Chemistry Reference
In-Depth Information
Scheme 2.11 Electron-withdrawing groups increase acidity by increasing anionic stability.
As illustrated in Scheme 2.11, electron-withdrawing groups are readily recognized when
the group places either a partial or formal positive charge adjacent to an acidic center.
This placement of a partial positive charge allows greater delocalization of the negative
charge that develops when the acidic proton is removed. Through this increased delocaliza-
tion of the developing negative charge, the stability of the developing anion increases, thus
increasing the acidity of the target proton.
As illustrated in Scheme 2.12, electron-donating groups are readily recognized when the
group places either a partial or formal negative charge adjacent to an acidic center. This
placement of a negative charge forces destabilization of the negative charge that develops
when the acidic proton is removed. To illustrate, imagine trying to force two magnets to
meet at their negative poles. As the negative poles get closer, the repulsive forces between
the magnets increase. As with magnets, two negative charges on adjacent atoms result in a
destabilizing situation. By decreasing the stability of a developing negative charge, the
stability of a developing anion decreases, thus decreasing the acidity of the target proton.
Functional groups were defined and discussed in Chapter 1 (Section 1.2). In that
discussion, the concept was presented that functional groups can be either electron-
withdrawing groups or electron-donating groups. In fact, all inductive effects result from
the introduction of functional groups to organic molecules. Furthermore, through an
understanding of the characteristics of the various functional groups, one can predict
Scheme 2.12 Electron-donating groups decrease acidity by decreasing anionic stability.
