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Summarycis-and trans-(3,6-D2)-1 ,Ccyclohexadienes l a and l b have been synthesized from cis-3,4-dichlorocyclobutene (5). Aromatization to benzene with DDQ is cis-stereospecific with an uncertainty of 5 % . This result is discussed in relation to concerted or stepwise mechanisms for aromatization of 1,4-dihydroaromatics with 2,3-dichloro-5,6-dicyano-p -benzoquinone (DDQ).Introduction. -Oxidation of hydrocarbons with quinones proceeds via hydride transfer from the substrate to the quinone (mechanism A , Scheme 1) [l]. In agreement with this description, reaction rates of substituted toluenes with 2,3-dichIoro-5,6-dicyano-p-benzoquinone (DDQ) correlate with o giving a p-value of -4.3 [2]. Accordingly, substrates capable of forming stable carbenium ions upon hydride transfer exhibit enhanced reactivity. For example, cycloheptatriene reacts 200 times faster than 1,3-cycloheptadiene, and 1,2,3-triphenylcyclopropene ca. lo5 times faster than triphenylmethane [3] [4]. Similarly, vicinal dehydrogenation is believed to proceed in a stepwise manner (mechanism B ) involving sequential hydride and proton loss [5]. The partial cis-stereospecificity observed for dehydrogenation of acenaphthene has been ascribed to formation of an ion pair as reactive intermediate [6]. In contrast, dehydrogenation of 1 ,Ccyclohexadiene (1) and its benzannulated homologues does not fit this mechanistic picture. The reactivity of these compounds is usually about 100 times higher than that of 1,2-dihydroaromatics or similar model compounds, and it approaches that of cycloheptatrienes, provided that loss of two H-atoms from the allylic positions is possible [3-51. The enhanced rate of 1,6dihydroaromatics can be understood if it is assumed that part of the aromatic stabilization of the product is already reflected in the transition state of the reaction. This mechanistic hypothesis requires concerted loss of two H-atoms from the I,Cpositions, either in a cyclic mechanism (C) involving two cis-H-atoms, or in a cis-or trans-conjugate 1,Celimination (D). The cyclic mechanism C was proposed by Stoos & R o h k , when they found that cis-3,6-dimethyl-1 ,Ccyclohexadiene (2) reacts ca. 20 times faster than the trans-isomer 3 and 700 times faster than the 3,3-dimethyl derivative 4 [4]. Although the preference for cis-

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.