Chemical ionization mass spectrometry. Mechanisms in ester spectra

Abstract

CHs's on tertiary C's), 1.95 (s, 1, H on C with -NHC02Et). The two methyl groups on each of the four secondary insertion products should each appear as a doublet. The singlet listed at 0.87 is the envelope of these closely spaced, unresolved doublets. The primary insertion product was not present in sufficient quantity for analysis and assignment was based on retention time and selectivity data.The stereospecificty and selectivity were then calculated from the peak areas in the usual manner.The absolute yield was determined by using acetophenone as an external standard. Total moles of insertion product was then calculated from the area/mol for acetophenone using the calibration factor of 0.78 for the products relative to acetophenone. It was assumed that all the insertion products have the same detector sensitivity.

Energetics and structural effects in the fragmentation of protonated esters in the gas phase

“…However, for the n-butyl formate and i-butyl formate, formation of the primary butyl cations (AH0 --40 kcal mol-I) is thermochemically unfavourable compared to formation of HC02H2+ (AH0 = 25 kcal mol-I). It appears that in these two cases fragmentation must be accompanied by [1,2]-H shifts to form the more stable sec-butyl and tert-butyl structures respectively.…”

“…It appears likely that there is significant rearrangement to the tert-butyl and sec-butyl structures, respectively, during fragmentation of these protonated esters. These rearrangements can occur by relatively simple [1,2]-H shifts in the developing cation. For all the butyl acetates there is significant formation of the CH3-CEO+ ion; this may represent fragmentation of the ether oxygen protonated species.…”

“…The partitioning of the excess energy between the neutral acetic acid and alkyl ion appears to be determined at this stage. A simple [1,2]-H shift in the alkyl ion leads to the secondary ion interacting with acetic acid by iondipole forces; this structure b, which will be more stable than a , may then undergo simple fission of the weak bond to acetic acid yielding the alkyl ion or may transfer a proton to acetic acid yielding the protonated acid and an appropriate olefin. Isotopic labelling studies (6) in n-propyl acetate show that the H transferred from the propyl group does not originate from a specific position of the alkyl group but may come from any position.…”

“…H+ -, R'C=O+ + ROH Reaction [I], involving formation of the acylium ion by acyl carbon-oxygen bond cleavage, and reaction [2], involving formation of the alkyl carbonium ion by alkyl carbon-oxygen bond cleavage, are analogous to the AAcI and AAII mechanisms for fragmentation of esters in concentrated acid solutions (8). Reaction [3] has no direct solution analogue.…”

“…For the secondary acetate, 3-methyl-2-pentyl acetate, the A H 0 values are very similar, yet the C5Hll+ ion is the dominant ion in the mass spectra. This strongly indicates a [1,2]-H shift to yield the more stable tert-pentyl ion. Collin and Herman (25) have reported an activation energy of -2 kcal mol-' for this isomerization from studies of the protonation of 3-methyl-1-pentene in radiolysis experiments.…”