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Initiation of radical chain reactions of thiol compounds and alkenes without any added initiator: thiol-catalyzed cis/trans isomerization of methyl oleate.

Chemistry (Weinheim an der Bergstrasse, Germany) (2012-05-18)
Ursula Biermann, Werner Butte, Rainer Koch, Patrice A Fokou, Oǧuz Türünç, Michael A R Meier, Jürgen O Metzger
ABSTRACT

A kinetic study of the dodecanethiol-catalyzed cis/trans isomerization of methyl oleate (cis-2) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis-2, respectively, and an overall kinetic isotope effect k(H)/k(D) of 2.8 was found. The initiation was shown to be a complex reaction. The electron-donor/-acceptor (EDA) complex of dodecanethiol (1) and cis-2 formed in a pre-equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule-assisted homolysis (MAH) of the sulfur-hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E(A)=82 kJ  M(-1). Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well-known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf-life stability of cis-unsaturated thiol compounds and of mixtures of alkenes and thiol compounds.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Methyl oleate, 99%
Supelco
Methyl oleate, analytical standard
Sigma-Aldrich
Methyl oleate, technical grade, 70%
Sigma-Aldrich
Methyl elaidate, ≥99% (capillary GC)
Supelco
Methyl elaidate, analytical standard
Supelco
cis-9-Octadecenoic acid methyl ester, certified reference material, 10 mg/mL in heptane

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