The complex of manganese(Ill) with trans-1,2-diaminocyclohexane-tetraacetic acid was prepared and its formula shown to be KMnIII CyDTA(H₂O), where CyDTA is an abbreviation for trans-1,2-diaminocyclohexanetetraacetic acid anion. A mechanistic study of the oxidation of 2,4,6-tri-t-butylphenol with this complex was carried out in methanol under three conditions: (1) in the presence of acid; (2) in the presence of base; (3) in the absence of acid and base.
Under acidic conditions, the oxidant is HMnIIICyDTA(H₂O) and the
rate-determining step of the reaction involves a hydrogen atom abstraction
from the O-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic
isotope effect of 4.8 was obtained at 25°. The activation parameters are
ΔH* =15.7 Kcal.mole-¹ and ΔS* = -10.4 e.u. The small effect observed for
substitution in the para position is consistent with the radical character
of the reaction. A ℓ value of 0.39 was obtained from a Hammett plot of
log k₃/k₃H versus σp.
Under basic conditions, the oxidant is MnIIICyDTA(OH)²- and there are two competitive reactions: one that involves electron-transfer from
2,4,6-trl-t-butylphenoxy anion to MnIIICyDTA(OH)²- and another that
involves hydrogen atom abstraction from 2,4,6-tri-t-butylphenol by
MnIIICyDTA(OH)²-. The activation parameters are ΔH* = 10.2 Kcal.mole-¹,
ΔH* = -23 e.u. for the former reaction; ΔH* =10.2 Kcal.mole-¹ ,
ΔH* = -29 e.u. for the latter reaction. A kinetic isotope effect of 1.9 was obtained for the latter reaction at 25°. The rate of electron-transfer
from 2,4,6-tri-t-butylphenoxy anion to MnIIICyDTA(OH)²- is 104 times faster than the rate of hydrogen atom abstraction from 2,4,6-tri-t-butyl-
phenol by MnIII CyDTA(OH)²-. The effect on reaction rate for substitution in the para position is large, a ℓ value of 2.3 being obtained from a Hammett plot of log k₉/k₉H versus σ-p (k₉ is the rate constant for the reaction between 2,4,6-tri-t-butylphenol and MnIIICyDTAC(OH)²-).
Under neutral conditions, the oxidant is MnIIICyDTA(H₂0)- and the rate-determining step of the reaction involves hydrogen atom abstraction from the O-H group of 2,4,6-tri-t-butylphenol by the oxidant. A kinetic isotope effect of 5.9 was obtained at 25°. The activation parameters are ΔH* =10.9 Kcal.mole-¹ and ΔH* = -39 e.u. The small effect observed for substitution in the para position is consistent with the radical character of the reaction, a ℓ value of -0.07 being obtained from a Hammett plot of log k₃a/k₃aH versus σp. The compound 3,5-di-t-butyl-4-hydroxybenzaldehyde, however, has an anomalously high rate. This anomaly is also observed under acidic conditions.
The oxidising power of MnIIICyDTA(OH)²- is 360 times greater than that of MnIIICyDTA(H₂0)-with respect to hydrogen atom abstraction from 2,4,6-tri-t-butylphenol. This difference in oxidising strength suggests that an 'inner-sphere'-type mechanism.is involved in the reactions. The difference in oxidising strength between MnIIICyDTA(OH)²- and
MnIIICyDTA(H₂0) is even greater in 95% dimethyl sulfoxide solvent -5.4 x 107.
Oxidation of 2,4,6-tri-t-butylphenol with potassium ferrlcyanide in methanol in the presence of hydrochloric acid is shown to be a reversible reaction. The lack of data on the ionization constants of H₄Fe(CN)₆ and H₃Fe(CN)₆ prevented a detailed study of the equilibria involved. / Science, Faculty of / Chemistry, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/44728 |
| Date | January 1972 |
| Creators | Poh, Bo Long |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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