Gas-phase acidities of cycloheptatrienes. Effects of alkyl groups on the stability of carbanions

Eur. J. Mass Spectrom. 8, 359–366 (2002)
DOI: 10.1255/ejms.499

Gas-phase acidities of cycloheptatrienes. Effects of alkyl groups on the stability of carbanions

Masaaki Mishima^*
Institute for Fundamental Research of Organic Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan. E-mail: mishima@ms.ifoc.kyushu-u.ac.jp
Tomomi Kinoshita,^* Yoshitaka Hattori and Ken’ichi Takeuchi
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

ABSTRACT:

The gas-phase acidities of 7-alkyl substituted cycloheptatrienes have been determined in a FTMS by a proton-transfer equilibrium method, DG°_acid (kJ mol^–1): methyl 1520.0, ethyl 1516.7, n-propyl 1513.3, I-propyl 1512.1, n-butyl 1510.4. The effects of alkyl groups on the acidity of cycloheptatriene is linearly correlated with the polarizability parameters (s_a) of substituents, giving a r_a of –55.3 (kJ s_a^–1 unit). The magnitude of r_a is half of that for RCH₃ and is significantly larger than that of the fluorene series. These results suggest that the extent of the charge-delocalization in a carbanion plays an important role in determining the susceptibility of the stability of the carbanion to substituent polarizability effects. In addition, comparison of r_a values of acidities of a series of elemental hydrides shows that the numerical value of r_a decreases in order, C > O»N > S, and that the change of r_a is related to the stability of the respective parent anions (R=H) rather than the atomic electronegativity at the deprotonation center.

Keywords: gas-phase acidity, cycloheptatriene, carbanion, substituent effect, polarizability, FT-ICR

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