Pathway for the loss

Hydrolytic and reductive dehalogenation are discussed in the context of pathways for the loss of halogen before ring fission (see later in this section). 

A hypothetical pathway for the loss of H20 from D-[6- H]-4FG is proposed in Figure 10. The kinetic data... 

Scheme 735. A representation of bimolecular elimination pathways for the loss of HCl from (+)-2-chlorohexane [H3C(C1)CH(CH2)3CH3] to prodnce 1-hexene [H2C=CH(CH2)3CH3] and (E)- and (Z)-2-hexene [( ,Z)-CH3CH=CH(CH2)CH3].The base RO is an alkoxide anion. The ratio of products is a function of the alkoxide in the particnlar alcohol from which it is derived. Only antiperiplanar elimination is shown (see Bartsch, R. A. Bnnnett, J. F. J. Am. Chem. Soc., 1968,90,408 and Bartsch, R. A. Bnnnett, J. F. J. Am. Chem. Soa, 1969,91,1376).

A combined FIK and deuterium labelling study has revealed three pathways for the loss of methyl from the molecular ion of pent-3-en-2-ol, CH3CH=CHCH(0H)CH3. This is shown by the relative rate curves for loss of CHj and CDj from the molecular ion of pent-3-en-2-ol-l,l,l-d3, CH3CH=CHCH(0H)CD3 in Figure 5. 

Figure 3 Multistep reaction pathway for the loss of H and of H2O from ionized propanol. The figure is adapted from a study by Booze and Baer Booze JA and Baer T (1992) Dissociation dynamics of energy selected propanol ions from a a-type ion structure. The Journal of Physical Chemistry 96 5715-5719.

The citrate cycle is the final common pathway for the oxidation of acetyl-CoA derived from the metabolism of pyruvate, fatty acids, ketone bodies, and amino acids (Krebs, 1943 Greville, 1968). This is sometimes known as the Krebs or tricarboxylic acid cycle. Acetyl-CoA combines with oxaloacetate to form citrate which then undergoes a series of reactions involving the loss of two molecules of CO2 and four dehydrogenation steps. These reactions complete the cycle by regenerating oxaloacetate which can react with another molecule of acetyl-CoA (Figure 4). 

The pathway for the degradation of metribuzin that has been used as a herbicide has been established, and attention has been directed to the occurrence of the metabolites in leachate from agricultural soil in Denmark. Although the parent metribuzin was not found, the metabolites produced by loss of the N-amino group and the thiomethyl groups were produced and contributed to ground-water contamination (Kjaer et al. 2005). 

Degradation and evaporation seem to be the major pathways for acrolein loss in water smaller amounts are lost through absorption and uptake by aquatic organisms and sediments (USEPA 1980 Reinert and Rodgers 1987). The half-time persistence of acrolein in freshwater is 38 h at pH 8.6 and 50 h at pH 6.6 degradation is more rapid when initial acrolein concentrations are less than 3000 pg/L (Bowmer and Higgins 1976). Nordone et al. (1998) show a half-time persistence of 2.9 to 11.3 h at initial nominal concentrations of 20 pg/L, and 27.1 to 27.8 h at 101 pg/L. At pH 5, acrolein reacts by reversible hydrolysis to produce an equilibrium mixture with 92% beta-hydroxy-... 

There are several possible explanations to account for this apparent lack of toxicity. Some geranylgeranylated Ras-related proteins might compensate for the loss of Ras function (see, e.g., [46]). Alternatively inhibition of farnesyl transferase may reduce Ras activity below the level required for transformation, yet allow sufficient Ras activity for maintaining normal cell viability [47]. Alternatively, a different signaling pathway may be activated when Ras is not anchored to the plasma membrane. 

Subsequent loss of H from CHs should not occur. It is noteworthy that even such a simple fragmentation scheme offers two independent pathways for the generation of CH (reactions 6.5 and 6.6). 

Note The McLafferty rearrangement and the RDA reaction have several features in common i) both belong to the rearrangement type of fragmentations, although the name conceals this fact in case of the latter, ii) both represent pathways for alkene loss from molecular ions, and iii) both are highly versatile in structure elucidation. 

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

TATP displayed less reactivity with metals than HMTD, but it was less thermally stable. Sublimation (the direct transition of a material from the solid to the gaseous state) was the main pathway for weight loss in TATP. Even at room temperature, notable weight loss could be witnessed within a day. The high sensitivity and thermal instability of TATP precluded its use in any practical applications. Although of limited academic interest, it never took hold in the explosives community. 

An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process B), in which a covalent bond along the polymer main chain is broken and a stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for example, homolytic carbon-carbon cleavage, have obviously high activation energies. The stress-free rates of these reactions are therefore typically extremely low. 

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