Introduction to Addition Reactions

If necessary, review the suggested sections to prepare for this chapter. 

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In the previous chapter, we learned how to prepare alkenes via elimination reactions. In this chapter, we will explore addition reactions, common reactions of alkenes, characterized by the addition of two groups across a double bond. In the process, the pi (it) bond is broken  

Some addition reactions have special names that indicate the identity of the two groups that were added. Several examples are listed in Table 9.1. 

TABLE 9.1 SOME COMMON TYPES OF ADDITION REACTIONS 

Because the C-C 7t bond of an alkene is much weaker than a C-C 7 bond, the characteristic reaction of alkenes is addition the n bond is broken and two new a bonds are formed. 

The red electron-rich region of the n bond is located above and below the plane of the molecule. Because the plane of the alkene depicted in this electrostatic potential plot is tipped, only the red region above the molecule is visible. 

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This chapter concludes a survey of the reactions of carbonyl compounds which started way back in Chapter 6 with an introduction to addition reactions at the C=0 group, and moved on through the following stages ... 

Introduction to Addition Reactions Reactions of Alkenes—The characteristic reaction of alkenes is the addition of one substituent to each of the carbon atoms in the double bond (reaction 27.7). A variety of addition reactions are possible hydrogenation addition of hydrogen halides (reaction 27.8) hydration (reaction 27.9), producing a vicinal dihalide or a geminal dihalide and halogenation (reaction 27.10). In these reactions, a transfer of electron density occurs from the tt bond of the alkene to an electrophilic atom. In all these reactions— except halogenation— the electrophilic atom tends to add to... 

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. 

Introduction by addition reactions a. Addition of amines to cyclopropanones... 

Chapter 18 discussed the acyl addition reactions of several nucleophiles with the carbonyl unit of aldehydes and ketones. As pointed out in Chapter 16, carboxylic acids and their derivatives also contain a carbonyl unit. Although these acid derivatives react with nucleophiles via attack at the acyl carbon, the presence of a leaving group attached to the acyl carbon leads to a subsequent reaction that is not possible with aldehydes and ketones. Acid derivatives react with nucleophiles via acyl substitution. This reaction pathway proceeds by an intermediate called a tetrahedral intermediate. The introduction to this reaction, presented in Chapter 16 (Section 16.8), will be explained and expanded here. 

The introduction of additional alkyl groups mostly involves the formation of a bond between a carbanion and a carbon attached to a suitable leaving group. S,.,2-reactions prevail, although radical mechanisms are also possible, especially if organometallic compounds are involved. Since many carbanions and radicals are easily oxidized by oxygen, working under inert gas is advised, until it has been shown for each specific reaction that air has no harmful effect on yields. 

As in the case of the steroids, introduction of additional nuclear substituents yields morphine analogs of increased potency. The more important of these are derived from one of the minor alkaloids that occur in opium. Thebaine (14), present in crude opium in about one-tenth the amount of morphine, exhibits a reactive internal diene system that is well known to undergo various addition reactions in a 1,4 manner (e.g., bromination). Thus, reaction with hydrogen peroxide in acid may be visualized to afford first the 14-hydroxy-6-hemiketal (15). Hydrolysis yields the isolated unsaturated ketone (16). Catalytic reduction... 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

The checkers used a dry ice condenser during the introduction of ammonia to the reaction flask and replaced it with an air condenser before the addition of sodium. 

In the historical introduction to this book (Sec. 1.1) it was mentioned that the discoverer of diazo compounds, Peter Griess, realized quite early (1864 a) that these species could react with alkali hydroxides. Thirty years later Schraube and Schmidt (1894) found that the primary product from the addition of a hydroxide ion to a diazo compound can isomerize to form a secondary product. In this section we will discuss the equilibria of the first acid-base process of aromatic diazonium ions. In the following section additional acid-base reactions will be treated in connection with the isomerism of addition products of hydroxide ions to diazonium ions. 

This section will provide details of recent efforts to polymerize phosphaalkenes. It will begin with an introduction to the factors that must be considered when attempting to polymerize P=C bonds. In addition, a historical context will be provided since, perhaps ironically, it was so-called polymerization reactions that plagued early efforts to prepare compounds possessing heavier element multiple bonds. Finally, it will close with the first successful polymerization of a P=C bond to give poly(methylenephosphine)s. 

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