Charge-site initiated cleavage

C. Charge-Site-Initiated Cleavage Inductive Cleavage... 

The bond adjacent to a heteroatom can be broken by a charge-site-initiated reaction, that is by attraction of an electron pair from this bond, and we talk about an induced cleavage (i) ... 

For a localized charge site, the initiation of the cleavage takes place by the attraction of the electron pair forming the bond charge site initiation mechanism). 

As mentioned earlier [see reaction (6.43)], cyclic ketones undergo three distinct fragmentation pathways elimination of an alkene via cleavage of two 0-bonds in the ring, charge site-initiated reactions, and H-transfer reactions. 

This concept is basically a means to force consideration of possible decomposition pathways and their resulting product stabilities and steric requirements. The limits of its simplifying assumptions, however, should be recognized in mass-spectral interpretation. Sometimes, especially for multiple-bond cleavages, either radical or charge-site initiation produces the same products the cyclic ion decompositions of Equation 4..10 can also be written (Equation 8.33) as a charge-site reaction. The substantial peak at m/z 56 in the spectrum of 2-methyl-... 

Another common one-bond cleavage is charge-site-initiated or inductive cleavage, often indicated in a fragmentation mechanism by the symbol i. Inductive cleavage involves the attraction of an electron pair by an electronegative heteroatom that ends up as a radical or as a closed-shell neutral... 

Note The term a-cleavage for this widespread radical-site initiated process with charge retention can be misleading, because the bond cleaved is not directly attached to the radical site, but to the next neighboring atom. 

Both the radical site and the charge site of the hydroxyl group are intermediate in their capability for reaction initiation, so that mass spectra of alcohols exhibit many of the types of reactions outlined in Chapters 4 and 8. Tertiary alcohols contain the largest, and primary alcohols the smallest, total abundance of oxygen-containing ions in their spectra. In all but the spectra of 1-alkanols, a-cleavage (Equation 4.17) is the most useful characteristic reaction. In the spectrum of 3-methyl-3-heptanol (Figure 9.5) this produces the important peaks at... 

One particularly intriguing aspect of Cl for molecular weight determination for polymers is the appearance of the quasimolecular ion (MH+). Cl leads to less fragmentation compared to EL Proton attachment preferentially occurs on heteroatoms, and the initial fragmentation is at the charge site normally by heterolytic cleavage (inductive effect). 

---