amounts of H-A and A- exist in the solution. Put another way the pK is the pH at which H-A is one-half deprotonated in the reference solution.
The concept is not just applied to neutral substances or substances that are commonly recognized as Bronsted acids. Basic molecules are also characterized by pKa values but these are the pKas of the corresponding fully protonated base.
then Ka=[H+] [B] / [H-B+; ] pKa is –log Ka = -log [H+] –log [B] + log [H-B+]
Thus pKa= pH –log [B] + log [H-B+]
The solvent systems commonly used are aqueous sulphuric acid for measuring the pKa of strong acids. The pKas of strong acids are usually negative numbers. The more negative the number, the stronger the acid. The strongest commonly known acid is hydrogen iodide.
Water is the solvent used for measuring the pKa of moderate acids and DMSO is common for measuring the pKas of weak acids such as the important class of carbon acids (hydrogens bonded directly to a carbon).
As an example of the proper interpretation of the pKa acidity of a particular proton, we can note the two pKas important for the common solvent methanol, The pKa of Me-OH2 is -2.5 and the pKa of the hydrogen bonded to oxygen in neutral methanol, Me-OH, is 15.5. What the first number says is that at a pH of -2.5 (something like molar sulphuric acid), methanol in the solution is one-half protonated. The second number tells me that even in the strongest aqueous base (pH 14) methanol is not yet half deprotonated.
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Traditional Bronsted Acids
|
pKa
|
|
HI
|
-10
|
|
HBr
|
-9
|
|
HCl
|
-8
|
|
CF3SO3H
|
-5.1 (-5.9)
|
|
HClO4
|
-5.0
|
|
FSO3H
|
-4.8 (-6.4)
|
|
PhSO3H
|
-2.8
|
|
H2SO4
|
-2.8
|
|
HNO3
|
-1.3
|
|
CF3CO2H
|
-0.6
|
|
CCl3CO2H
|
-0.5
|
|
H3PO4
|
2.1
|
|
HF
|
3.2
|
|
H2CO3
|
3.7
|
In the same vein, one can look at the pKas of common protic solvents.
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Acids with Deprotonated Form as Common Bases
|
Approx pKa
|
|
Isopropanol (isopropoxide)
|
17.1
|
|
t-butanol (t-butoxide)
|
18.0
|
|
cyclopentadiene (cyclopentandienyl anion)
|
18.1
|
|
acetylene (acetylide)
|
25.0
|
|
triphenylmethane (triphenylmethide)
|
30.6
|
|
diisopropylamine (diisopropylamide)
|
39.0
|
|
ammonia (amide)
|
41.0
|
|
benzene (phenyl lithium)
|
43.0
|
|
ethane (like butyl lithium)
|
50.0
|
|
methane (methyl lithium)
|
58 ± 5
|
|
|
|
The table below shows the pKas of different common carboxylic acids.
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Acidity of Carboxylic Acids
|
pKa
|
Reference Solvent
|
|
CF3CO2H
|
-0.6
|
water
|
|
CCl3CO2H
|
-0.5
|
water
|
|
HO2CCO2H
|
1.25
|
water
|
|
Cl2HCO2H
|
1.35
|
water
|
|
FCH2CO2H
|
2.60
|
water
|
|
ClCH2CO2H
|
2.86
|
water
|
|
O2N-Ph-CO2H
|
3.44
|
water
|
|
HCO2H
|
3.75
|
water
|
|
HO-CO-OH
|
3.70
|
water
|
|
PhCO2H
|
4.20
|
water
|
|
PhCO2H
|
11.0
|
DMSO
|
|
CH3CO2H
|
4.76
|
water
|
|
CH3CO2H
|
12.3
|
DMSO
|
|
H2N-Ph-CO2H
|
4.92
|
water
|
|
H2N-CO2H
|
9.8
|
water
|
|
|
|
|
The pkas of the conjugate acids of solvents are a guide to how active a Bronsted acid will be in different solvents, The more negative the pKa the more reactive the proton will be.
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Conjugate Acids of Possible Solvents
|
pKa
|
|
CH3SCH3
|
-6.99
|
|
CH3O-Ph
|
-6.5
|
|
Ph-COOEt
|
-6.2
|
|
CH3OCH3
|
-3.8
|
|
(CH3)2CO
|
-2.85
|
|
CH3OH
|
-2.5
|
|
H2O
|
-1.7
|
|
CH3SOCH3
|
-1.5
|
|
NH2(CS)NH2
|
-1.26
|
|
H ( CO )NH2
|
-0.48
|
|
CH3(CO)NH2
|
0.1
|
|
NH2(CO)NH2
|
0.5
|
|
NH2CH2CO2H
|
2.35
|
Probably the most valuable table is the one showing carbon acids alongside other reference acids. This helps when deciding how strong a base is required for a particular deprotonation.
|
Organic acids
|
pKa
|
|
PhSO2H
|
1.2
|
|
PhCH=NHOH (a protonated oxime)
|
2.0
|
|
MeSO2H
|
2.3
|
|
HNO2
|
3.4
|
|
MeCOSH
|
3.4
|
|
H2CO3
|
3.7
|
|
P{hCH2NH2OH (a protonated hydroxylamine)
|
4.9
|
|
PhNHMe2 (protonated dimethylaniline)
|
5.1
|
|
PyrH (protonated pyridine)
|
5.2
|
|
MeNH2OH (protonated N-methyl hydroxylamine)
|
6.0
|
|
Thiophenol
|
6.5
|
|
H2S
|
7.0
|
|
phthalimide
|
8.3
|
|
PhB(OH)2
|
9.0
|
|
acetylacetone
|
9.0
|
|
ammonium ion
|
9.2
|
|
succinimide
|
9.6
|
|
NH4CO2 (carbamic acid zwitterions)
|
9.8
|
|
Phenol
|
10.0
|
|
Nitromethane
|
10.0
|
|
bicarbonate
|
10.2
|
|
Thiophenol (in DMSO)
|
10.3
|
|
Ethanethiol
|
10.6
|
|
Cyclohexyl ammonium
|
10.7
|
|
triethyl ammonium
|
10.8
|
|
2-carboxyethyl cyclohexanone (beta keto ester)
|
10.9
|
|
acetone enol
|
11.0
|
|
Diethylammonium
|
11.0
|
|
Dicyanomethane
|
11.4
|
|
Hydrogen peroxide
|
11.6
|
|
2-indanone
|
12.2
|
|
PhCH2NO2
|
12.3
|
|
CF3CH2OH
|
12.4
|
|
Methyl cycanoacetate
|
12.8
|
|
guanadinium ion
|
13.4
|
|
ethylacetoacetate (DMSO)
|
14.2
|
|
imidazole
|
14.5
|
|
methanol
|
15.5
|
|
water
|
15.7
|
|
ethanol
|
15.9
|
|
Phenylacetone
|
15.9
|
|
acetaldehyde
|
16.5
|
|
2-nitropropane
|
16.9
|
|
isopropanol
|
17.1
|
|
t-butanol
|
18.0
|
|
cyclopendadiene
|
18.1
|
|
thioacetamide
|
18.5
|
|
acetone
|
19.2
|
|
nenzylcyanide
|
21.9
|
|
diphenylamine
|
23.5
|
|
chloroform
|
24
|
|
phenyl methylketone
|
24.7
|
|
acetylene
|
25
|
|
acetamide
|
25.5
|
|
urea
|
26.9
|
|
3-pentanone (DMSO)
|
27.1
|
|
Phenyl methyl sulfone (DMSO)
|
29.0
|
|
ethyl acetate
|
30.5
|
|
triphenylmethane
|
30.6
|
|
2-phenyl-1,3-dithiane (DMSO)
|
30.7
|
|
Dimethylsulfone (DMSO)
|
31.1
|
|
acetonitrile (DMSO)
|
31.2
|
|
diphenylmethane (DMSO)
|
32.3
|
|
N,N-diethylacetamide (DMSO)
|
34.5
|
|
diisopropylamine (THF)
|
35.7-39
|
|
ammonia
|
41
|
|
toluene (DMSO)
|
43
|
|
benzene (CHA)
|
43
|
|
ethylene
|
44
|
|
propylene
|
47.1-48.0
|
|
ethane
|
about 50
|
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