Desired products

Consider the process illustrated in Fig. 1.2. The process requires a reactor to transform the FEED into PRODUCT (Fig. 1.2a). Unfortunately, not all the FEED reacts. Also, part of the FEED reacts to form BYPRODUCT instead of the desired PRODUCT. A... 

Since process design starts with the reactor, the first decisions are those which lead to the choice of reactor. These decisions are among the most important in the whole design. Good reactor performance is of paramount importance in determining the economic viability of the overall design and fundamentally important to the environmental impact of the process. In addition to the desired products, reactors produce unwanted byproducts. These unwanted byproducts create environmental problems. As we shall discuss later in Chap. 10, the best solution to environmental problems is not elaborate treatment methods but not to produce waste in the first place. 

In general terms, if the reaction to the desired product has a higher order than the byproduct reaction, use a batch or plug-flow reactor. If the reaction to the desired product has a lower order than the byproduct reaction, use a continuous well-mixed reactor. 

By considering only those raw materials which undergo reaction to undesired byproduct, only the raw materials costs which are in principle avoidable are considered. Those raw materials costs which are inevitable (i.e., the stoichiometric requirements for FEED which converts into the desired PRODUCT) are not included. Raw materials costs which are in principle avoidable are distinguished from those which are inevitable from the stoichiometric requirements of the reaction. ... 

The graph gives the yields that the refiner would obtain at the outlet of the atmospheric distillation unit allowing him to set the unit s operating conditions in accordance with the desired production objectives. 

This highly flexible process allows the best optimization of yields in desirable products and it features a high degree of selectivity. 

The metal envelope of a metal-ceramic tube can be adapted to tbe shape desired. Production is standard technology. 

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

We have seen (Section I) that there are two types of loops that are phase inverting upon completing a round hip an i one and an ip one. A schematic representation of these loops is shown in Figure 10. The other two options, p and i p loops do not contain a conical intersection. Let us assume that A is the reactant, B the desired product, and C the third anchor. In an ip loop, any one of the three reaction may be the phase-inverting one, including the B C one. Thus, the A B reaction may be phase preserving, and still B may be attainable by a photochemical reaction. This is in apparent contradiction with predictions based on the Woodward-Hoffmann rules (see Section Vni). The different options are summarized in Figure 11. 

Figure 11. Three typical loops for the case where A is the reactant and B—the desired product. Loops in which a conical intersection may be found are (a) and (c). A loop that does not encircle a conical intersecdon is (h). In loop (a) the A B reacdon is phase inverting, and in loops (b) and c) it is phase preserving.

I want to transform a given starting material. A, into a desired product, P how can 1 do this (Figure 10.3-la) ... 

Solid organic compounds when isolated from organic reactions are seldom pure they are usually contaminated with small amounts of other compounds ( impurities ) which are produced along with the desired product. Tlie purification of impure crystalline compounds is usually effected by crystallisation from a suitable solvent or mixture of solvents. Attention must, however, be drawn to the fact that direct crystallisation of a crude reaction product is not always advisable as certain impurities may retard the rate of crystallisation and, in some cases, may even prevent the formation of crystals entirely furthermore, considerable loss of... 

The theoretical yield in an organic reaction is the amount which would be obtained under ideal conditions if the reaction had proceeded to completion, i.e., if the starting materials were entirely converted into the desired product and there was no loss in isolation and purification. The yield (sometimes called the actual yield) is the amount of pure product which is actually isolated in the experiment. The percentage yield is... 

In order to obtain an improved yield of the desired product, an excess over the proportion required by the chemical equation of one (or more) of the reactants is often used. In a given preparation, the selection of the reagent to be employed in excess will depend upon a number of factors these include its relative cost and ease of removal after the reaction, and... 

Following the isolation of a desired product. The isolation of a desired substance by a purification procedure such as distillation or chromatography may be followed by a determination of the infraied spectrum. It is not essential to know what the compound is in this... 

A suspension of 3.90 g (19.6 mmol) of p-(bromomethyl)benzaldehyde (2.8) and 4.00 g (31.7 mmol) of sodium sulfite in 40 ml of water was refluxed for two hours, after which a clear solution was obtained. The reaction mixture was cooled on an ice bath resulting in precipitation of some sodium sulfite. After filtration, the solvent was evaporated. Ethanol was added to the remaining solid and the suspension was refluxed for 10 minutes. After filtering the hot solution, the filtrate was allowed to cool down slowly to -18 °C whereupon sodium (p-oxomethylphenyl)methylsulfonate (2.9) separated as colourless crystals. The extraction procedure was repeated two more times, affording 2.29 g (10.3 mmol, 53%) of the desired product. H-NMR (200 MH D2O) 5(ppm) =4.10 (s,2H) 7.44 (d,2H) 7,76 (d,2H) 9.75 (s,lH). 

These programs systematically determine which bonds could be broken or formed in order to obtain the desired product. This results in generating a very large number of possible synthesis paths, many of which may be impossible or impractical. Much work has been done to weed out the unwanted synthesis routes. One major strength of this technique is that it has the capacity to indicate previously unexplored reactions. 

A second scheme uses a database of known chemical reactions. This more often results in synthesis routes that will work. However, this occurs at the expense of not being able to suggest any new chemistry. This method can also give many possible synthesis routes, not all of which will give acceptable yield or be easily carried out. The quality of results will depend on the database of known reactions and the means for determining which possible routes are best. These are often retro synthetic algorithms, which start with the desired product and let the researcher choose from a list of possible precursors. 

Note 1. If commercial BuLi in hexane is used with diethyl ether or THF as cosolvent, a dark brown reaction mixture is formed, from which the desired product can be isolated in lower yields. 

An alternative procedure involves use of alkyl nitrites and traps the desired product as an acetal[16],... 

Two steps take place in the synthesis, as in the case of trimethine dyes. The first intermediate is a 2-anilinobutadieny] thiazolium, which then reacts with a second mole of 2-methylthiazolium salt or another molecule of a different ring according to the desired product either a symmetrical or asymmetrical dye (method B). 

The conversion of esters to hydrazides and of hydrazides to the sulfonyl derivatives occurs in good yield in the McFadyen-Stevens synthesis, but the decomposition of sulfonyl derivatives gives low yields of the desired products, for example, thiazole hydrazide (28) with 10% excess of PhSOjCl in pyridine gave a 75% yield of l-phenylsulfonyl-2-(4-methyl-5-thiazo ecarbonyl)hydrazine (29) (66). The Newman-Caflish modification of the McFadyen-Stevens synthesis gave 37% 4-methyl-5-thiazole-carboxaldehyde (30) (Scheme 27). 

The ability to recognize when oxidation or reduction occurs is of value when decid mg on the kind of reactant with which an organic molecule must be treated to convert It into some desired product Many of the reactions to be discussed m subsequent chap ters involve oxidation-reduction... 

Begin by asking the question What kind of compound is the target molecule and what methods can I use to prepare that kind of compound The desired product has a bromine and a hydroxyl on adjacent carbons it is a vicinal bromohydrin The only method we have learned so far for the preparation of vicinal bromohydrms involves the reaction of alkenes with Bi2 m water Thus a reasonable last step is... 

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