A Summary of Organic Reaction Mechanisms

As we said in Chapter 5, there are three fundamental reaction types polar reactions, radical reactions, and pericyclic reactions. Let s review each to see how the reactions we ve covered fit the different categories. 

Polar reactions take place between electron-rich reagents (nucleophiles/ Lewis bases) and electron-poor reagents (electrophiles/Lewis acids). These reactions are heterolylic processes and involve species with an even number of electrons. Bonds are made when a nucleophile donates an electron pair to an electrophile bonds are broken when one product leaves with an electron pair. 

The polar reactions we ve studied can be grouped into five categories  

Electrophilic Addition Reactions (Sections 6.8 and 6.9 Review Table 1, reactions la-Id, 2a-2b) 

Alkenes react with electrophiles such as HBr to 3ueld saturated addition products. The reaction occurs in two steps. The electrophile first adds to the alkene double bond to yield a carbocation intermediate, w hich reacts further to yield the addition product. 

---

A Summary of the Kinds of Organic Reactions A Summary of Organic Reaction Mechanisms Problems 653... 

Interlude A Summary of Organic Reaction Mechanisms, foUowing Chapter 14, summarizes the relatively few types of reaction mechanisms that drive the majority of organic reactions, thereby encouraging understanding over memorization. 

Over the past half-dozen years, many laboratories have focused their efforts on the development of chiral hydrogen bond donors that function as catalysts for enantioselective organic reactions. One of the earliest successes in this area came from Jacobsen and co-workers, who reported the use of peptide-like chiral urea-based catalysts for the hydrocyanation of aldimines and ketoimines [40, 41]. Several other laboratories have also reported highly enantioselective transformations catalyzed by a chiral hydrogen bond donor. The following sections provide a summary of the many developments in hydrogen bond-catalyzed enantioselective reactions, along with a discussion of mechanisms and selectivity models. 

As already mentioned, also for the other oxygenated Cl compounds, i.e. formaldehyde [118, 138-147] and methanol [148-154], as well as for larger organic molecules, dynamic instabilities are reported. Many of them are compiled in Ref. [154], for formaldehyde oxidation on Rh and Pt [147] and methanol oxidation on Pt [155] the oscillations could be clearly identified as HN-NDR type oscillations. However, in view of the number of reaction steps involved in these oxidation reactions and of the possible complexity of the interaction of the supporting electrolyte with the dynamics even in the much simpler formic acid oxidation, it is not astonishing that any quantitative considerations should still be missing. There are some attempts to qualitatively explain the observed phenomena with reaction mechanisms that go beyond the simple dual-path model described above. However, at the time being, they are quite speculative. Therefore I shall not discuss them in more detail in this article. A summary of these works can be found in [156],... 

Structure and mechanism in photochemical reactions. The reactions of geminal radical pairs created in bulk polymers are presented by Chesta and Weiss in Chapter 13. Of the many possible chemical reactions for such pairs, they are organized here by polymer and reaction type, and the authors provide solid rationalizations for the observed product yields in terms of cage versus escape processes. Chapter 14 contains a summary of the editor s own work on acrylic polymer degradation in solution. Forbes and Lebedeva show TREPR spectra and simulations for many main-chain acrylic polymer radicals that cannot be observed by steady-state EPR methods. A discussion of conformational dynamics and solvent effects is also included. 

HI is a strong reducing acid with a negative pH. Even though it is a common reagent in organic chemistry, corrosion data of materials in HI acid at elevated temperatures are limited. Table 4.6 shows a summary of the available data. Noble and refractory metals have shown low corrosion rates, but the temperatures at which the data were taken are lower than those in the Bunsen reaction environment. The corrosion mechanism of H2SO4 depends on temperature and concentration. Within the... 

The identification of compound X as an acyl nitrogen compound fits in with observed facts of ozone formation in nitrogen dioxide-organic compound photochemical reactions. A summary of the mechanism follows. 

Unfortunately, this picture got corrupted when data were shown of good ESI performance for organic bases in positive-ion mode from basic solutions. This so-called wrong-way-around electrospray indicates that yet another mechanism will be operative. Nebulization of analyte solutions was initially adopted in LC-MS to achieve a gentle transfer of neutral molecules from the liquid phase to the gas phase by soft desolvation, which is a process similar to the processes described by the charge-residue model, but now for neutral species. Gas-phase ion-molecule reactions between these neutral analyte molecules and ion-evaporated buffer ions, for instance, NH/, will also lead to protonated molecules. It appears that this gas-phase chemical ionization rather than the liquid-phase process is just another process involved in ESI. A summary of the ionization processes is given in Figure 2. 

In this section, a summary of the chemical principles involved with membrane reactors for desulfurization are overviewed. The details will be covered in the following sections. Electrochemical desulfurization technologies assisted by membranes have been extensively explored for the removal of sulfur that exists in sulfur compounds in fossil fuels and in SO2 form in flue gas. In principle, SO2 can be absorbed by an aqueous electrolyte solution and then electrochemically converted into species such as sulfate, hydrogen sulfide, and sulfur, among others, by oxidation or reduction processes, whereas the sulfur compounds in fossil fuels can be similarly removed. The universal reaction mechanism of the electrochemical cathodic reduction of organic sulfur compounds in gasoline and diesel is shown in Eqn (14.1) (Lam et al., 2012) ... 

The article is organized as follows. In the next Section we present a brief outline of the theoretical background for the present work. Section 3 contains summaries of the SC models for the electronic mechanisms of the gas-phase Diels-Alder reaction between butadiene andethene [11] and the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12]. In Section 4 we provide, for the first time, a description of the SC model for the electronic mechanism of the gas-phase disrotatory electrocyclic ring-opening of cyclohexadiene. Conclusions and final comments are presented in Section 5. 

In his summary of the mechanism in Physical Organic Chemistry , Hammett (1940a) opted for nucleophilic displacement on the groimds that the added ionic nucleophiles lead to an increase in reaction rate. However, none of the early studies precluded the possibility that the observed rate enhancement was the result of a salt effect. Recently AJbery and Bell (1961) have studied the reaction in moderately concentrated solutions of perchloric and hydrochloric acids. The rates are... 

---