Addition reactions, of alkenes

3 Stereochemistry of Alkene Hydrogenation 230 6.15 Stereochemistry of Halogen Addition 250 

4 Electrophilic Addition of Hydrogen Halides to 6.16 Mechanism of Halogen Addition to Alkenes  

5 Regioselectivity of Hydrogen Halide Addition 6.17 Conversion of Alkenes to Vicinal Halohydrins 253 

7 Carbocation Rearrangements in Hydrogen Halide 6.21 Reactions of Alkenes with Alkenes  

8 Addition of Sulfuric Acid to Alkenes 239 Ethylene and Propene The Most Important 

Fill in the blanks below. To verily that your answers are eorrect, look in your textbook at the end of Chapter 9. Eaeh of the sentences below appears verbatim in the section entitled Review of Concepts and Vocabulary. 

Addition reactions are thermodynamically favorable at temperature and disfavored at  

Hydrohalogenation reactions are regioselective, because the halogen is generally placed at the substituted position, called addition. 

The regioselectivity of an ionic addition reaction is determined by the preference for the reaction to proceed through.  

Acid-catalyzed hydration is inefficient when ate possible. 

We have already studied one addition reaction of alkenes— hydrogenation— in which a hydrogen atom is added at each end of a double (or triple) bond. In this chapter we shall study other alkene addition reactions that do not involve the same mechanism as hydrogenation. We can depict this type of reaction generally, using E for an electrophilic portion of a reagent and Nu for a nucleophilic portion, as follows. 

Some specific reactions of this type that we shall study in this chapter include addition of hydrogen halides, sulfiiric acid, water (in the presence of an acid catalyst), and halogens. Later we shall also study some specialized reagents that undergo addition reactions with alkenes. 

Two characteristics of the double bond help us understand why these addition reactions occur  

An addition reaction results in the conversion of one tt bond and one a bond (Sections 1.12 and 1.13) into two o- bonds. The result of this change is usually energetically favorable. The energy released in making two a bonds exceeds that needed to break one cr bond and one tt bond (because tt bonds are weaker), and, therefore, addition reactions are usually exothermic  

The electrons of the tt bond are exposed. Because the ir bond results from overlapping p orbitals, the tt electrons lie above and below the plane of the double bond  

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 

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Once we grasp the idea of stereoisomerism m molecules with two or more chirality cen ters we can explore further details of addition reactions of alkenes... 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

How do we know that the carbocation mechanism for electrophilic addition reactions of alkenes is correct The answer is that we don t know it s correct at least we don t know with complete certainty. Although an incorrect reaction mechanism can be disproved by demonstrating that it doesn t account for observed data, a correct reaction mechanism can never be entirely proved. The best we can do is to show that a proposed mechanism is consistent with all known facts. If enough facts are accounted for, the mechanism is probably correct. 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

Click Coached Problems for a self-study module on addition reactions of alkenes. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Several other types of addition reactions of alkenes are also of importance and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes are covered in Section 2.6 and cycloadditions involving concerted mechanisms are encountered in Sections 6.1 to 6.3. Free radical addition reaction are considered in Chapter 11. 

The addition reaction of alkenes and phenylmercuric bromide typically occurs at about 80°C. Phenylmercuric iodides are somewhat more reactive and may be advantageous in reactions with relatively unstable alkenes.167... 

The decolorisation of bromine, usually in CC14 solution, is one of the classical tests for unsaturation, and probably constitutes the most familiar of the addition reactions of alkenes. It normally proceeds readily in the absence of added catalysts, and one is tempted to assume that it proceeds by a simple, one-step pathway ... 

Summary of Key Reactions Summary of Addition Reactions of Alkenes... 

A summary of addition reactions of alkenes with 1-methylcyclopentene as the organic substrate. A bond designated means that the stereochemistry of the group is unspecified. For brevity the structure of only one enantiomer of the product is shown, even though racemic mixtures would be produced in all instances in which the product is chiral. 

In many addition reactions of alkenes generating two chiral units, the configuration at the C —C double bond is translated into the relative configuration at the newly created stereogenic centers. Thus, knowledge of the stereochemical course of the addition reaction (s> n or anti)... 

Alkenes are electron-rich species. The double bond acts as a nucleophile, and attacks the electrophile. Therefore, the most important reaction of alkenes is electrophilic addition to the double bond (see Section 5.3.1). An outline of the electrophilic addition reactions of alkenes is presented here. 

Routes to benzo-fused derivatives of 1,4-dioxanes, 1,4-oxathianes and 1,4-dithianes make use of anions or dianions of the appropriate 1,2-disubstituted benzene. An alternative approach to the synthesis of 1,4-benzodioxanes involves Diels-Alder addition reactions of alkenes across the quinone function of 1,2-benzoquinones, e.g. (352) — (353). 

The importance of such reactions to synthetic organic chemistry is paramount. It is our intention in this and the following chapter to show the great diversity, utility, and specificity of addition reactions of alkenes and alkynes. 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

In an addition reaction of alkenes, the double bond is broken and two new groups are added, each forming a single bond to a different carbon of what was the double bond. 

These addition reactions are similar to the addition reactions of alkenes. However, the reaction is much slower for an alkyne, because alkynes are less reactive. Alkynes can be expected to be more nucleophilic, as they are more electron rich in the vicinity of the multiple bond, that is, six electrons in a triple bond as compared to four in a double bond. However, electrophilic addition to an alkyne involves the formation of a vinylic carbocation (Following fig.). This carbocation is much less stable than the carbocation intermediate formed during electrophilic addition to an alkene. 

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