Gas phase ion-molecule chemistry : 1. geometric dependence in the acidity of the amines 2. reversal of the gas phase acidity of the long chain alcohols (1-pentanol and beyond)

Author

Priscilla Rose Higgins

Date of Award

12-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemistry

Major Professor

John E. Bartmess

Abstract

The Ion Cyclotron Resonance mass spectometer has been used to study gas phase ion-molecule reactions.

1,1,1,3,3,3-Hexamethyldisilazane was found to be more acidic than expected in the gas phase by 15 kcal/mol, relative to other functional groups, based on the solution phase data. Its conjugate base will not deprotonate simple ketones in the gas phase, although that is the common use of the lithiated salt in the condensed phase. Theoretical calculations have shown that there is a geometry change upon removal of the hydrogen at the nitrogen center. The Si-N-Si bond angle became linear. This strengthening of acidity in the gas phase and the geometry change must be due to one of the following: polar, polarizability, resonance or rehybridization effect. In order to determine which effect was the principal contributor to the strengthening in the gas phase acidity and the change in geometry, a geometrically constrained molecule was chosen, 1,1,3,3-tetramethyl-1,3-disila-2-aza-cyclopentane, and its acidity was measured. 1,1,3,3-Tetramethyl-1,3-disila-2-aza cyclopentane was found to be 10.5 kcal/mol more weakly acidic than the hexamethyldisilazane. By comparing the theoretical calculations of the silylamines and alkylamines, it is postulated the strengthening in acidity of hexamethyldisilazane must be due to rehybridization at the nitrogen center.

As for the acidities of the aliphatic alcohols, it has been shown in the kinetic method, which is a method that measures the acidities of the ions formed from fragmenting dimer ions, that there was a linear strengthening in acidity of the branched and primary alcohols (carbon lengths up to four). The long chain primary alcohols (carbon lengths greater than four), showed a monotonic strengthening in acidity, but not a linear one. In the equilibrium method, which is a method that measures the acidities of free ions, the long chain alcohols did not show a monotonic strengthening in acidity as was shown in the kinetic method. The acidities of the long chain alcohols by our method, equilibrium method, were as follow at 25°C: C₃ < C₄ < C₉ < C₇ < C₈ C₅ < C₆. This non-monotonic trend in acidities among the long chain alcohols might be due to the alkoxides forming cyclic structures. The tail-end methyl group is capable of approaching the anionic site with increasing carbon length. However, our results also showed the long chain alcohols (1-pentanol and beyond) were less acidic relative to propionitrile. When using the ΔS values for linear and cyclizing alcohols to calculate ΔG for the long chain alcohols, the long chain alcohols should have been more acidic than the carbon acids. Therefore, something else is occurring in the long chain alcohols, in which there is a need for further studies.

Recommended Citation

Higgins, Priscilla Rose, "Gas phase ion-molecule chemistry : 1. geometric dependence in the acidity of the amines 2. reversal of the gas phase acidity of the long chain alcohols (1-pentanol and beyond). " Master's Thesis, University of Tennessee, 1995.
https://trace.tennessee.edu/utk_gradthes/11135