14: Substituent Effects

(9/94)(11,12/96)(11,12/04,01/05) Neuman Chapter 14
Figure 14.25
Remember that the C-R bond does not actually break. The 2nd resonance structure indicates
that the C-R electron pair overlaps with the C=C π system as we showed in Chapter 8.
Aryl groups such as phenyl rings can also donate electron density by resonance to a π
system as we show in Figure 14.26.
Figure 14.26
We include all aryl (Ar) groups in the +R category (Table 14.09), but occasionally they act as
-R groups.
-R Groups. All -R substituents that we show in Table 14.09 can accept electron donation
by resonance from a π system. We show this in the general example in Figure 14.27 where
we represent S by Y=Z.
Figure 14.27
For example, the general group Y=Z is an O=N bond in NO2, the O=C bond in R(C=O) and
CO2H, the N≡C bond of the CN group, and a C=C bond in aryl groups. We show specific
resonance structures for these substituents in section 14.4 that deals with Electrophilic
Aromatic Substitution reactions.
Correspondence between I and R Properties. All -R substituents are also -I substituents.
However, a similar analogy does not hold for +R substituents. With the exception of CR3
groups (alkyl groups), all +R substituents in Table 13.9 are actually -I substituents.
The atom in S that is directly bonded to the substrate is usually more electronegative than H
causing the S group to be -I. At the same time, that atom generally has an unshared pair of
18