Atomization efficiency

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

On the base of the developed mathematical models was developed regression model of the atomizer efficiency via main design pai ameters such as linear dimensions and operation temperatures. 

Carbonylation of methanol to acetic acid is fully discussed in Chapter 9. Another carbonylation process using a phosphine ligand to control the course of the reaction is a highly atom efficient route to the widely used monomer methyl methacrylate (Scheme 4.19). In this process the catalyst is based on palladium acetate and the phosphine ligand, bisphenyl(6-methyl-2-pyridyl) phosphine. This catalyst is remarkably (>99.5%) selective for the 2-carbonylation of propyne under the relatively mild conditions of <100 °C and 60 bar pressure. 

Overall this equates to the direct oxidation of ethene in a 100% atom efficient process. 

The plausible mechanism is based on the proposal by Jana and coworkers (Scheme 14). In this case, the sp-hybridized vinyl cation can be attacked by halide, instead of water, to give the ElZ isomer of the alkenyl halide. Compared with the systems using stoichiometric Lewis acid and strong base to prepare substituted alkenyl halides, the present method would provide an excellent alternative due to the environmentally benign system and atom efficiency. 

Diols are applied on a multimilhon ton scale as antifreezing agents and polyester monomers (ethylene and propylene glycol) [58]. In addition, they are starting materials for various fine chemicals. Intimately coimected with the epoxidation-hydrolysis process, dihydroxylation of C=C double bonds constitutes a shorter and more atom-efficient route to 1,2-diols. Although considerable advancements in the field of biomimetic nonheme complexes have been achieved in recent years, still osmium complexes remain the most efficient and reliable catalysts for dihydroxylation of olefins (reviews [59]). 

B - atomization efficiency, and volume of the cavity of the graphite tube (ml), provided that the analyte diffuses slowly out of the cavity (26). 

Sheldon has introduced several indicators to measure the efficiency and environmental impact of a reaction. The atom efficiency is the molecular weight of the desired product divided by the total molecular weight of all products. For example the conventional oxidation of a secondary alcohol... 

Explain the concepts of atom efficiency and environmental friendliness. What is an E-factor Which processes usually have the highest E-factors Explain what catalysis means on the different length scales indicated in Eig. 1.8. 

Aregioselective catalytic system for the allylic substitution of non-symmetric allyl carbonates by carbon and nitrogen nucleophiles based on [ Bu N][Fe(NO)(CO)3] and PPhj was developed (Scheme 2.26). The high regioselectivity was ascribed to the slow a-allyl- to Jt-aUyl-isomerisation relative to the rate of substitution. However, the use of high excess of the pro-nucleophile and DMF solvent are drawbacks on the atom efficiency and functional group tolerance of the system. 

Telomerisation is an important, atom efficient reaction, which generates functionalised dienes from 1,3-diene feedstocks. The reaction, which typically employs a palladium based catalyst, comprises coupling two molecnles of a conjugated 1,3-diene... 

Cycloaddition reactions catalysed by transition metal complexes are an important tool in the construction of a wide range of carbo- and hetero-cyclic systems, such as benzene, pyridines, triazoles, etc. [7]. In general, these reactions are extremely atom-efficient and involve the formation of several C-C bonds in a single step. Among the innumerable possible catalytic systems for the cycloaddition reaction the NHC-metal complexes have received special attention [7c]. 

Sheldon, R.A. (2000) Atom Efficiency and Catalysis in Organic Synthesis. Pure Applied Chemistry, 72(7), 1233-1246. 

However, one category of selectivity is largely ignored by organic chemists the atom selectivity , or what is variously called atom economy (Trost, 1991, 1995), atom efficiency, or atom utilization solvents (Sheldon, 1992, 1992a, 1993, 1994, 1996, 1997, 1997a). The complete disregard of this parameter is the root cause of the waste problem in fine chemicals manufacture. 

The atom utilization or atom efficiency concept is a useful tool for rapid evaluation of the amount of waste that will be generated by alternative routes to a particular product. It is calculated by dividing the molecular weight of the desired product by the sum total of the molecular weights of all the substances produced in the stoichiometric equation of the reaction(s) in question. The comparison is made on a theoretical (i.e. 100% chemical yield) basis. Fig. 2.8 shows a simple illu.stration of the concept for ethylene oxide manufacture. 

Figure 2.8. Atom efficiencies of routes to ethylene oxide.

The classical chlorohydrin route has an atom efficiency of 25% and is better described as a calcium chloride process, with ethylene oxide as the major by-product. In other words, even if... 

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

Another elegant example of a highly atom efficient process is the use of palladium-cataly.sed amidocarbonylation for the one-step synthesis of amino acid derivatives from an aldehyde, CO, and an amide (Eqn. (10)) (Beller et al., 1997, 1999). 

In a process developed by Hoffmann La Roche (Roessler, 1996) for the anti-Parkinsonian drug, lazabemide, palladium-catalysed amidocarbonylation of 2,5-dichloropyridine replaced an original synthesis that involved eight steps, starting from 2-methyl-5-ethylpyridine, and had an overall yield of 8%. The amidocarbonylation route affords lazabemide hydrochloride in 65% yield in one step, with 100% atom efficiency (Fig. 2.22). 

The same reasoning applies to the synthesis of pure enantiomers as to organic synthesis in general processes should be atom efficient and have low E factors, i.e. involve catalytic methodologies. This is reflected in the increasing attention being focused on enantioselective catalysis, using either enzymes or chiral metal complexes. 

It is thus obvious that the direct transformation of a simple alkene into an amine would be a more economic process, since it would suppress at least one step without formahon of co-products (atom efficiency) [Scheme 4-1, paths (c) and (d)]. 

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