9: Electrophilic Addition Reactions of Alkenes and Alkynes
- Page ID
- 170480
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- 9.1: Prelude to Alkenes and Alkynes
- Carbon-carbon double and triple bonds undergo a wide variety of addition reactions in which one of the multiple bonds is broken and two new bonds to carbon are formed. 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.
- 9.2: The Reactivity of Multiple Carbon-Carbon Bonds
- In the early days of organic chemistry, alkenes were described as "unsaturated" because, in contrast to the "saturated" alkanes, they were found to react readily with substances such as halogens, hydrogen halides, oxidizing agents, and so on. Therefore, the "chemical affinity" of alkenes was regarded as unsatisfied or "unsaturated". One reason alkenes and alkynes react more readily than alkanes is because the carbon-carbon bonds of a multiple bond are individually weaker than normal carbon-carb
- 9.3: Electrophilic Additions to Alkenes
- The reactions of alkanes discussed previously are homolytic processes, which means that the bonds are made and broken through radical or atomic intermediates. In contrast, the reactions of alkyl halides involve heterolytic bond cleavage and ionic reagents or products. Many important reactions of alkenes are heterolytic reactions because the electrons in the alkene double bonds are more exposed and accessible than the electrons in an alkane C−C bond
- 9.4: Orientation in Addition to Alkenes
- Addition of an unsymmetrical substance such as HX to an unsymmetrical alkene can give two products. If the ratio of the products is determined by the ratio of their equilibrium constants, the reaction is under "equilibrium (or thermodynamic) control". and the reaction is reversible. When a reaction is carried out under conditions in which it is not reversible, the ratio is determined by the relative rates of formation of the various products and the reactions are under "kinetic control."
- 9.5: Electrophilic Hydration to Make Alcohols
- Electrophilic hydration is the act of adding electrophilic hydrogen from a non-nucleophilic strong acid (a reusable catalyst, examples of which include sulfuric and phosphoric acid) and applying appropriate temperatures to break the alkene's double bond. After a carbocation is formed, water bonds with the carbocation to form a 1º, 2º, or 3º alcohol on the alkane.
- 9.6: Electrophilic Addition Reactions of Alkynes
- The alkynes behave in many ways as if they were doubly unsaturated alkenes. However, alkynes are substantially less reactive than corresponding alkenes toward many electrophiles. A simple but reasonable explanation is that the carbocation formed from the alkyne is less stable than that from the alkene because it cannot achieve the sp2 hybrid-orbital configuration expected to be the most stable arrangement for a carbocation.
- 9.7: Hydrogenation with Heterogeneous Catalysts
- Addition of hydrogen to a multiple bond is hydrogenation. It is applicable to almost all types of multiple bonds and is of great importance in synthetic chemistry, particularly in the chemical industry.
- 9.8: Heats of Hydrogenation
- Catalytic hydrogenation is useful for analytical and thermochemical purposes. The analysis of a substance for the number of carbon-carbon double bonds it contains is carried out by measuring the uptake of hydrogen for a known amount of sample. Measurement of the heat evolved in the hydrogenation of alkenes gives information as to the relative stabilities of alkenes.
- 9.9: Addition of Boron Hydrides to Alkenes. Organoboranes
- Hydroboration and the many uses of organoboranes in synthesis were developed largely by H. C. Brown and co-workers. In our discussion, we shall give more detail on hydroboration itself, and then describe several useful transformations of organoboranes.
- 9.11: More Electrophilic Additions
- Most alkenes react readily with ozone, even at low temperatures, to yield cyclic peroxidic derivatives known as ozonides. Ozonization of alkenes has been studied extensively for many years, but there is still disagreement about the mechanism (or mechanisms) involved because some alkenes react with ozone to give oxidation products other than ozonides. Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide.
- 9.12: Addition Reactions of Dienes
- Addition reactions of isolated dienes proceed more or less as expected from the behavior of simple alkenes. Thus, if one molar equivalent of 1,5-hexadiene is treated with one equivalent of bromine a mixture of 5,6-dibromo-1-hexene, 1,2,5,6-tetrabromohexane and unreacted diene is obtained, with the dibromo compound being the major product (about 50%).
- 9.13: Alkenes and Alkynes II (Exercises)
- These are the homework exercises to accompany Chapter 11 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).