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10.7: Nucleophilic Addition Reactions

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  • 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. For example, consider the addition of some weakly acidic reagent \(\ce{HX}\) to an alkene. In the presence of a strong base \(\left( ^\ominus \ce{OH} \right)\), \(\ce{HX}\) could give up its proton to form the conjugate base \(\ce{X}^\ominus\), which is expected to be a much better nucleophile than \(\ce{HX}\):

    \[\ce{H}:\ce{X} + ^\ominus \ce{OH} \rightleftharpoons \ce{H_2O} + :\ce{X}^\ominus\]

    What can follow with an alkene is an ionic chain reaction with the following two propagating steps. First, the nucleophile attacks at carbon to form a carbon anion (carbanion) intermediate (Equation 10-8). Second, electrophilic transfer of a proton from \(\ce{HX}\) to the carbanion forms the adduct and regenerates the nucleophile (Equation 10-9). The overall reaction is the addition of \(\ce{HX}\) to the double bond:

    Net reaction:

    The \(\ce{HX}\) reagent can be water, an alcohol \(\left( \ce{ROH} \right)\), a thiol \(\left( \ce{RSH} \right)\), an amine \(\left( \ce{RNH_2} \right)\), or hydrogen cyanide \(\left( \ce{HCN} \right)\) or other carbon acids (i.e., compounds with acidic \(\ce{C-H}\) bonds). However, nucleophilic addition of these reagents to simple alkenes rarely is encountered. To have nucleophilic addition the double bond must be substituted with strongly electron-withdrawing groups such as carbonyl-containing groups, \(\ce{NO_2}\), \(\ce{C \equiv N}\), or positively charged ammonium or sulfonium groups. However, alkynes generally are more reactive towards nucleophiles than they are toward electrophiles. For example, with a base catalyst, 2-hexen-4-yne adds methanol across the triple bond, leaving the double bond untouched:

    (Nonetheless, the double bond seems to be necessary because a corresponding addition is not observed for 2-butyne, \(\ce{CH_3C \equiv CCH_3}\).)

    Many nucleophilic addition reactions have considerable synthetic value, particularly those involving addition of carbon acids, such as \(\ce{HCN}\), because they provide ways of forming carbon-carbon bonds. More of their utility will be discussed in Chapters 14, 17, and 18.


    • 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."