|
Eur. J. Mass Spectrom. DOI: 10.1255/ejms.499 |
|
Gas-phase acidities of cycloheptatrienes. Effects of alkyl groups on the stability of carbanions | ||
Masaaki Mishima* |
||
ABSTRACT: | ||
The gas-phase acidities of 7-alkyl substituted cycloheptatrienes have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry using a proton-transfer equilibrium method, DG0acid (kJ mol1): methyl 1520.0, ethyl 1516.7, n-propyl 1513.3, i-propyl 1512.1, n-butyl 1510.4. The effect of alkyl groups on the acidity of cycloheptatriene is linearly correlated with the polarizability parameters (sa) of substituents, giving a ra of 55.3 (kJ sa1 unit). The magnitude of ra is half of that for RCH3 and is significantly larger than that for 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 ra values of acidities of a series of elemental hydrides shows that the numerical value of ra decreases in the order, C > O » N > S, and that the change of ra 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 |
You can now buy this paper on-line in PDF format; it costs only £11.75. Just click on the BUY on-line button. You can pay on-line through a secure server and get access immediately.
© IM Publications Any problems? E-mail . |
![]() |