Types of organic reaction

1 Types of organic reactions - key organic reaction types 

Towards the end of Chapter 10 we introduced a classic example of a nucleophilic substitution reaction, namely the hydrolysis of a halogenoalkane by a warm aqueous solution of sodium hydroxide. As a result of the polarization of the carbon-halogen bond, the carbon atom is an electron-deficient centre susceptible to attack by a nucleophile such as the hydroxide ion (OH ). Primary halogenoalkanes are thought to undergo a substitution mechanism that involves a single reaction step. This one-stage reaction involves the simultaneous attack of the nucleophile and departure of the halide ion. We will use as an example the reaction between bromomethane and sodium hydroxide solution  

Kinetic studies show that this reaction is a single-step reaction in which the halc enoalkane and hydroxide ion are both involved. The rate expression for the reaction is found experimentally to be  

In addition to hydroxide ions, other common nucleophiles are water molecules, ammonia molecules, amine molecules and cyanide ions. Each of these nucleophiles reacts to replace the halogen atom, which leaves as the halide ion. 

The singie-step mechanism envisaged for an Snj2 reaction 

Displacement (substitution). An atom or group of atoms in a molecule or ion is replaced by another atom or group. 

Addition. Two molecules combine to yield a single molecule. Addition frequently occurs at a double or triple bond and sometimes at small-size rings. 

Elimination. This is the reverse of addition. Two atoms or groups are removed from a molecule. If the atoms or groups are taken from adjacent atoms (-elimination), a multiple bond is formed if they are taken from other than adjacent atoms, a ring results. Removal of two atoms or groups from the same atom (a-elimination) produces a carbene. 

Rearrangement. Bonds in the molecule are scrambled, converting it to its isomer. 

Oxidation-reduction (redox). These reactions involve transfer of electrons or change in oxidation number. A decrease in the number of H atoms bonded to C and an increase in the number of bonds to other atoms such as C, O. N, Cl. Br. F. and S signals oxidation. 

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Research on ligand effects in aqueous solution has mainly focused on two types of organic reactions ... 

Boron trifluoride [7637-07-2] (trifluoroborane), BF, was first reported in 1809 by Gay-Lussac and Thenard (1) who prepared it by the reaction of boric acid and fluorspar at duU red heat. It is a colorless gas when dry, but fumes in the presence of moisture yielding a dense white smoke of irritating, pungent odor. It is widely used as an acid catalyst (2) for many types of organic reactions, especially for the production of polymer and petroleum (qv) products. The gas was first produced commercially in 1936 by the Harshaw Chemical Co. (see also Boron COMPOUNDS). 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

Organic chemical reactions can be organized broadly in two ways—by what kinds of reactions occur and by how those reactions occur. Let s look first at the kinds of reactions that take place. There are four general types of organic reactions additions, eliminations, substitutions, and reammgements. 

Although the discussion to this point has been concerned with the explanation of the behavior of Bronsted acids as catalysts, there is an enormous range of reactions in which catalysis by acids and bases occurs. Many of the important types of organic reactions involve catalysis by acids or bases. In this section, several reactions will be mentioned, but the mechanistic details will not be presented in this book on inorganic chemistry. The discussion is intended to show the scope of catalysis by acids and bases. 

As has already been mentioned, boron halides are electron-deficient molecules. As a result, they tend to act as strong Lewis acids by accepting electron pairs from many types of Lewis bases to form stable acid-base adducts. Electron donors such as ammonia, pyridine, amines, ethers, and many other types of compounds form stable adducts. In behaving as strong Lewis acids, the boron halides act as acid catalysts for several important types of organic reactions (see Chapter 9). 

The global parameters help understanding the behavior of a system and lead to applicable and useful principles such as the principle of maximum hardness (MHP) [1], In this chapter, however, our main focus is to introduce the working formula of local reactivity parameters, their actual computations, and practical ways of application to different types of organic reactions. In this process, we mention briefly some of the relevant global reactivity parameters and their calculations as well just to have continuity in the subject matter. 

Thus we can say that in the whole field of cationic polymerization there are still very few studies which prove unambiguously the participation of ions as chain-carriers, that there are many more which provide strong circumstantial evidence, and that the theory has been accepted so widely in large measure because it fits in well with the current views on many types of organic reactions. This brief review of some important lines of evidence concerning the nature of the chain-carrier in cationic polymerizations has been occasioned by the new findings, to be described in subsection 2.2, which have made it necessary to reappraise the whole question. 

There are a number of important types of organic reactions, including combustion, substitution, addition (such as hydrogenation, and halogenation.), condensation, as well as many others. 

What do these reactions have in common They are both examples of organic reactions. In this section, you will take a quick look at the main types of organic reactions. You will concentrate on simply recognizing these types of organic reactions. In the next section, you will examine the reactions of specific functional groups and learn how to predict the products of organic reactions. 

Addition reactions, substitution reactions, and elimination reactions are the three main types of organic reactions. Most organic reactions can be classified as one of these three types. 

The following Sample Problems show how to identify different types of organic reactions. 

Oo In your own words, describe each type of organic reaction. Include an example for each type. 

This type of organic reaction is important and deserves some comments. The addition of one electron to a molecnle generates an anion-radical. This results in an increase in its reactivity. Particularly, the bond dissociation energies in anion-radicals are much smaller than those in the corresponding nentral molecnles. Thns, snbstitntion reactions proceed more easily in the case of anion-radicals. The reactions of the type are good examples of this feature. Two possible schemes (a andb) for the reaction conrse are listed as follows ... 

The complex ion catalyzes various types of organic reactions including oxidation of ethylene to acetaldehyde in aqueous solution (the Wacker Process) ... 

The presence of radical anions as intermediates has been clearly established in many organic reactions and has been the subject of theoretical studies due to their importance in different types of organic reactions including DNA dam-agei8t,i82 jn these intermediates, the it-acceptor can be linked to the C-X bond... 

The rationale of classification by reaction types is that different functional groups may show the same kinds of reactions. Thus, as we have just seen, alcohols, carboxylic acids, and amines all can accept a proton from a suitably strong acid. Fortunately, there are very few different types of organic reactions — at least as far as the overall result that they produce. The most important are acid-base, substitution, addition, elimination, and rearrangement reactions. Some examples of these are given below, and you should understand that these are descriptive of the overall chemical change and nothing is implied as to how or why the reaction occurs (also see Section 1-11). 

Due to high efficiency and in some cases even enantioselectivity, solid state reactions have recently attracted considerable attention to various types of organic reactions [194-197]. The solid-state Michael reactions between 4-arylidene-3-methyl-l-phenyl-5-pyrazolones 267a-f and indole gave rea-... 

The topic of catalysis with Nafion has recently been reviewed in detail (56). Apart from using Nafion-H primarily as a solid superacid catalyst, a number of reports have described the use of functionalized Nafion derivatives by metal cation exchange to achieve various types of organic reaction. These include a bifunctional catalyst (acid and cation site), a heterogeneous perfluorosulfonate salt (only cation sites), and a trifunctional... 

Because the activation energies are small and the stereoelectronic requirements are not difficult to meet, most acid-base reactions are very fast in comparison to other types of organic reactions. Therefore, it is usually not necessary to be concerned with the rates of acid-base reactions. In organic reactions that have mechanisms involving several steps, including an acid-base step, one of the other steps in the mechanism usually controls the rate. 

Several other photocatalytic environmentally friendly processes may also be considered. Photocatalytic synthesis of chemicals may serve as a clean alternative route to traditional synthetic methods (see section 6.5). Palmisano et al. reviewed various types of organic reactions that could be achieved by heterogeneous photocatalysis... 

Schomacker compared the use of nonionic microemulsions with phase transfer catalysis for several different types of organic reactions and concluded that the former was more laborious since the pseudo-ternary phase diagram of the system had to be determined and the reaction temperature needed to be carefully monitored [13,29]. The main advantage of the microemulsion route for industrial use is related to the ecotoxicity of the effluent. Whereas nonionic surfactants are considered relatively harmless, quaternary ammonium compounds exhibit considerable fish toxicity. 

With only six electrons in the positive carbon s valence shell, a carbocation is a powerful electrophile (Lewis acid), and it may react with any nucleophile it encounters. Like other strong acids, carbocations are unlikely to be found in basic solutions. Carbocations are proposed as intermediates in many types of organic reactions, some of which we will encounter in Chapter 6. 

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