Carbon-Halogen Bond Scission and Rearrangement of .beta.-Halohydrins on the Rh(111) Surface
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Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) studies were performed in order to investigate the decomposition behavior of β-halohydrins (XCH2CH2OH, X = F, Cl, Br, I) on Rh(111). The goal of these experiments was to synthesize and to characterize oxametallacycles (-CH2CH2O-) on the Rh(111) surface. The halohydrins did not follow the expected pathway, but a new reaction analogous to the pinacol rearrangement to acetaldehyde was discovered. 2-Iodoethanol and 2-bromoethanol decomposed via this path to release methane at 267 and 252 K, respectively, with about 25% selectivity. 2-Chloroethanol decomposed via a pathway in which methane was liberated at 260 K with a selectivity of 18%, while 2-fluoroethanol decomposition did not produce methane. H2 and CO were also observed as desorbing products during the TPD experiments, while carbon was also deposited on the Rh(111) surface. Halogen atoms desorbed at high temperature in these experiments. The decomposition of ethylene glycol on Rh(111) was also studied, since it has the same molecular structure as the β-halohydrins, but with an OH group replacing the halogen. Only CO and H2 were detected as desorbing products during ethylene glycol decomposition. No volatile methane was detected, nor was any carbon deposited on the metal surface. All of the carbon and oxygen atoms in the ethylene glycol desorbed in the form of molecular carbon monoxide; C-O bond scission did not occur. The reaction pathways and products observed during decomposition of the β-halohydrins and ethylene glycol demonstrate the importance of the β-CX bond strength (where X = I, Br, Cl, F, OH) in determining the reaction pathway and consequently the reaction products. © 1994 American Chemical Society.
author list (cited authors)
Brown, N. F., & Barteau, M. A.