Charge-Site Rearrangements

Even-electron acyclic ion decompositions in which the charge site retains its location and the electrons remains paired, both of which are usually desirable 

Unknown 4.20. Predict the principal peaks in the mass spectrum of N-methyl-N-isopropyl-N-butylamine. 

Even-electron ions can also undergo H rearrangements which do not involve the charge site (nor an unsaturated heteroatom), such as reaction 4.44. The resulting peaks should be used with caution for assigning functional group 

One of the above mechanisms should help in the elucidation of Unknown 4.21. 

The summary inside the back cover is intended to underscore the relationships of the common types of reactions which ions can undergo, using examples from this chapter. You should use this as a checklist when you are attempting to predict the spectrum of a molecule to use it effectively, however, you will have to understand the material of the preceding sections. 

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Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

These findings may be rationalized in terms of the scheme outlined below An active site of intermediate acidity will not completely remove the hydroxyl group from the oxime and after migration of the R group the intermediate (I) is formed. If basic sites are present on the catalyst surface then a reaction with the partially charged carbon may occur and the rearrangement would not proceed. In the absence of basic sites rearrangement may proceed as follows ... 

If the two membrane surfaces have the same number of negatively charged sites (as they normally do) from which H can dissociate, then y, and y) are identical so also are a and a). By substituting Equations 23-9 and 23-10 into Equation 23-8. substituting the equalities y, = ji and a = a>. and rearranging the resulting equation, we find that... 

Charge-site reactions (one bond cleaved without rearrangement)... 

Two bonds cleaved. The first step of an OE" " rearrangement (Equations 8.70 and 8.74) separates the radical and charge site so that they can act indepen-... 

The carbocations formed as intermediates when allylic halides undergo Stvfl reactions have their positive charge shared by the two end carbons of the allylic system and may be attacked by nucleophiles at either site Products may be formed with the same pattern of bonds as the starting allylic halide or with allylic rearrangement... 

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