Formation, reactivity, and relative stability of clustered alkoxide ions by cyclotron resonance spectroscopy

Abstract

Ion cyclotron resonance spectroscopy has been used to study the general gas-phase reaction of alkoxide ions (RBO-) with alkyl formates (HCOORb) for R = CH3, C&, and i-CbHT. These systems are shown to lead in all cases to the formation of a complex ion, (R,Rb02H)-, and to produce alkoxide displacement (RbO-) only when the electron affinity of RbO . is equal to or larger than that of R,O.. Evidence is presented that the complex ions correspond to alkoxide ions "solvated" by a single neutral molecule of alcohol and presumably held by hydrogen bonding, The "solvated" alkoxide ions can react further with alcohols to establish the intrinsic clustering ability of alcohols by the formation of a preferential "solvated" species. The order of solvating ability is shown to follow the scale of gas-phase acidity, t-C4H90H > i-CaH70H > on cyclotron resonance spectroscopy (icr) has been I successfully used in gas-phase studies of ion-molecule reactions to point out the importance of solvent effects in determining reactivity in solution. For example, the acidity order of simple alcohols in the gas phase is the reverse of the order in so1ution.'P2 Likewise, the acidities of the hydrocarbons toluene and propylene are greater than the acidity of water in the gas phase.3 The basicities of the amines follow thewhereas the solution order does not obey this simple trend. Since equilibria and kinetics are generally complex functions of intrinsic effects and solvation, these studies are particularly relevant for the separation of these factors and the understanding of the correct relationship between structure and reactivity. This paper.reports the application of icr to a new and unique method for the indirect preparation of gasphase alkoxide ions, "solvated" by a single neutral molecule of alcohol. This method is based on the general gas-phase reaction of alkoxide ions with alkyl formates in the pressure range of to 5 X 10-5 Torr, where bimolecular collisions are the only important processes. The "solvated" alkoxide ions thus formed can be further reacted with other neutral molecules to establish the order of preference of neutrals as solvating agents. In this study, the relative order of intrinsic solvating ability of the simple alcohols is determined by icr and related to hydrogen-bonding ability.The trend observed for relative stability of "solvated" species parallels closely the extensive measurements recently carried out by high-pressure mass spectrometry on gas-phase ionic equilibria of C1-solvated by different hydrogen-bonding neutral^.^ Furthermore, the relationship established in ref 5 by Kebarle between gas-phase acidity and the hydrogen-bonding capability of the "solvent" molecule is further explored for the case of the simple alcohols. Experimental SectionAll the experiments were carried out in a Varian V-5900 icr spec-(1) J. I. Brauman and L. K. Blair, J. Amer. Chem. SOC., 90, 6561 (2) J. I. Brauman and L. K. Blair, ibid., 92, 5986 (1970). (3) J. 1. Brauman and L. K. Blair, ibid., 93, 4315 (1971). (4) J. I. Brauman, J. M. Ri...

Gas-phase pathways for ester hydrolysis

“…The product R O -+ (CH,),CCONH, -+ (CH,),CCOOR' -(CH,),CCONH-+ R'OH (8)(CH,),CCOO-+ R'NHL( 9 ) (or olefin + NH3) (M-1)-+ NH, R = CF3, C H 3 0 , C2H50 Alkyl pivalates display a much more complex reactivity pattern as described in the set of reactions 7-10.…”

Gas-phase ionic reactions of benzyl and methoxide anions

“…Charge-localized ions reacted to give mainly ␣-elimination, whereas chargedelocalized ions reacted exclusively at the carbonyl group. Pioneering work by Riveros and co-workers 15,16 on alkyl formate reactions with alkoxides and by Johlman and Wilkins 17 on hydroxide-alkyl formate systems showed that when different pathways are available, ␣-elimination is a major pathway. Brickhouse and Squires 6 have also shown that there are significant differences in the reactivity of charge localized and delocalized ions with 6,6-dimethylfulvene.…”

“…Although the Riveros reaction, equation (1), is the major reaction pathway for methoxide, 11,15,16 Table 1 shows that in the reaction between benzyl anion and various formates, no S N 2 or Riveros reaction products were observed. For ethyl and especially tert-butyl formate, we observe secondary Riveros reaction products:…”

Additivity of substituent effects on the acidity of alcohols

“…The same applies to substitutions with phenyl [5,6] and trifluoromethyl groups. [7,8] Experimental results are given below. This work is aimed at getting new experimental information by means of Fourier Transform ICR (FT-ICR) [9][10][11][12][13][14][15][16][17] and computational data on the eventual additivity of these effects as well as to determine under the same experimental conditions the effect of multiple phenyl substitutions.…”