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International Journal of Mass Spectrometry 228 (2003) 297306 The role of hydride migration in the mechanism of alcohol

Summary: International Journal of Mass Spectrometry 228 (2003) 297­306
The role of hydride migration in the mechanism of alcohol
elimination from protonated ethers upon chemical ionization
Experiment and theory
Julius Ben Ari, Miriam Karni, Yitzhak Apeloig, Asher Mandelbaum
Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry,
Technion--Israel Institute of Technology, 32000 Haifa, Israel
Received 3 January 2003; accepted 17 March 2003
Dedicated to Helmut Schwarz, a great scientist and a wonderful human being, "a cemented cistern that does not
lose a drop" [Pirkey Avoth (Ethics of Fathers), chapter 2, mishnah 8], on the occasion of his 60th birthday.
An enhanced elimination of methanol under isobutane-chemical ionization (CI) conditions, resulting in highly abundant
[MH­CH3OH]+ ions, has been observed in several primary and secondary methyl ethers having a tertiary -position (methine),
as compared with those with -methylene. This elimination is stereospecific in stereoisomeric 2-methyl-1-methoxycyclohex-
anes and in other ethers affording significantly more abundant [MH­CH3OH]+ ions in the cis-isomers than in their trans-coun-
terparts. These findings suggest involvement of a 1,2-hydride migration from the - to -position in the course of the al-
cohol elimination from the MH+ ions of the above cis-ethers, resulting in stabilized tertiary carbocation structures. The
possible pathways of methanol elimination from protonated cis-2-methyl-1-methoxycyclohexane were explored by density
functional calculations at the B3LYP/6-31+G(d,p) level of theory. The transition states for MeOH elimination involving
1,2-hydride migration were located and the activation energy of the process was evaluated. The activation barrier of the alco-


Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology


Collections: Chemistry