Aromatic compounds: fragmentation

Fragmentation Fragmentation occurs on both sides of the carbonyl group. For example, in acetophenone, the major ions occur at masses 77, 105, and 120 (see Figure 20.3). Ions at m/z 39, 50, and 51 also suggest the presence of an aromatic ring. Aromatic compounds, such as quinone, tetralone, and anthraquinone, readily lose CO. 

A few routes to new silenes, usually involving flash vacuum pyrolysis at high temperatures, have been reported. Silenes were proposed as the result of the thermal expulsion of trimethylmethoxysilane, or a similar volatile fragment, from the starting material but frequently, proof that the silenes proposed to account for the observed products were in fact formed was not provided.116 119 The other thermal route employed was the retro-Diels-Alder regeneration of a silene from an adduct with an aromatic compound—often a 9,10-anthracene or 1,4-naphthalene adduct or, in some cases, a 1,4-benzene adduct, as illustrated in Eq. (19).120... 

In Fig. 4b fragment ions can be observed, which correspond to 16 Da upwards relative to Pr271 and 32 Da to the parent compound fragments. These data suggest that a double hydroxylation of 4-MBC occurred. The two hydroxyl groups were attached, as in the previous case, in the aromatic moiety of the compound. 

Ortho-Effect. The ortho-effect is one of the most widely known structural phenomena in organic chemistry. It is widely used in organic chemistry for synthetic purposes. The mass spectra of the majority of ort/jo-substituted aromatic compounds possess significant differences in comparison with the spectra of their meta- and para-isomers. A classic example of the ortho-effect in mass spectrometry involves fragmentation of alkylsalicylates. The intense peaks of [M - ROH]+ ions dominate in the El spectra of these compounds. These peaks are absent in the spectra of their meta- and para-isomers. The reaction leading to these ions may be represented by Scheme 5.12. 

The reduction of polymers can be carried out by using a diimide, generated in situ. The precursor for diimide can be p-toluenesulfonyl hydrazide (TSH), the reaction temperature is between 110-160 °C and the solvents are high boiling aromatic compounds. Possible side-reactions are cis-trans isomerization of 1,4-dienes, attachment of hydrazide fragments to the polymer, degradation and cyclization of the polymer. 

The stability of the aromatic system leads to fragments containing aromatic species. The following tips help you interpret the mass spectral data of aromatic compounds ... 

The aliphatic fragmental values were used to calculate the aromatic fragmental values using the solubility data of liquid aromatic compounds. 

The aqueous solubility of 436 aliphatic and aromatic compounds, both liquid and solid, was calculated with Equation 3.48, using fragmental solubility constants (V Mita et al., 1986). Results... 

All of the group 5 heterobenzenes show mass spectra typical of aromatic compounds (Table 4). The compounds show intense molecular ions which are the base peaks for all the heterobenzenes except bismabenzene. Important fragmentation involves loss of either C2H2 or HCE from the molecular ion. Loss of HCE becomes relatively less important for the heavier heterobenzenes. This observation may be explicable in... 

Under the usual commercial hydrodesulfurization conditions (elevated temperatures and pressures, high hydrogen-to-feedstock ratios, and the presence of a catalyst), the various reactions that result in the removal of sulfur from the organic feedstock (Table 4-3) occur. Thus, thiols as well as open chain and cyclic sulfides are converted to saturated and/or aromatic compounds depending, of course, on the nature of the particular sulfur compound involved. Benzothio-phenes are converted to alkyl aromatics, while dibenzothiophenes are usually converted to biphenyl derivatives. In fact, the major reactions that occur as part of the hydrodesulfurization process involve carbon-sulfur bond rupture and saturation of the reactive fragments (as well as saturation of olefins). 

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