Synthesis problems

The goal of URT is to obtain reflectivity images from back-scattered measurements. This consists in a Fourier synthesis problem, and the first task is to correctly cover the frequency space of the "object" r. Let for simplicity the dimension of the physical space be 2. 

How do chemists find a pathway to the synthesis of a new organic compound They try to find suitable starting materials and powerful reactions for the synthesis of the target compound. Thus, synthesis design and chemical reactions are deeply linked, since a chemical reaction is the instrument by which chemists synthesize their compounds synthesis design is a chemist s major strategy to find the most suitable procedure for a synthesis problem. 

One of the hits found in the Chem Inform reaction database is shown in the window for reaction substructure searches in Figure 10.3-55. It fits the synthesis problem perfectly, since in the synthesis direction it forms the coumarin ring system directly, in one step. 

Combinatorial. Combinatorial methods express the synthesis problem as a traditional optimization problem which can only be solved using powerful techniques that have been known for some time. These may use total network cost direcdy as an objective function but do not exploit the special characteristics of heat-exchange networks in obtaining a solution. Much of the early work in heat-exchange network synthesis was based on exhaustive search or combinatorial development of networks. This work has not proven useful because for only a typical ten-process-stream example problem the alternative sets of feasible matches are cal.55 x 10 without stream spHtting. 

As can be seen from Fig, 3.7, the pinch decomposes the synthesis problem into two regions a rich end and a lean end. The rich end comprises all streams or parts of streams richer than the pinch composition. Similarly, the lean end includes all the streams or parts of streams leaner than the pinch composition. Above the pinch, exchange between the rich and the lean process streams takes place. External MSAs are not required. Using an external MSA above the pinch will incur a penalty of eliminating an equivalent amount of process lean streams from service. On the other hand, below the pinch, both the process and the external lean streams should be used. Furthermore, Fig. 3.7 indicates that if any mass is transferred across the pinch, the composite lean stream will move upward and, consequently, external MSAs in excess of the minimum requirement will be used. Therefore, to minimize the cost of external MSAs, mass should not be transferred across the pinch. It is worth pointing out that these observations are valid only for the class of MEN problems covered in this chapter. When the assumptions employed in this chapter are relaxed, more general conclusions can be made. For instance, it will be shown later that the pinch analysis can still be undertaken even when there are no process MSAs in the plant. The pinch characteristics will be generalized in Chapters Five and Six. 

Unlike this example, some cases involve the construction of less intuitively apparent MEN configurations. Chapters Five and Six provide systematic rules for matching streams and configuring the network. Furthermore, many MEN synthesis problems require the screening of multiple external MS As. This issue is addressed by the next example. 

Having identified the values of all the flowrates of lean streams as well as the pinch location, we can now minimize the number of mass exchangers for a MOC solution. As has been previously mentioned, when a pinch point exists, the synthesis problem can be decomposed into two subnetworks, one above the pinch and one below the... 

Figure 8.1 Schematic representation of the REAMEN synthesis problem.

The CHARMEN synthesis problem can be stated as follows Given a number Nr of waste (rich) streams and a number Ns of lean streams (frtiysical and reactive MSAs), it is desired to synthesize a cost-effective network of physical and/or reactive mass exchangers which can preferentially transfer certain undesirable species from the waste streams to the MSAs. Given also are the flowrate of each waste stream, G,, its supply (inlet) composition, yf, and target (outlet) composition, y/, and the supply and target compositions, Xj and jc for each MSA. In addition, available for service are hot and cold streams (process streams as weU as utilities) that can be used to optimize the mass-exchange temperatures. 

TherefOTe, the CHARMEN synthesis problem can be formulated by combining the HEN formulation of Section 9.1 and the REAMEN synthesis equations developed in Chapter Eight after adjustment to incorporate the notion of substreams. For instance, the cost of MSAs can be expressed as follows ... 

Figure tO.l Schematic representation of the HISEN synthesis problem. 

The MEN software is based on the information described in Chapters Five and Six for developing algebraic and optimization-based solutions for the MEN synthesis problem. It can generate composition-interval diagrams, tables of exchangeable loads and optimization formulations for minimizing cost of MSAs. 

When undertaking any synthesis problem, you should look at the product, identify the functional groups it contains, and then ask yourself how those functional groups can be prepared. Alwaj7s work in a retrosynthetic sense, one step at a time. 

If you think some of the synthesis problems at the end of this chapter are hard, try devising a synthesis of vitamin B12 starting only from simple substances you can buy in a chemical catalog. This extraordinary achievement was reported in 1973 as the culmination of a collaborative effort headed by Robert B. Woodward of Harvard University and Albert Eschenmoser of the Swiss Federal Institute of Technology in Zurich. More than 100 graduate students and postdoctoral associates contributed to the w ork, which took more than a decade. 

One of the surest wavs to learn organic chemistry is to work synthesis problems. The ability to plan a successful multistep synthesis of a complex molecule requires a working knowledge of the uses and limitations of a great many organic reactions. Not only must you know which reactions to use, you must also know when to use them because the order in which reactions are carried out is often critical to the success of the overall scheme. 

During our previous discussion of strategies for working synthesis problems in Section 8.9, we said that ids usually best to work a problem backward, or retrosyntheticnlly. Look at the target molecule and ask yourself, "What is an immediate precursor of this compound " Choose a likely answer and continue working backward, one step at a time, until you arrive at a simple starting material. Let s try some examples. 

Sheehan s concentrated attack upon the penicillin synthesis problem began in 1948 and was conducted on a broad front. It was anticipated at the outset that the formidable penicillin V molecule would succumb to organic synthesis only in the event that new powerful and selective methods of organic synthesis are brought to bear on the problem. But, in addition, and perhaps more importantly, these new synthetic methods must be mild enough to contend with... 

Tsaparlis, G., Angelopoulos, V. (2000) A model of problem solving its operation, validity and usefulness in the ease of organie-synthesis problems. Science Education, 84, 131-153. 

In order to begin practicing synthesis problems, it is absolutely essential that you master all of the individual reactions that we have seen so far. You must learn how to walk before you can start to run. Therefore, we will first focus on one-step synthesis problems. Once you feel comfortable with the individual reactions, then we can start stringing them together in various sequences to form synthesis problems. 

Both H and R can attack a ketone or aldehyde to give an alcohol. The main difference is the effect on the carbon skeleton. With H, the carbon skeleton does not change at all. But with R, the carbon skeleton gets larger. We are forming a C—C bond. We will soon see that this is very important for synthesis problems. For now, let s focus on how we can make R in the first place. After all, a negative charge on a carbon atom is not very stable (and therefore not trivial to make). 

This problem illustrates an important point we have seen two perfectly correct answers to this problem. In fact, from now on, we will rarely encounter synthesis problems that have only one solution. More often, we will find synthesis problems that have more than one acceptable answer. 

When the reagents are not shown, then you have a synthesis problem ... 

It can only take three or fonr steps before the problem can get quite difficult. If you convert the sequence above into a synthesis problem, it would look like this ... 

If you are having tronble with synthesis problems when you first encounter them, the worst thing you can do is to give up and say Oh, well. I m not good at synthesis problems. As the conrse moves on, this attitnde will slowly kill your grade in the course. To see why this is so, let s compare organic chemistry to a game of chess. 

There are a few techniques that will make you feel more comfortable with synthesis problems, and there are exercises that you can go through to increase your proficiency in doing synthesis problems. That s what this chapter is aU about. 

As we mentioned earlier, one-step syntheses are the first synthesis problems you will encounter. They will never be more difficult than predicting products. Before you can move on to multistep syntheses, you first need to feel comfortable with one-step syntheses. 

For now, skip forward a few pages. We have some techniques to go over that will help you solve synthesis problems. 

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