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The rates of 1,2-hydrogen shifts for a series of carbonium ions have been determined at various temperatures from n.m.r. line broadenings. For the cations studied the rates vary over more than eight orders of magnitude ; they show no correlation with the basicity of the parent unsaturated compounds. It is concluded that the magnitude of the partial positive charge at the carbon atom to which the hydrogen is to migrate largely determines the reaction rate. This leads to a model where in the transition state the positive charge is retained at the carbon atoms, and the hydrogen remains o-bonded to the carbon atoms.* All chemical shifts are given in p.p.m. downfield from tetramethylsilane. t ortho, meta and para with respect to the protonated carbon.

Perylene, naphthacene, anthracene and some substituted anthracenes are oxidized to the dipositive ions by SbF5 (neat or in SO2CIF). In FSO3H‐SbF5 dication formation takes only place with the easily oxidizable arenes perylene and naphthacene. The dication of anthracene has also been obtained by bromide abstraction from 9, 10‐dibromo‐9, 10‐dihydroanthracene. The PMR spectra of the dications are reported. Like in the dianions, the sum of the charge‐induced shifts varies considerably from one arene to the other.

KONINKLIJKE/SHELL-LABORATOMUM, AMSTERDAMIn carbonium ions the relative orientation of the vacant p-orbital of the electron-deficient carbon atom and the C-H and C-R bonds at the adjacent carbon atom ("orbital orientation") is a very important factor for the rates of 1,2-hydride and alkyl shifts and of cleavage of /? C-C bonds. A large number of examples of this effect can be found among the reactions of directly observable carbonium ions. In some cases it causes the rate of a 1,2-shift or a P-cleavage in a cyclic carbonium ion to be several orders of magnitude lower than in acyclic carbonium ions.

KONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAMA PMR spectroscopic study is described of the protonation of a series of /I-dicarbonyl compounds (Table 1) in HF-SbFs. At high acidity they form 1,3-dicarbonium ions, except in the case of compounds VI and VII. At lower acidity monocations are formed : diketones give allylic type monocations, and lteto esters, dicarboxylic esters and acids give alkoxyhydroxy and dihydroxycarbonium ions, respectively.Between these two groups there is a marked difference in spectroscopic behavlour of solutions containing comparable amounts of mono-and dications. Equilibration of mono-and dications is "slow" with the diketones and "fast" with the other compounds.For the compounds studied, the equilibrium constants KII* =: [dicationli [monocation] [HzF.] vary from less than 10-for tri-and hexafluoroacetylacetone to more than I O7 for ethyl acetoacetate. The monocation-dication equilibrium systems are useful as indicators and buffers at high acidity; they are unreactive towards alkylcarbonium ions. A lower limit has been established for the basicity of isobutene, which turns out to be much more basic than benzene. IntroductionWe recently communicated the formation of mono-and dications of some 1 ,3-diketones 1. Diprotonation of acetylacetone, benzoylacetone and dibenzoylmethane was found to take place in H F containing an excess of SbF5 (HSbFs), yielding dications with structure D , while under less acidic conditions (in H F without excess of SbF5, or in HFS03 or H2S04) monocations were formed having the allyl-cation structure A :

A kinetic investigation has bcen made of the isomerization of 2-methylpentane with liquid HF-SbF:, as catalyst at temperatures of from --20 to -t 20°C. The isomerization proceeds i n three distinct stages: first 3-methylpcntane is formed, then the equilibriuni mixture of 2-and 3-methylpentanc isornerizes to equilibrium with 2,3-dimethylbutane, and finally that mixture reacts to give n-hexane and neohcxane. respectively, until thermodynamic equilibrium is reached. Rate constants at 0 " for these four rcactions are 0.42, 0.030, 0.0001 5, and 0.0007 s-l . (mole hcxane) . (mole SbFj) I , respectively. In the first reaction, mass transfer between acid and hydrocarbon phases is rate limiting; the rate-determining step of the second reaction is the intramolecular rearrangement of the intermediate tertiary carbonium ions, and of the last two reactions the intermolecular hydride-ion transfer between secondary and tertiary carbons. The rate constant for hydrideion transfer from a secondary carbon to a tertiary carbonitim ion is about 0.06 1 . mole I . s. 1 at 0 ( E \ == 14-15 kcal . mole I ) , and for that between two tertiary carbons is greater than 10 1 . mole . 5 -l . The rate of rearrangement of a tcrtiary ion to a secondary or tertiary ion with different degree of chain branching is about 0.1 s--1 at 0 " (E \ = 16-17 kcal . mole l). A mechanism for that type of rearrangement via a (protonated) cyclopropane ring is consistent with the data. In particular, it explains why n-butane does not isonierize to isobutanc under conditions whcrc n-pentane and rj-hexane are rapidly converted. -2 3 * . . J . M , Oelderik, E. L. Muckor, J. C. Plutteeuw, and A . van der Wiel, U.S.P. 3, 201, 494 (1965). D. M . Brouwer, Rec. Trav. Chini. 87, 210 (1968). Actually, the "catalyst" is a solution of alkylcarbonium ions in H F k f . ref. 3 ) , containing equimoiar amounts of carbonium ion and SbFe-. ~~_ _ _~~~ ~~ 724

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