Synthetic Reactions We Can Do So Far

Although there are relatively few groups we can attach to a benzene ring directly, we can often transform one group into another through chemical reactions. It is nitrobenzene that is the key to many of these transformations. Nitrobenzene itself is relatively unre-active, but the nitro group can be reduced in many ways to the amino group (Fig. 14.49). This transformation opens the way for the synthesis of a host of new molecules. 

Aminobenzene, generally known by its common name aniline (p.243), can be treated wath nitrous add (HONO) or its sodium salt (Na ONO) and HCl, to produce benzenediazonium chloride, which is explosive when dry, but a relatively safe material when wet (Rg. 14.50). A molecule of the structure R—N2 is called a diazomum ion. We win soon use them in a number of important reactions. 

FIGURE 14.50 Reaction of aniline with nitrous acid to give benzenediazonium chloride. 

Aniline derivatives are used to make azo dyes. Almost all dyes, including azo dyes and these natural dyes from the Fez Tannery in Morocco, are colored because they contain highly conjugated aromatic amines. 

PROBLEM 14.15 Draw resonance forms for benzenediazonium chloride. 

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Let US begin with a few examples in which we predict reactivity on mechanistic grounds. Then we shall turn to synthesis—the making of molecules. How do chemists develop new synthetic methods, and how can we make a target molecule as efQciently as possible The two topics are closely related. The second, known as total synthesis, usually requires a series of reactions. In studying these tasks, therefore, we will also be reviewing much of the reaction chemistry that we have considered so far. 

However, the question must always be asked as to whether these processes could have taken place on the primordial Earth in its archaic state. The answer requires considerable fundamental consideration. Strictly speaking, most of the experiments carried out on prebiotic chemistry cannot be carried out under prebiotic conditions , since we do not know exactly what these were. In spite of the large amount of work done, physical parameters such as temperature, composition and pressure of the primeval atmosphere, extent and results of asteroid impacts, the nature of the Earth s surface, the state of the primeval ocean etc. have not so far been established or even extrapolated. It is not even sure that this will be possible in the future. In spite of these difficulties, attempts are being made to define and study the synthetic possibilities, on the basis of the assumed scenario on the primeval Earth. Thus, for example, in the case of the SPREAD process, we can assume that the surface at which the reactions occur could not have been an SH-containing thiosepharose, but a mineral structure of similar activity which could have carried out the necessary functions just as well. The separation of the copy of the matrix could have been driven by a periodic temperature change (e.g., diurnal variation). For his models, H. Kuhn has assumed that similar periodic processes are the driving force for some prebiotic reactions (see Sect. 8.3). 

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