I have an Emmons-Horner-like agent (triethylphosphonoacetate) that I would like to label with tritium. The alpha proton is readily exchangable in NaHCO3/D2O; I presume the same would work with NaHCO3/T2O?
More to the point, once the Deuterated or tritiated compound is made, how easily will the deuteron/tritium exchange with bulk solvent (assuming that is now in H2O)? I.e. is the pKa of this carbanion the same, for H-C, D-C, or T-C? I would presume that it must be more difficult to strip off a T-C rather than a H-C acidic proton?
Any help would be greatly appreciated!
Dear Florian,
C-H would be the weakest bond, then C-D, then C-T would be the strongest.
This is due to the zero point energy of the bond and the fact the zero point energy is proportional to:
[(mC + mH)/ (mC x mH)]1/2
Zero point energy is the minimum energy the quantum harmonic oscillator can have. Because the greatest mass isotope has the smallest zero point energy, the energy of the bond is greatest.
Kinetically, hydrogen will react at the fastest rate, followed by deuterium and then tritium (given a reaction such as a carbon atom or radical plus molecular hydrogen giving a C-H bond plus an •H atom). The most quickly formed product may not necessarily be the most stable.
Hoping this will be helpful,
Rafik
pKa definition is based on pH definition. In D2O the acidity is measured by pD, in T2O by dT. Your question is much more complex than you thought. You may read a book
Ph Measurements
by C. Clark Westcot
ISBN-13: 978-0127451503
ISBN-10: 0127451501
and the IUPAC recommendations:
https://www.iupac.org/publications/pac/2002/pdf/7411x2169.pdf
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