Hydrogen Migrations in Alkylcycloalkyl Radicals: Implications for Chain‐Branching Reactions in Fuels

Abstract

A thorough understanding of the oxidation chemistry of cycloalkanes is integral to the development of alternative fuels and improving current fuel performance. An important class of reactions essential to this chemistry is the hydrogen migration; however, they have largely been omitted from the literature for cycloalkanes. The present work investigates all of the hydrogen migration reactions available to methylcyclopentane, ethylcyclopentane, methylcyclohexane, and ethylcyclohexane. The kinetic and thermodynamic parameters have been studied by a combination of computational methods and compared to their corresponding n-alkyl and methylalkyl counterparts to determine the effect that the cycloalkane ring has on these reactions. In particular, although the alkylcycloalkyl activation energies for the dominant 1,4, 1,5, and 1,6 H-migration are higher than in n-alkyl and methylalkyl radicals, because several of the rotors needed to form the transition state are locked into place as part of the cycloalkane ring, the A-factors are higher for the alkylcycloalkyl reactions, making the rates closer to the noncyclic systems, at higher temperatures. The results presented here suggest that the relative importance of each H-migration pathway differs from the trends predicted by either the n-alkyl or methylalkyl radical systems. Of particular interest is the observation that since the barrier height of the 1,4 H-migration is only 3-5 kcal mol(-1) higher than the 1,5 H-migration in the methyl and ethylcycloalkyl radicals, compared to a difference of roughly 7 kcal mol(-1) in similar reactions for both the n-alkyl and methylalkyl radicals, the 1,4 H-migrations in alkylcycloalkyl radicals will be more important in the overall mechanism than would be predicted based on the n-alkyl and methylalkyl radicals. These results have important combustion model implications, particularly for fuels with high cycloalkane content.

Kinetics of the hydrogen abstraction ·C2H5 + alkane → C2H6 + alkyl reaction class: an application of the reaction class transition state theory

“…It is well known that tunneling is of great importance for the light particles transfer [28,[39][40][41][42][43][44][45][46][47][48]. As may be seen from Fig.…”

New Members of the National Academy of Sciences Royal Society Wolfson Research Merit Awards

“…He has reported in Angewandte Chemie on the reactivity of small radicals such as HO 2 . and O 2 .− at the air–water interface,2a and in Chemistry—A European Journal on hydrogen migrations in alkylcycloalkyl radicals 2b…”

Neue Mitglieder der National Academy of Sciences Wolfson‐Forschungspreise der Royal Society

“…In der Angewandten Chemie beschrieb er die Reaktivität einfacher Radikale wie HO 2 . und O 2 .− an der Luft‐Wasser‐Grenzfläche2a und in Chemistry—A European Journal Wasserstoffwanderungen in Alkylcycloalkylradikalen 2b…”