7.4 Nucleophilic Addition and Nucleophile Elimination Steps

In Section 7.2, we introduced the S_N2 step, in which a nucleophile bonds to an atom containing a suitable leaving group. A nucleophile can also bond to an atom that is involved in a polar π bond—that is, a π bond that is part of a double or triple bond connecting atoms with significantly different electronegativities, such as those in carbonyl groups (C═O), imine groups (C═N), and cyano groups (C≡N). Specifically, as shown in Equations 7-9 through 7-11, the nucleophile forms a bond to the less electronegative atom and the π bond breaks, becoming a lone pair on the more electronegative atom. A nucleophile adds to the polar π bond in these steps, so they are called nucleophilic addition steps.

The reverse of nucleophilic addition (Equations 7-12 and 7-13) is also commonly encountered in organic chemistry.

In both examples, a lone pair of electrons from a more electronegative atom forms a π bond to a less electronegative atom. A leaving group is simultaneously expelled to avoid exceeding an octet on the less electronegative atom. In the products, the leaving group ends up with a lone pair of electrons and an excess of negative charge, both of which are characteristics of a nucleophile. As a result, we call these nucleophile elimination steps.

Nucleophilic addition and nucleophile elimination steps are typically found in multistep mechanisms. In Chapters 17 and 18, for example, we will learn reactions in which proton transfer steps are coupled with nucleophilic addition steps. In Chapters 20 and 21, we will see how nucleophilic addition and nucleophile elimination steps in the same mechanism can result in a substitution.

The electron-rich to electron-poor nature of a nucleophilic addition step is fairly straightforward. The nucleophile in a nucleophilic addition step, which has an excess of negative charge, is relatively electron rich, and the less electronegative atom of the polar π bond is relatively electron poor. Thus, the curved arrow drawn from the nucleophile to the polar π bond represents the flow of electrons from an electron-rich site to an electron-poor site:

The second curved arrow, drawn from the center of the double (or triple) bond to the electronegative atom (X), is necessary to avoid exceeding an octet on the less electronegative atom (C).

YOUR TURN 7.7

SHOW ANSWERS

For the following nucleophilic addition step, label the pertinent electron-rich and electron-poor sites. Add the appropriate curved arrows and, in the box provided, draw the product. Identify the curved arrow that is drawn from the electron-rich site to the electron-poor site.

In nucleophile elimination (the reverse step), the more electronegative atom (X) is relatively electron rich: It bears either a full negative charge or a partial negative charge. The less electronegative atom (typically C) is relatively electron poor. Thus, the curved arrow that originates from a lone pair of electrons on X and points to the bonding region between C and X represents the flow of electrons from an electron-rich site to an electron-poor site:

Once again, a second curved arrow, to represent the breaking of the C—L bond to the leaving group (L), is necessary to avoid exceeding an octet on the less electronegative atom (C).

YOUR TURN 7.8

SHOW ANSWERS

For the following nucleophile elimination step, label the pertinent electron-rich and electron-poor sites. Add the appropriate curved arrows and identify the curved arrow that is drawn from the electron-rich site to the electron-poor site.

problem 7.13 Draw the appropriate curved arrows and the products for each of the following nucleophilic addition steps.

problem 7.14 Draw the appropriate curved arrows necessary for each of the following nucleophile elimination steps to produce a ketone, and draw the resulting ketone.