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10: Alkenes and Alkynes I - Ionic and Radical Addition Reactions

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  • With few exceptions, the multitude of reactions discussed in introductory texts are classified as ionic reactions. By this we mean that nucleophilic and electrophilic sites in reacting molecules bond to each other. Furthermore, charged species such as carbocations, carbanions, conjugate acids and conjugate bases are often intermediates on the reaction path, the overall transformation taking place in two or more discrete steps. Ionic reactions normally occur in solution, and changes in solvents may have dramatic consequences. Free-radical addition is an addition reaction in organic chemistry involving free radicals. The addition may occur between a radical and a non-radical, or between two radicals.

    • 10.1: 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.
    • 10.2: Physical and Spectroscopic Properties of Alkenes and Alkynes
      In general, the physical properties of alkenes are similar to those of alkanes. Like the continuous-chain alkanes, the 1-alkenes form a homologous series of compounds that show regular changes in physical properties with increasing chain length. The boiling points, melting points, and densities of the simple alkynes are somewhat higher than those of the corresponding alkanes or alkenes, and these properties also show regular changes as the chain length is increased.
    • 10.3: 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
    • 10.4: 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
    • 10.5: 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."
    • 10.6: 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.
    • 10.7: Nucleophilic Addition Reactions
      When a stepwise ionic addition reaction involves nucleophilic attack at carbon as a first step, it is described as a nucleophilic addition. Reactions of this type often are catalyzed by bases, which generate the required nucleophile.
    • 10.8: Radical-Chain Addition Reactions to Alkenes
      The addition of hydrogen bromide to unsymmetrical alkenes involves two reaction mechanisms, each giving a different product.  These are the ionic addition and the radical-chain addition mechanisms.
    • 10.9: Polymerization of Alkenes
      One of the most important technical reactions of alkenes is their conversion to higher-molecular-weight compounds or polymers. A polymer is defined as a long-chain molecule with recurring structural units.
    • 10.E: Alkenes and Alkynes I (Exercises)
      These are the homework exercises to accompany Chapter 10 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
    • 10.10: Alkylation of Alkenes
      Addition of a saturated hydrocarbon to an alkene to yield a saturated hydrocarbon of higher molecular weight is known as alkylation. Such reactions are used by the petroleum industry to produce medium-molecular-weight hydrocarbons from smaller molecules. The key to the mechanism of hydrocarbon alkylation was provided by the discovery by P. D. Bartlett, in 1940, that a carbocation can react rapidly with a hydrocarbon having a tertiary hydrogen to yield a new carbocation and a new hydrocarbon

    Contributors and Attributions

    • John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."