CO Loss from Phenols

Phenols exhibit a strong molecular ion peak often representing the base peak in their spectra. The most characteristic fragment ions of phenols are caused by loss of CO from the molecular ion [107], and subsequent H loss, thereby forming [M-28] and [M-29] ions, respectively (Fig. 6.31), the identity of which have been ascertained by HR-MS as [M-CO] and [M-CHO] . [108] This initially unexpected fragmentation proceeds via ketonization of the molecular ion prior to elimination of CO. The mechanism has been verified by D-labeling. These experiments also revealed that only about one third of the H cleaved off from the 

Note The above mechanism of CO loss from phenols is perfectly analogous to HCN loss from aniline and other aminoarenes (Chap. 6.14.2). 

Example Ethyl loss clearly predominates over methyl loss in the El mass spectrum of 2-(l-methylpropyl)phenol. It proceeds via benzyhc bond cleavage, the products of which are detected as the base peak at m/z I2I and m/z 135 (3%), respectively (Fig. 6.32a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8%) is completely due to the C isotopic contribution to the peak at m/z 121. From the HR-EI spectrum (Fig. 6.32b) 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 CgH90 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. 

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The mass spectrum of aniline has been known since the early days of mass spectrometry. [122] Initially, the observed [M-27] ion has been interpreted in terms of HCN loss (Fig. 6.56a). The mechanism for loss of the elements of [H, N, C] from aminoarenes is perfectly analogous to CO loss from phenols (Chap. 6.9.1). [231] More recently, it could be demonstrated that loss of hydrogen isocyanide, HNC, occurs rather than losing the more stable neutral species HCN, a behavior typical of ionized pyridine. [222]... 

Note The detection of CO loss from molecular or fragment ions usually indicates the presence of carbonyl groups. However, it is less indicative of molecular structure than the highly specific reactions discussed before, because a multitude of rearrangement processes can be effective. These might even lead to CO loss in cases where no carbonyl group exists, e.g., from phenols. 

Although the ketene ring 30 is found to be only 53 kJmoC above 21 and by far more stable than acetyl ion 29, it turns out that the CO loss from an indirect process, finally giving the five-membered ion 31, namely 29 30 —> (CO - - 31), constitutes a substantially more difficult route. It is apparent from Figure 32 that the cyclic isomer 29 could be a possible intermediate in the CO-eliminative process of phenol cation 21. Nevertheless,... 

Figure 34 illustrates the lowest energy rearrangement path for the CO-loss process of ionized phenol. It involves, in a first step, the enol-keto conversion 21-22. Starting from 22, a ring opening leads to structure 34 which, in turn, by ring closure produces ion 41. A direct and concerted isomerization 22 41 was not found. The CO loss from 41... 

FIGURE 34. Schematic representation of the (CsHsO)" potential energy surface showing the lowest energy path for CO loss of phenol radical cation. Relative energies given in kJ mol were obtained from B3LYP/6-311++G(d,p)+ZPE calculations. Adapted from Reference 452 with permission... 

The dissociation of the styrene ion involves a transition state very similar to the reactant. The large negative entropy of activation results from the fact that the internal energy content of the transition state is reduced, compared to the reactant ion, by an amount equal to the critical energy. The transition state structure for the CO loss from the phenol cation is also close to the precursor but some vibrational modes loosen up, resulting in a slightly positive activation entropy. 

The loss ofHCN and H CN from aniline is similar to the elimination of CO and CHO from phenol. 

It may be noted that the energy amount involved in the CO-loss process is by far smaller than that needed for a deprotonation of phenol cation as mentioned above, namely 857 kJmoC. This suggests that the ease with which a deprotonation of phenol radical cations occurs in different solutions was likely to arise from either a specific participation of the solvent molecules in the supermolecule or a strong continuum effect. 

The ionized dihydroxybenzenes 39" behave similarly (Scheme 17). Note that, in this series, one of the phenolic hydroxyl groups acts as a hydrogen acceptor and the other as an H donor. The El mass spectrum of catechol (o-39) exhibits a significant ortho effect. While the intensity of the [M — H20]" peak in the El spectrum of o-39 is no greater than ca 15%B, the spectra of resorcinol (m-39) and hydroquinone (p-39) both show negligibly smah [M — H20] + peaks ( 2%B). It is likely that water loss from the intermediate 47 generates again bicyclic [M — H20]" ions, i.e. ionized benzoxirene 48. And, notably, the CO losses does not parallel the ortho effect of the water elimination in this series, as it is the most pronounced ion the case of m-2>9. 

Phenols A conspicuous molecular ion peak facilitates identification of phenols. In phenol itself, the molecular ion peak is the base peak, and the M — 1 peak is small. In cresols, the M - 1 peak is larger than the molecular ion as a result of a facile benzylic C—H cleavage. A rearrangement peak at m/z 77 and peaks resulting from loss of CO (M - 28) and CHO (M -29) are usually found in phenols. 

Polysaccharide pyrolysis at 375-520°C is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of HsO, CO, C02, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an... 

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