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Quantification and Analysis of
Acid- and Base-Catalysis in
Organic Reactions
Some Preliminaries
Definition of pH
A concise summary of thermodynamics
(W. J. Moore in Physical Chemistry)
1. The First Law say you can’t win;
the best you can do is break even.
2. The Second Law says you can break
even only at absolute zero.
3. The Third Law says you can never reach
absolute zero.
Never mind!
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Cross-check
1) H. Sigel, A. D. Zuberbühler, O. Yamauchi, Anal. Chim. Acta, 1991, 255, 63.
Measure pH with a glass electrode.
The glass electode must be calibrated!
(See provider instructions, e.g., Metrohm)
Remarkably, glass electrode readings (pH)meas are closer to
pcH+ that to paH+! 1)
Your results should hold within:
(pH)meas – (pcH+)calc = 0.03 ± 0.02
The practical approach
Choose a well-known (preferably monoacid) buffer with
known pKa near (±1 unit) to that pKa which you want to determine.
Titrate with 0.1 M strong base starting with 0.1 M HA plus dye or
with 0.1 M strong acid starting with buffer salt plus dye.
Record spectra and get pH readings between small additions of
titrant.
Global analysis of the spectral series (number of components)
and fit of an appropriate titration function.
pKa,c = 4.18 ± 0.02
Rate constants of keto–enol conversions
J. Wirz, Adv. Phys. Org. Chem., Vol. 44, 2010, 325
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Can we measure kE and kK
separately?
kobs
K
log(kobs)
Buffer catalysis
Extrapolation to zero buffer concentration is required to determine rate
constants of wholly aqueous solutions for pH–rate profiles.
The individual contributions of the general acid and general base components
of the buffer are obtained for a plot of kbu vs. the buffer ratio xHA = cHA/(cHA+cA–).
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Helv. Chim. Acta 2001, 84, 1441
pH-rate profile: o-
nitrotoluene
aci-decay
o-Nitrobenzyl methyl ether: pH-profile
hemi decay
cyclo decay
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Intercepts of buffer dilution plots
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Bronsted equation Bronsted α variation with ∆Go
Marcus theory
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pKa
E = 11.4 ± 0.1
kH+ = (5.4 ± 1.0) × 104 M–1 s–1
k0 = 400 ± 60 s–1
kH+ = (9.5 ± 1.0) × 105 M–1 s–1
But: Why does base catalysis saturate?
Change in rate-det. step!
Conclusions
• When will YOU do flash photolysis?
• beware of artefacts
• buffer catalysis
• experts in reading pH–rate profiles
• equilibrium constants KE spanning
30 orders magnitude
• assignments of elementary reactions
• LFER