Fragmentation (chemistry)

Fragmentation of molecules produced during the mass during the ionization of the analyte. You may wish to do to obtain data for structure determination as in the electron impact ionization. The following are the main fragmentation reactions are enumerated.

Alpha cleavage

Heteroatoms promote the cleavage of the constant alpha to them bond. By electron impact a hetero atom loses an electron from a free, non -bonding electron pair and becomes a radical. After ionization of the hetero atom an electron moves from the sigma bond of a directly to the heteroatom adjacent carbon atom ( the alpha- carbon atom) to the heteroatom and, together with the single electron of the heteroatom binding between alpha - carbon atom and heteroatom from. As a second bond between carbon atom and hetero atom is established and the hetero atom, the single electron is re- paired. As a result, at the same time the original binding of the alpha -carbon atom is released and its binding partner, in turn, to a radical.

Compounds with heteroatoms that are affected by this rule include amines, alcohols, ethers, thiols, sulfides and halides. Here, the effect is of zerfallsdirigierende

Thus, the fragments of amines in the spectrum extremely intense and can be clearly seen, the fragments of iodides, however, are only very weakly. This is due to the ability of the hetero atom to stabilize the positive charge. Since hetero atoms have a high electronegativity, presents certain difficulties to stabilize positive charge. As a result, the homolytic cleavage may best be caused by hetero atoms, the electronegativity similar to that of carbon is - this results in the strong peaks with nitrogen, the electronegativity of the carbon is most similar.

In several successive fragmentation reactions an alpha- cleavage takes place only once, as a homolytic cleavage is very energy-intensive in the cationic product of an alpha- cleavage. In the literature only a few exceptions are known.

Benzyl Allylspaltung

Aromatics, which have double bonds result in similar heteroatoms to Alpha - divisions. Benzylspaltungen here are marked as Allylspaltungen because the energy gain is larger for the former.

When a double bond is present in one molecule may be assumed that the localization of the charge on the latter. Most likely, then the formation of a Allylions by homolysis or heterolysis by allyl radical. In theory this could be found out by the resulting mass as in open-chain molecules, the position of the double bond. Practically this is not possible, since the fragmentation reaction preceded by numerous isomerization reactions. This isomerization can be considered as hydrogen rearrangements. In alicyclic compounds such isomerizations are rare. Unique results provides the split contrast to aromatics, as they are very stable. The cleavage of the benzyl bond is strongly favored, causing the Benzylion is generated. This submerged in nearly all compounds containing benzyl as intense signal at m / z 91.

Retro -Diels -Alder reaction

In the retro-Diels -Alder reaction, a six-membered ring is entzyklisiert with a double bond ( double split ) and you get an ene and diene component. The diene is the preferred carrier. The retro -Diels -Alder reaction can be in the molecular ion, as also take place in a fragment. The reaction yields, provided that no nitrogen is present, even-numbered fragments from even-numbered molecular ion, what stands out among the otherwise mostly odd fragments. The entzyklisierte ring can, however, must contain no heteroatoms. The reaction itself reacts very sensitive to minor structural changes; during a certain compound forms a strong peak in the mass spectrum, it may be that a very similar compound is almost impossible to detect in the mass spectrum.

The Diels -Alder reaction, the initial ionization takes place in the double bond in each case. In the following two reaction pathways are possible, in which the charge either remains at the original location or postponed.

McLafferty rearrangement

The McLafferty rearrangement gamma constant H atom is transferred to a least doubly bonded atom through a six-membered transition state. The alpha- bond is cleaved to a neutral free and moved the double bond. The time required for the McLafferty rearrangement double bond must not be already present in the original molecule, but may also be formed only in a fragmentation reaction, for example by an alpha - cleavage. The acceptor for the proton can be, among others, a C = O, C = N or C = C bond. If the gamma- carbon atom no H atom be present, the reaction will not take place too. The McLafferty rearrangement is a reaction analogous to the Norrish type II reaction.

Neutral loss

In order to identify a mass spectrum, it is usually necessary to identify the M . Peak. The following table may help to contain some neutral ( uncharged ) fragments, which, inter alia, to can spin off from the molecular ion. Principle can also be applied to the table Sekundärfragmentationen, but it is most useful for spin-offs from the molecular ion, otherwise a mass difference that can be found in this table does not have to be necessarily incurred by a neutral loss.

In this way also suspicions about originally present in the molecule structures can be erected. Furthermore, it should be noted that a plurality of neutral loss can be consecutive.