Heteroatoms oxygen

As discussed in this book (Chapter 2, for example) a main difference between fossil fuels and biomass as feedstocks is that in the former case the functionalization of base chemicals obtained from the oil (ethylene, propylene, aromatics, etc.) occurs essentially by introduction of heteroatoms, while in the case of biomass-derived based chemicals (glycerol, for example) it is necessary to eliminate heteroatoms (oxygen, in particular). Consequently, the catalysts required to develop a petrochemistry based on bio-derived raw materials need to be discovered and cannot simply be translated from existing ones, even if the knowledge accumulated over many years will make this discovery process much faster than that involved in developing the petrochemical catalytic routes. 

Bifluorophores consisting of two different fluorescent dyes linked by a flexible spacer containing heteroatoms (oxygen, nitrogen or sulfur atoms) can bind cations. This results in a decrease of the distance between the two fluorophores and, consequently, to an increase in efficiency of photoinduced energy transfer between the two moieties (Figure 2.13) provided that the emission spectrum of the donor (D) overlaps the absorption spectrum of the acceptor (A).(36) The transfer efficiency depends on the distance according to Forster s theory ... 

The electrodecarboxylation of heterocycles having a carboxylic group in a-position to an heteroatom (oxygen, nitrogen) can lead to the corresponding methoxylated product when the electrolysis is performed in methanol [143,184-189], as shown in Scheme 101. 

Oxazole (pATa 0.8) and thiazole (pATa 2.5) are weak bases. The basicity of the nitrogen is reduced by the presence of the other heteroatom. Oxygen and sulfur provide a stronger electron-withdrawing inductive... 

For an acyclic chain containing a heteroatom, oxygen is senior to sulfur, sulfur to nitrogen, nitrogen to phosphorus, phosphorus to silicon, silicon to germanium, etc. 

The direct conversion deals with the straight hydrogenation of carbon monoxide to paraffins, olefins and heteroatom (oxygen, nitrogen) containing products. The indirect conversion invokes intermediates such as methanol, methyl formate and formaldehyde. The latter ones in a consecutive reaction can yield a variety of desired chemicals. For instance, acetic acid can be synthesized directly from CO/H2, but for reasons of selectivity the carbonylation of methanol is by far the best commercial process. 

Reductive Cleavage of Carbon-Heteroatom and Heteroatom-Oxygen Bonds... 

The azoles (oxazole, imidazole, and thiazole) are five-membered aromatic heterocycles that have two heteroatoms in the ring. One of the heteroatoms in each of these heterocycles is an sp2-hybridized nitrogen that contributes one electron to the 6n aromatic system and has a basic nonbonded lone pair. The other heteroatom (oxygen, nitrogen, or sulfur) contributes two electrons to the 6n system. The imidazole skeleton is present in the amino acid histidine. The thiazole ring occurs in thiamin (vitamin B. ... 

The most widely held view of coal structure pictures coal as groups of fused aromatic and hydroaromatic ring clusters, possibly linked by relatively weak aliphatic bridges. The ring clusters contain heteroatoms (oxygen, sulfur and nitrogen) and have a variety of attached functional groups. 

Making furans is even easier because the heteroatom (oxygen) is... 

In El the molecular ion is only weakly observed in the case of linear saturated hydrocarbons. The presence of branching usually entails the disappearance of this peak. However, an unsaturation, and especially an aromatic ring, makes the molecular ion peak more intense. The presence of electronegative saturated heteroatoms (oxygen, fluorine) normally prevents the observation of the molecular ion. In fact, its intensity depends on the groups that are present. Thus an aliphatic ester yields a weak or absent molecular ion, whereas an aromatic one usually yields an intense molecular ion peak. 

Figure 24 The mechanism of oxidation of heteroatom-containing compounds by P450 enzymes (a) The one-electron oxidation mechanism of heteroatom oxygenation (b) The one-electron and hydrogen atom abstraction mechanisms of heteroatom dealkylation...

A common characteristic of all these diselenides is the close proximity of a heteroatom (oxygen or nitrogen) to the selenium atom. It has been reported at several occasions that selenium can interact with nearby heteroatoms. In some... 

As indicated in the Introduction, only a brief survey of hetero electrocyclizations will be presented hoe. The reader should rdier to Marvell s excellent treatise or other sources for leading references. However, there are several unique features of die heterocyclizadon process that are worth considering in this limited survey. In particular, when the heteroatom is placed at Ae end of the triene system, cycliza-tions often prove to be exceedingly facile. To focus on diis feature of the reaction substrate, the following discussion will be restricted to electrocyclizations in which the heteroatom (oxygen and nitrogen since they are the most common) is located at the end of the triene. 

It is also possible for the heteroatom (oxygen in a carbonyl) to react as a base, attacking the electrophilic species. This species is most often a proton and the mechanism is... 

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