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Proposed mechanism of initiation of catechol polymerization at high pH.
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Figure 9: Proposed mechanism of initiation of catechol polymerization at high pH.

Mentions: Raman analysis (Figure 8) of catechol at different pH is also consistent with the formation of polycatechol. The spectra for lower pH (2,4,6) solution show in-plane and out-of plane CH deformation bands at 1040 and 774 cm-1. These peaks are absent at higher pH (8, 10, 12). A new peak associated with the phenoxy radical appears at 1494 cm-1. New peak at 1382 cm-1 associated with in plane C-H rocking vibration was also observed indicating retention of some C-H bonds in polycatechol. In addition, a new peak was observed at 1584 cm-1 indicating formation of tropones. From this evidence it appears polymerization is taking place by formation of C-C bonds between benzene rings following deprotonation of catechol. Pka for catechol is 9.5; thus at pH 8, approximately 3% of the catechol will be deprotonated, likely enough to start polymerization (figure 9).

The catalytic role of uranyl in formation of polycatechol complexes

Jha PK, Halada GP - Chem Cent J (2011)

Bottom Line: The effect of time and dissolved oxygen were also studied.The benzene ring was found to be intact after polymerization.No evidence for formation of ether bonds was found, suggesting polymerization was due to formation of C-C bonds between catechol ligands.

Affiliation: Department of Materials Science and Engineering and the Center for Environmental Molecular Science Stony Brook University, Stony Brook, New York 11794-2275, USA. pjha@ic.sunysb.edu.

ABSTRACT
To better understand the association of contaminant uranium with natural organic matter (NOM) and the fate of uranium in ground water, spectroscopic studies of uranium complexation with catechol were conducted. Catechol provides a model for ubiquitous functional groups present in NOM. Liquid samples were analyzed using Raman, FTIR, and UV-Vis spectroscopy. Catechol was found to polymerize in presence of uranyl ions. Polymerization in presence of uranyl was compared to reactions in the presence of molybdate, another oxyion, and self polymerization of catechol at high pH. The effect of time and dissolved oxygen were also studied. It was found that oxygen was required for self-polymerization at elevated pH. The potential formation of phenoxy radicals as well as quinones was monitored. The benzene ring was found to be intact after polymerization. No evidence for formation of ether bonds was found, suggesting polymerization was due to formation of C-C bonds between catechol ligands. Uranyl was found to form outer sphere complexes with catechol at initial stages but over time (six months) polycatechol complexes were formed and precipitated from solution (forming humic-like material) while uranyl ions remained in solution. Our studies show that uranyl acts as a catalyst in catechol-polymerization.

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