Planning a reaction

Before chemists can perform a reaction, just as in any profession, they need to plan exactly what they are going to do. If you were to ask practising chemists, they would all agree that time spent in planning a reaction is time well spent, and invaluable to the success of the experiment. 

What are the major points which you should consider when planning a reaction  

A list of most of the questions and points is given below. 

You also have to consider what you are trying to achieve during the reaction. Is the reaction probing some detailed reaction mechanism or is it preparatory — in other words, part of a long synthesis directed towards a desired product. An analytical chemist investigating a mechanism will have a very different set of priorities in planning a reaction compared to a synthetic chemist. 

Notice the style and the various conventions that are used. The aim of the experiment and the equation for the reaction are set out clearly at the start, followed by the method and finally the results. A note is also made of any safety precautions necessary. Note that amounts of substances are placed in brackets after the compounds they refer to and are given in grams (or mis if the compound is a liquid) and also (preferably) in numbers of moles this is conventional for formal reports and publications, so you may as well get used to it from the start. 

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Now read through Examples 2.1, 2.2 and 2.3 again and jot down some of the problems that organic chemists must tackle when planning a reaction. 

By now you should be able to see that in order to plan a reaction, even a fairly simple one, quite a lot of detailed knowledge might be required. In order to carry out a reaction successfully, it is necessary to understand how and why that reaction takes place. It is necessary to know about the reaction on a molecular level to know as much as possible about the detailed sequence of elementary reactions followed by the reactant molecules as they are converted into product molecules. As you saw in Part 1, this is called the reaction mechanism. 

What is required from a preparative viewpoint is a means of obtaining 100% elimination when an alkene is required, and 100% substitution when the substituted product is required. In practice this ideal situation can rarely be obtained, but there are guidelines that can be applied to maximize the yield of the desired product. A detailed discussion of the factors affecting the ratio of elimination to substitution is beyond the scope of this book. What we will do is discuss some of the more important points, and leave you with the set of rules that practising chemists apply when planning a reaction. 

There are molecules that do not possess a chiral center but generate a product with a chiral center after a chemical reaction. In terms of planning a reaction, it is important that we predict whether the product will have an iR)- or an (5)-configuration. To make this prediction, we must know from which face of the molecule the reagent will approach during the reaction. If it approaches from one face, the () )-enantiomer is generated if it approaches from the opposite face, the product is the (5)-enantiomer. 

Plan A reaction occurs if the reactant that is a metal in its elemental form (Mg) is located above the reactant that is a metal in its oxidized form (Fe ) in Table 4.5. If the reaction occurs, the Fe ion in FeCl2 is reduced to Fe, and the Mg is oxidized to Mg. ... 

Number of reaction steps in a synthesis plan a reaction step refers to the interval between a given isolated intermediate and the next consecutive isolated intermediate in a synthesis plan. 

When planning a reaction with an ester and an alkoxide ion it is important to use an alkoxide that has the same alkyl group as the alkoxyl group of the ester. 

Nevertheless, chemists have been planning their reactions for more than a century now, and each day they run hundreds of thousands of reactions with high degrees of selectivity and yield. The secret to success lies in the fact that chemists can build on a vast body of experience accumulated over more than a hundred years of performing millions of chemical reactions under carefully controlled conditions. Series of experiments were analyzed for the essential features determining the course of a reaction, and models were built to order the observations into a conceptual framework that could be used to make predictions by analogy. Furthermore, careful experiments were planned to analyze the individual steps of a reaction so as to elucidate its mechanism. 

This is a question of reaction planning. To answer such a problem reagents and reaction conditions have to be found that allow one to perform the desired transformation. Such a question is best answered by a query into a reaction database (see Section 5.12). 

Sulfonate esters are subject to the same limitations as alkyl halides Competition from elimination needs to be considered when planning a functional group transforma tion that requires an anionic nucleophile because tosylates undergo elimination reactions just as alkyl halides do... 

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. 

The ability to plan a sequence of reactions in the right order is particularly valuable in the synthesis of substituted aromatic rings, where the introduction of a new substituent is strongly affected by the directing effects of other substituents. Planning syntheses of substituted aromatic compounds is therefore an excellent way to gain confidence using the many reactions learned in the past few chapters. 

Problem 16.24 In planning a synthesis, it s as important to know what not to do as to know what to do. As whiten, the following reaction schemes have flaws in them. What is wrong with each ... 

Is the Experiment Properly Planned to Provide Significant Data One cannot directly measure a reaction rate. One is restricted to measurements of the concentrations of various species or of a physical property of the system as a function of time. Thus one must plan the experiments so as to obtain significant differences in the quantities that are observed in the laboratory. In a properly planned experiment the... 

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