McL with Double Hydrogen Transfer

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 [86] and has become known as McLafferty rearrangement with double hydrogen transfer (rlH)  

Labeling studies indicate that this process is by far not as site-specific as its better known counterpart [89,90]. Based on thermochemical data, evidence has been presented for a two-step mechanism that finally yields a carbonyl-protonated carboxylic acid and the unsaturated radical. While the final products are well described, the second step has not fully been elucidated [91,92]  

Note The ion series m/z 51,77, 105 is a reliable indicator for benzoyl substructures, e.g., from benzaldehyde, benzoic acid and its derivatives, acetophenone, benzophenone etc. In contrast to benzylic compounds, the peaks at m/z 39, 65, and 91 are almost absent. If a peak at m/z 105 and the conplete series m/z 39, 51, 65, 77, 91 are present, this strongly points towards the conposition [CgH9], and thus to phenylalkanes. In case of doubt, HR-MS is the method of choice for their differentiation. 

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs] ion has initially been attributed to a McLafferty rearrangemeait followed by loss of an al-koxy radical and final stabilization to a cyclic oxonium ion [94]. However, it has been revealed that four other pathways in total lead to its formation excluding the above one [95,96]. The most prominent fragmentation pathways are  

Note Phthalates, especially di-2-ethylhexyl phthalate (also known as dioctyl phthalate, DOP), are commonly used plasticizers in synthetic polymers. Unfortunately, they are extracted from the polymers upon exposure to solvents such as dichloromethane, chloroform, or toluene, e.g., from syringes, tubing, vials etc. Therefore, they are often detected as impurities. They are easily recognized from their peaks at m/z 149 (often base peak), m/z 167, and [M-(R-2H)] (m/z 279 in case of DOP). The molecular ion is often absent in their El spectra. 

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