Table 5.
PCET Thermochemistry of Hydroxylaminesa
| compound (ROH)b | solvent | E∘(RO•/−) | E∘(ROH•+/0) | pKa(ROH•+) | pKa(ROH) | E∘(V vs H2) | BDFE |
|---|---|---|---|---|---|---|---|
| H2NOH | H2O | 0.09290 | 1.3290 | [−6.7] | 13.7291 | 0.90 | 73.6 |
| TEMPOH | MeCN | [−1.85] | -- | -- | 41c | 0.6019 | 6619,33 |
| TEMPOH | THF | -- | -- | -- | -- | 0.5819 | 65.519 |
| TEMPOH | C6H6 | -- | -- | -- | -- | 0.508 | 63.833,g |
| TEMPOH | hexane | -- | -- | -- | -- | 0.508 | 63.4292,g |
| 4-oxo-TEMPOH | hexane | -- | -- | -- | -- | 0.412 | 61.2292,g |
| 4-oxo-TEMPOH | hexane | -- | -- | -- | -- | 0.542 | 64.2h |
| 4-oxo-TEMPOH | CCl4 | -- | -- | -- | -- | 0.594 | 65.6293,g |
| 4-oxo-TEMPOH | MeCN | -- | -- | -- | -- | 0.648 | 66.9294 |
| 4-MeO-TEMPOH | MeCN | -- | -- | -- | -- | 0.622 | 66.3294 |
| 4-OH-TEMPOH | hexane | -- | -- | -- | -- | 0.577 | 65.0292,g |
| 4-NH2-TEMPOH | hexane | -- | -- | -- | -- | 0.564 | 64.7292,g |
| 4-Cl-TEMPOH | hexane | -- | -- | -- | -- | 0.573 | 64.9292,g |
| tBu2NOH | MeCN | [−1.8]c | -- | -- | 39295,d | 0.551 | 64.7294 |
| NHPI | H2O | 1.2296 | -- | -- | 6.3297 | 1.6 | 89 |
| NHPI | MeCN | 0.30e | -- | -- | 22.9297,d | 1.69 | 90.9 |
| 4-AcO-NHPI | MeCN | -- | -- | -- | -- | 1.359 | 83.3f |
| 3-F-NHPI | MeCN | -- | -- | -- | -- | 1.346 | 83.0f |
| 4-Me-NHPI | MeCN | -- | -- | -- | -- | 1.328 | 82.6f |
| 4-MeO-NHPI | MeCN | -- | -- | -- | -- | 1.289 | 81.7f |
| 3-MeO-NHPI | MeCN | -- | -- | -- | -- | 1.315 | 82.3f |
| 3,6-(MeO)2-NHPI | MeCN | -- | -- | -- | -- | 1.281 | 81.5f |
Potentials for 1e− reductions are in V vs SHE if in aqueous solution and V vs Cp2Fe+/0 if they are in organic solution. Values for E∘(V vs H2) are in V; −eE∘ is the average free energy for addition. Uncited values of E∘(V vs H2) were calculated from other values in the row using Scheme 2 or eq 18. BDFEs are in kcal mol−1 and if uncited were calculated from E∘(V vs H2) using eq 18. Values in [square brackets] have been calculated from the other values in the row using Hess’ law.
TEMPOH = 1-hydroxy-2,2,6,6-tetramethyl-piperidine, NHPI = N-hydroxyphthalimide.
Estimated in ref 33.
Extrapolated from DMSO to MeCN using the method of Kütt and co-workers for OH acids.298
Determined at −10 °C in MeCN with respect to NHPI in ref 301. Modified relative to our value for BDFEMeCN(NHPI).
BDFE values calculated from the cited BDE.
This value was measured by our laboratory through equilibration with TEMPO• (see SI). We recommend this value (vs the prior report) because the equilibration method is a more sensitive measure of the relative BDFE (and E∘(V vs H2)). Furthermore, we discuss in Section 3.1.1 the evidence that this BDFE should change very little with solvent identity.