
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.

## References

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."

10.7: Nucleophilic Addition Reactions is shared under a not declared license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.