Reactions and Their Mechanisms

Virtually all organic reactions fall into one of four categories substitutions, additions, eliminations, or rearrangements. 

Substitutions are the characteristic reactions of saturated compounds such as alkanes and alkyl halides and of aromatic compounds (even though they are unsaturated). In a substitution, one group replaces another. For example, chloromethane reacts with sodium hydroxide to produce methyl alcohol and sodium chloride  

In this reaction a hydroxide ion from sodium hydroxide replaces the chlorine of methyl chloride. We shall study this reaction in detail in Chapter 6. 

Additions are characteristic of compounds with multiple bonds. Ethene, for example, reacts with bromine by an addition. In an addition all parts of the adding reagent appear in the product two molecules become one  

Eliminations are the opposite of additions. In an elimination one molecule loses the elements of another small molecule. Elimination reactions give us a method for preparing compounds with double and triple bonds. In Chapter 7, for example, we shall study an important elimination ealled dehydrohalogenation, a reaction that is used to prepare alkenes. In dehydrohalogenation, as the word suggests, the elements of a hydrogen halide are eliminated. An alkyl halide becomes an alkene  

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In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

In the last chapter we saw the importance of nnderstanding mechanisms. We said that mechanisms are the keys to understanding everything else. In this chapter, we wiU see a very special case of this. Students often have difficulty with substitution reactions—specifically, being able to predict whether a reaction is an Sn2 or an SnI. These are different types of substitution reactions and their mechanisms are very different from each other. By focusing on the differences in their mechaiusms, we can understand why we get Sn2 in some cases and SnI in other cases. 

The use of an electron-rich trivalent phosphorus center for addition to or substitution at an electrophilic site is a long-established approach to the formation of carbon-phosphorus bonds. The classical studies of the Michaelis-Arbuzov, Michaelis-Becker, Abramov, Pudovik, and related reactions and their mechanisms and synthetic utilities have been thoroughly reviewed. In this chapter, we present only a brief introduction to these reactions and provide several examples of their more facile uses from the older literature. More attention is given to relatively recent developments regarding such reactions that are seen as improvements in their general utility. 

This review article summarizes the broad area of electroorganic synthesis, (selected electroorganic synthetic reactions, with a special emphasis on those that have been commercialized or investigated in pilot plants) and selected applications of electrochemical techniques for waste-water and effluent treatment. There are a number of modern textbooks and updated reviews [4-53] of electroorganic chemistry that include much more detail on organic reactions and their mechanisms than it is appropriate to discuss here. 

In developing models for treating chemical reactivity we have been guided by the concepts used by the organic chemist in discussing the causes of organic reactions and their mechanisms. Examples of the more prominent effects are shown in Figure 2. 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Organic chemistry, especially Organic 11, is all about the reactions and their mechanisms. If you don t know the reactions, you can t pass. You need to know the reactions by name, the reactants, the products, and the conditions, along with the reaction mechanism. Practice, practice, practice. Use flash cards. Quiz each other in your study groups. Write, write, write. Group reactions by product, by type, and so on. Do them forward and backward. Know those reactions ... 

Numerous glucocorticosteroids for topical application are available. Essentially they all suppress the symptoms of inflammatory and hypersensitivity reactions and their mechanism of action is similar. Their indications include seborrhoeic and atopic dermatitis, phototoxic reactions, psoriasis, chronic discoid lupus, hypertrophic lichen planus and alopecia areata. However it has to be kept in mind that the use of corticosteroids for these conditions in most cases only gives symptomatic relieve and that the problem tends to recur on cessation of therapy. Traditionally topical corticosteroid formulations are grouped according to approximate relative efficacy. This efficacy is determined by both the potency of the agent and the concentration in which the corticosteroid is used. 

A. A. Frimer. Chemical Reviews, volume 79, American Chemical Society (1979), p. 3S9. The reactions and their mechanisms of alkenes with singlet oxygen are the subjects of this review. 

With chemical model reactions, valuable insights can be gained about enzymatic reactions and their mechanisms. In most cases, modeling of the active center is the main target, so that simple model compounds for the decisive groups in and around the active site are sought, with which mechanism, substrate breadth, or other reaction parameters can be tested. 

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