Page 361 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

342 In general, the dissection of substituent effects need not be limited to the
resonance and polar components that are of special prominence in reactions of aromatic
CHAPTER 3 compounds. Any type of substituent interaction with a reaction center might be
Structural Effects on indicated by a substituent constant characteristic of the particular type of interaction and
Stability and Reactivity
a reaction parameter indicating the sensitivity of the reaction series to that particular
type of interaction. For example, it has been suggested that electronegativity and polar-
izability can be treated as substituent effects that are separate from polar and resonance
effects. 134 This gives rise to the equation

k
log = + + + (3.50)

R R
F F

k
0
where is the polar, is the resonance, is the electronegativity, and is the

F
R

polarizability substituent constant. In general, we emphasize the resonance and polar
components in our discussion of substituent effects.
The Hammett substituent constants in Table 3.26 provide a more quantitative
interpretation of substituent effects than was given in Scheme 3.1, where substituents
were simply listed as EWG or ERG with respect to resonance and polar components.
The values of and provide comparative evaluations of the separate resonance and
I
R
polar effects. By comparing and , individual substituents can be separated into
I
R
four quadrants, as in Scheme 3.5. Alkyl groups are electron releasing by both resonance
and polar effects. Substituents such as alkoxy, hydroxy, and amino, which can act as
resonance donors, have negative and values, but when polar effects are dominant
+
p
these substituents act as EWGs, as illustrated by the positive and values. A
m I
third group of substituents act as EWGs by both resonance and polar interactions. This
group includes the carbonyl substituents, such as in aldehydes, ketones, esters, and
amides, as well as cyano, nitro, and sulfonyl substituents. Of the common groups, only
trialkylsilyl substituents are electron withdrawing by resonance and electron donating
by polar effects, and both effects are weak.
3.6.2. Application of Linear Free-Energy Relationships to Characterization
of Reaction Mechanisms
Let us now consider how linear free-energy relationships can provide insight into
reaction mechanisms. The choice of benzoic acid ionization as the reference reaction
for the Hammett equation leads to > 0 for EWGs and < 0 for ERGs, since EWGs
favor the ionization of the acid and ERGs have the opposite effect. Further inspection
of the Hammett equation shows that will be positive for all reactions that are favored
by ERGs and negative for all reactions that are favored by EWGs. If the rates of a
reaction series show a satisfactory correlation, both the sign and magnitude of provide
information about the TSs and intermediates for the reaction. In Example 3.3 (p. 340),
the value for hydrolysis of substituted methyl benzoates is +2 38. This indicates
that EWGs facilitate the reaction and that the reaction is more sensitive ( > 1) to
substituent effects than the ionization of benzoic acids. The observation that the reaction
is favored by EWGs is in agreement with the mechanism for ester hydrolysis discussed
in Section 3.4.4. The tetrahedral intermediate is negatively charged. Its formation
should therefore be favored by substituents that stabilize the developing charge. There
is also a ground state effect working in the same direction. EWG substituents make
134
R. W. Taft and R. D. Topsom, Prog. Phys. Org. Chem., 16, 1 (1987).