Temperature Dependence of the Isobutane Chemical Ionization Mass Spectra of Open‐chain and Cyclic Alcohols; Structural, Stereochemical and Molecular‐Size Effects. A reevaluation of the isobutane chemical ionization of alcohols

Abstract

SummaryThe temperature dependence of the isobutane chemical ionization (CI.) mass spectra of 54 open-chain, cyclic and unsaturated C5-to Clo-alcohols was studied at temperatures ranging from 60 to 250", and enthalpy changes were calculated for the corresponding main reactions of typical alcohols.The CI. reactivity is controlled by the temperature and the substrate structure as usual, and in addition, by the molecular size. The combination of thermal, structural and substrate-size effects leads to the following main conclusions.At low-reactivity conditions, i.e. at 150" or less, the alcohols with less than 11 C-atoms give four distinct types of spectra, with ( M -OH)+ usually as the base peak. The characteristic ions are MC4H$ and (M-H)' for primary, M H + and (MC4H9-H20)' for secondary, (MC4H9-H20)+ for tertiary and allyl-type alcohols. Configurational assignments of stereoisomeric alcohols are also possible, by means of steric compression and shielding effects. The MH+/(M-OH)+ ratio in the spectra of epimeric methylcyclohexanols is at least 3 to 4 times higher for the isomers with mainly axial OH-group conformation compared to the equatorial isomers. Stereospecific ( M -H)+ ions are apparently formed from trans-2-methylcyclopentanol and endo-norbornan-2-01 by a favorable abstraction of the unshielded H ((1)-atoms versus normal behavior of the other epimers.While the spectra recorded at 200" show almost exclusively ( M -OH)+ ions, those at 250" give nevertheless some C-skeleton information through the temperature dependent decomposition of the ( M -OH)+ ions.

Stereospecific Ion—Molecule Reactions of Anabolic-Type Steroid Tertiary Alcohols with Proton-Donating Cations

“…The shielding effect of the bulky methyl group in 2methylcyclopentanols was suggested to explain the difference in the ease of dehydrogenation of cis-and trans-isomers in isobutane or ammonia plasma. 18 In our recent work, 19 steric hindrance to the attack of a proton-donating cation from the axial region was involved to explain the easier dehydration of the protonated equatorial OH group vs the axial group in the case of conformationally rather stable 4alkyl-4-hydroxy-trans-decahydroquinolines.…”