Substituent Effect on the Stability of 1-Phenylethyl Cation in the Gas Phase

MISHIMA Masaaki, Usui Satoshi, Fujio Mizue, TSUNO Yuho

doi:10.1246/nikkashi.1989.1269

The gas-phase basicities (GB) were determined for a series of styrenes based on protontransfer equilibrium method using a pulsed ion cyclotron resonance (ICR) mass spectrometer. The LArSR analysis of the substituent effect on the GB gave an r of 1.14 and a p of 13.6<BR>δGB=ρ(σ°+rδδR+) ( 1 )<BR>The r value higher than unity\ indicated clearly a greater resonance demand for 1-phenylethyl cation than that for a-cumyl cation while the p value was comparable to that of the latter. The exalted resonance demand might be attributed to the lower stability of 1-phenylethyl cation than a-cumyl cation. This trend in the r value is consistent with the observations for the relevant benzylic carbocations. Most importantly, the r value for gas-phase stability of the 1-phenylethyl cation has been found to be identical in magnitude with the r value for the solvolysis of the corresponding substrates (r=1.15). The same is generally true for a series of benzylic carbocations. The identity of th e r value between the SN 1 solvolysis and gas-phase carbocation stability suggests that the intramolecular charge delocalization in the transition state should be quite close to that of the carbocation intermediate. It is concluded that the r value of 1.15 for the 1-phenylethyl solvolysis must be intrinsic resonance demand characteristic of the parent 1-phenylethylc ation i tself. The LArSR correlationw ith r = 1.15 for the solvolysisi n aq. ethanol and aq. acetone suggests a mechanistic involvement of solvent nucleophilic assistance for substrates less reactive than m-cyano derivative.

Substituent Effect on the Stability of 1-Phenylethyl Cation in the Gas Phase

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