Stabilities of α-Cumyl Cations in the Gas Phase

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  • 気相における置換クミルカチオンの安定性
  • キソウ ニ オケル チカン クミル カチオン ノ アンテイセイ
  • Stabilities of .ALPHA.-cumyl cations in the gas phase.

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Abstract

The gas-phase basicities (GB) of 2-(m, p-substituted phenyl)propenes were determined based on proton-transfer equilibrium method using a pulsed ion cyclotron 'resonance (ICR)mass spectrOmeter. The substituent effect on the GB was correlated linearly by σ+ to give a ρ of 13. O. The correlation whichlcovered a wide range of 35 kcal from p-NMe, to 3, 5-(CF3)2 group revealed that a+ values determined in solution for most of substituents were applicable to the gas-phase substituent effects without modification accounting for solvation of substituents. This result suggested that the resonance demand (r) of LArSR Eq. ( 1 ) for the gas-phase stability of α-cumyl (1-methyl71-phenylethyl) cation was identi cal with an r of 1.00 for the solvolysis of a-cumyl chlorides.<BR>δGB=ρ(σ°+rδδR+) ( 1 )<BR>The identity of the r value between in the gas phase and in solution was also observed for basicities of a series of benzoyl compounds. The charge delocalization in the aryl ring within a cation will remain essentially the same in the gas phase as in solution. The solvation of cation reduces the central charge to lower the response to substituent perturbation, essentially without change in intramolecular charge delocalization. The present result leads is to the conclusion that the charge delocalization in the SN 1 transition state in the solvolysis should be quite close to that of the carbocation intermediate. The same conclusion was given by the results for a series of relevant benzylic systems.<BR>On the basis of the substituent effect on the gas-phase stability of α-cumyl cation, the substituent effects of gas-phase basicities of benzaldehydes and chloride ion affinities of benzyl cations were analyzed to give r values of 1.00 and 1.29, respectively. The exalted r value of 1.29 for benzyl cation might be attributed to the lower stability than a-cumyl cation. The variation of the r value with the system is in complete agreement with the basic concept of varying resonance demand, supporting the validity of the LArSR Eq.

Journal

  • NIPPON KAGAKU KAISHI

    NIPPON KAGAKU KAISHI 1989 (8), 1262-1268, 1989-08-10

    The Chemical Society of Japan

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