A Mechanism for an Organic Reaction

In this reaction the electron pair of the tt bond of the alkene is used to form a bond between one carbon of the alkene and the proton donated by the strong acid. Notice that two bonds are broken in this process the tt bond of the double bond and the bond between the proton of the acid and its conjugate base. One new bond is formed a bond between a carbon of the alkene and the proton. This process leaves the other carbon of the alkene trivalent, electron deficient, and with a formal positive charge. A compound containing a carbon of this type is called a carbocation (Section 3.4). As we shall see in later chapters, carbocations are unstable intermediates that react further to produce stable molecules. 

It is a general rule that any organic compound containing oxygen, nitrogen, or a multiple bond will dissolve in concentrated sulfuric acid. Explain the basis of this rule in terms of acid-base reactions and intermolecular forces. 

In Chapter 6 we shall begin our study of organic reaction mechanisms in earnest. Let us consider now one mechanism as an example, one that allows us to apply some of the chemistry we have learned in this chapter and one that, at the same time, will reinforce what we have learned about how curved arrows are used to illustrate mechanisms. 

Dissolving tert-bvXy alcohol in concentrated (coned) aqueous hydrochloric acid soon results in the formation of tert-hvXy chloride. The reaction is a substitution reaction  

That a reaction has taken place is obvious when one actually does the experiment. t rt-Butyl alcohol is soluble in the aqueous medium however, tert-huXyl chloride is not, and consequently it separates from the aqueous phase as another layer in the flask. It is easy to remove this nonaqueous layer, purify it by distillation, and thus obtain the tert-huXy chloride. 

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Yet, in order to write a mechanism for an organic reaction, not all topics need to be understood to the same level. For example, thermodynamics and kinetics... 

The reactions in this chapter are organized according to the overall transformation achieved. An organization by mechanistic type (insertions, transmetallations, etc.) was considered and rejected. The purpose of this text is to teach the student how to go about drawing a mechanism for an unfamiliar reaction. A student who does not already know the mechanism for a reaction will find it easier to narrow down the possible mechanisms by considering the overall transformation rather than by trying to determine the mechanistic type. An organization by metal (early, middle, or late) was also considered and rejected, because it would obscure the important similarities between the reactions of different metals. 

Fig. 1 Typical redox reaction mechanism for an organic cathode material with a caibonyl redox center...

Nevertheless, there are many instances where electrochemical corrosion mechanisms may play a primary role in affecting the service performance of bonded joints. It should be noted that such mechanisms of attack involve both the presence of (a) anodic sites, where reaction with the metallic substrate occurs and electrons are generated, and (b) cathodic sites, where the electrons are consumed. The major reaction leads to the generation of hydroxyl ions, and the liquid present at these sites will become strongly basic and so possess a relatively high pH. Thus, typically an aqueous (electrolyte) layer needs to be present, since, without such an aqueous film, no electrical current can flow from the anodic to the cathodic sites. These aspects are illustrated, for example, by the schematic electrochemical corrosion mechanism for an organic coating on a steel substrate shown in Fig. 4, which is discussed in detail in Section 2.S.2.2. 

One of the most popular methods for studying the mechanism of an organic reaction is to investigate the influence of electronic factors on the rate of the reaction. The method was developed by L. P. Hammett and originated from considering the acidity constants for a series of benzoic acids with different groups in the para or meta positions (Scheme 8.1) [1], and is now generally referred to as a Hammett study [2]. 

It is now useful to show how steady-state solutions can be obtained in several other cases. We shall also present two shortcuts to simplify the algebraic steps in the derivations for complicated situations. The first example is the S>j 1 mechanism for the substitution reaction of an organic halide RX by a nucleophile Y ... 

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

The introduction of a substituent in an organic compound may affect its reactivity in a given reaction. A number of quantitative relationships have been suggested in connection with the effect of substituents on the rate constant of the reaction. Such structure-reactivity co-relations are helpful in predicting the reactivity of organic compounds in various reactions and also in verifying the reaction mechanism. One such useful relationship was proposed by Hemmett, which relates the equilibrium and rate constants for the reaction of meta and para substituted benzene derivatives. 

Several heme-containing proteins, including most peroxidases 12), have been observed to exhibit a low level of catalatic activity, with the chloroperoxidase from Caldariomyces fumago exhibiting the greatest reactivity as a catalase (13-15). Despite the fact that there is as yet only one such example to consider, it provides an alternate mechanism for the catalatic reaction and is addressed in this review. It was first characterized for its ability to chlorinate organic substrates in the presence of chloride and hydrogen peroxide at acid pH, but was later found... 

The fact that there are two different and independent mechanisms controlling product distributions - thermodynamic and kinetic - is why some chemical reactions yield one distribution of products under one set of conditions and an entirely different distribution of products under a different set of conditions. It also provides a rationale for why organic chemists allow some reactions to cook for hours while they rush to quench others seconds after they have begun. 

The mechanism that accounts for the oleofinic oxidation by hydroxyl radicals is the hydrogen abstraction. Moreover Cl radicals may also be an important mechanism for chlorinated organics. When the C-Cl bond is broken by photolysis, a Cl radical is released and can initiate additional oxidation reactions through a chain mechanism as follows ... 

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