Formation of the N-Iodopyridinium Cation in an Alkane Environment. Mid-Infrared Linear Dichroism Investigation of the Pyridine-Iodine System in Stretched Polyethylene.

Jens Spanget-Larsen 

Dept. Life Sciences and Chemistry, Roskilde University (RUC), Universitetsvej 1, Roskilde DK-4000, Denmark

Abstract

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It is well known that pyridine (py) and iodine (I2) form a 1:1 molecular complex, py-I2. Within the framework of a student project at Roskilde University [1], we have recently investigated the formation and mid-infrared linear dichroism of the complex in a stretched polyethylene matrix. However, the observed spectra displayed not only the expected transitions due to py and py-I2, but also a third, prominent series of sharp transitions, evidently due to at least one additional product. On the basis of a comparison with literature spectra, these transitions can be assigned to the N-iodo­pyridinium cation, pyI+. This assignment is consistent with the observed polarization data.

Ionic side reactions such as py-I2 → pyI+ + I-, etc., are easily observed in binary pyridine-iodine mixtures and in polar solvents, but not so easily in non-polar media. We are thus surprised to see the apparent efficiency with which the pyI+ cation is formed in a non-polar medium like polyethylene. With excess iodine, the peaks assigned to pyI+ dominate the observed mid-infrared spectrum (see below). We suspect that a driving force for the ionic reaction is the formation of polyiodide anions in the channels of the polymer medium. Further investigation of the phenomenon is in progress.

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[1] H. S. van Deurs, M. L. M. Espersen, R. Nissen, M. H. Rasmussen, C. J. S. Schönbeck, Student Project Report, Roskilde University, June 2006 (in Danish).

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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 2, by Jens Spanget-Larsen
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-24 11:32
Revised:   2009-06-07 00:44
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