3.4: Substituent Effects in the Reactivity of Aromatic Rings

Objectives

After completing this section, you should be able to

1. describe the two ways in which a substituent influences the electrophilic substitution of a monosubstituted aromatic compound.
2. classify each of the following substituents as being either activating or deactivating with respect to electrophilic aromatic substitution: \(\ce{\sf{-NH2}}\), \(\ce{\sf{-OH}}\), \(\ce{\sf{-NHR}}\), \(\ce{\sf{-NR2}}\), \(\ce{\sf{-OR}}\), \(\ce{\sf{-NHCOR}}\), alkyl (R), phenyl, \(\ce{\sf{R3N+}}\), \(\ce{\sf{-NO2}}\), \(\ce{\sf{-CN}}\), \(\ce{\sf{-COR}}\), \(\ce{\sf{-CO2H}}\), \(\ce{\sf{-CO2R}}\), \(\ce{\sf{-CHO}}\), halogens.
3. list a given series of substituents (selected from those given in Objective 2) in order of increasing or decreasing ability to activate or deactivate an aromatic ring with respect to electrophilic substitution.
4. explain, in general terms, the factors that determine whether a given substituent will activate or deactivate an aromatic ring with respect to electrophilic substitution.
5. list a given series of aromatic compounds in order of increasing or decreasing reactivity with respect to electrophilic substitution.
6. explain the inductive effects displayed by substituents such as nitro, carboxyl, alkyl and the halogens during electrophilic aromatic substitution reactions.
7. explain the resonance effects displayed by substituents such as nitro, carbonyl-containing, hydroxy, alkoxy and amino groups during electrophilic aromatic substitution reactions.

 

Reactivity of the Aromatic Ring

Substituted rings are divided into two groups based on the type of the substituent that the ring carries:

• Activated rings: the substituents on the ring are groups that donate electrons.
• Deactivated rings: the substituents on the ring are groups that withdraw electrons.

Activating and Deactivating Effects

What makes a group either activating or deactivating? The common characteristic of all activating groups is that they donate electrons to the ring, thereby making the ring more electron-rich, stabilizing the carbocation intermediate, and lowering the activation energy for its formation. Conversely, the common characteristic of all deactivating groups is that they withdraw electrons from the ring, thereby making the ring more electron-poor, destabilizing the carbocation intermediate, and raising the activation energy for its formation.

Compare the electrostatic potential maps of benzaldehyde (deactivated), chlorobenzene (weakly deactivated), and phenol (activated) with that of benzene. As shown in Figure \(\PageIndex{2}\), the ring is more positive (yellow-green) when an electron-withdrawing group such as –CHO or –Cl is present and more negative (red) when an electron-donating group such as –OH is present.

Figure \(\PageIndex{2}\): Electrostatic potential maps of benzene and several substituted benzenes show that an electron-withdrawing group (–CHO or –Cl) makes the ring more electron-poor, while an electron-donating group (–OH) makes the ring more electron-rich.

The withdrawal or donation of electrons by a substituent group is controlled by an interplay of inductive effects and resonance effects. The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:

 

Inductive Effect

An inductive effect is the withdrawal or donation of electrons through a σ bond due to electronegativity. Halogens, hydroxyl groups, carbonyl groups, cyano groups, and nitro groups inductively withdraw electrons through the σ bond linking the substituent to a benzene ring. This effect is most pronounced in halobenzenes and phenols, in which the electronegative atom is directly attached to the ring, but is also significant in carbonyl compounds, nitriles, and nitro compounds, in which the electronegative atom is farther removed. Alkyl groups, on the other hand, inductively donate electrons. This is the same hyperconjugative donating effect that causes alkyl substituents to stabilize alkenes  and carbocations.

 

Resonance Effect

A resonance effect is the withdrawal or donation of electrons through a \(\pi\) bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring. Carbonyl, cyano, and nitro substituents, for example, withdraw electrons from the aromatic ring by resonance. The \(\pi\) electrons flow from the ring to the substituent, leaving a positive charge in the ring. Note that substituents with an electron-withdrawing resonance effect have the general structure –Y=Z, where the Z atom is more electronegative than Y.

Conversely, halogen, hydroxyl, alkoxyl (–OR), and amino substituents donate electrons to the aromatic ring by resonance. Lone-pair electrons flow from the substituents to the ring, placing a negative charge on the ring. Substituents with an electron-donating resonance effect have the general structure $\text{–Ÿ}$, where the Y atom has a lone pair of electrons available for donation to the ring.

This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift. If the atom bonded to the ring has one or more non-bonding valence shell electron pairs, as do nitrogen, oxygen and the halogens, electrons may flow into the aromatic ring by p-π conjugation (resonance), as in the middle diagram. Finally, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, as in the right-hand diagram. Note that in the resonance examples all the contributors are not shown. In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.

 

Inductive and Resonance may not Agree

One further point: inductive effects and resonance effects don’t necessarily act in the same direction. Halogen, hydroxyl, alkoxyl, and amino substituents, for instance, have electron-withdrawing inductive effects because of the electronegativity of the –X, –O, or –N atom bonded to the aromatic ring but have electron-donating resonance effects because of the lone-pair electrons on those –X, –O, or –N atoms. When the two effects act in opposite directions, the stronger effect dominates. Thus, hydroxyl, alkoxyl, and amino substituents are activators because their stronger electron-donating resonance effect outweighs their weaker electron-withdrawing inductive effect. Halogens, however, are deactivators because their stronger electron-withdrawing inductive effect outweighs their weaker electron-donating resonance effect.

In the case of the nitrogen and oxygen activating groups displayed in the top row of the previous diagram, electron donation by resonance dominates the inductive effect and these compounds show exceptional reactivity in electrophilic substitution reactions. Although halogen atoms have non-bonding valence electron pairs that participate in p-π conjugation, their strong inductive effect predominates, and compounds such as chlorobenzene are less reactive than benzene. The three examples on the left of the bottom row (in the same diagram) are examples of electron withdrawal by conjugation to polar double or triple bonds, and in these cases the inductive effect further enhances the deactivation of the benzene ring. Alkyl substituents such as methyl increase the nucleophilicity of aromatic rings in the same fashion as they act on double bonds.

 

Net Effect

For instance, in aromatic nitration, an –OH substituent makes the ring 1000 times more reactive than benzene, while an –NO₂ substituent makes the ring more than 10 million times less reactive, getting the classification of strong activator and strong deactivator in that order. Chlorine makes the ring only 30 times less reactive than benzene, for such halogens are considered mild deactivators with increasing strength along the group.

 

Classification of Substituents

Examples of activating groups in the relative order from the most activating group to the least activating:

-NH₂, -NR₂ > -OH, -OR> -NHCOR> -CH₃ and other alkyl groups

with R as alkyl groups (C_nH_2n+1)

Examples of deactivating groups in the relative order from the most deactivating to the least deactivating:

-NO₂, -CF₃> -COR, -CN, -CO₂R, -SO₃H > Halogens

with R as alkyl groups (C_nH_2n+1)

The order of reactivity among Halogens from the more reactive (least deactivating substituent) to the least reactive (most deactivating substituent) halogen is:

F> Cl > Br > I

The order of reactivity of the benzene rings toward the electrophilic substitution when it is substituted with a halogen groups, follows the order of electronegativity. The ring that is substituted with the most electronegative halogen is the most reactive ring (less deactivating substituent) and the ring that is substituted with the least electronegative halogen is the least reactive ring (more deactivating substituent), when we compare rings with halogen substituents. Also the size of the halogen effects the reactivity of the benzene ring that the halogen is attached to. As the size of the halogen increase, the reactivity of the ring decreases.

Exercises

Exercise \(\PageIndex{1}\)

Draw the resonance structures for benzaldehyde to show the electron-withdrawing group.

Answer

Exercise \(\PageIndex{2}\)

Draw the resonance structures for methoxybenzene to show the electron-donating group.

Answer

Exercise \(\PageIndex{3}\)

Use Figure \(\PageIndex{1}\) to explain why Friedel–Crafts alkylations often give polysubstitution but Friedel–Crafts acylations do not.

Answer

Alkylbenzenes are more reactive than benzene itself, but acylbenzenes are less reactive.

Exercise \(\PageIndex{4}\)

An electrostatic potential map of (trifluoromethyl)benzene, C₆H₅CF₃, is shown. Would you expect (trifluoromethyl)benzene to be more reactive or less reactive than toluene toward electrophilic substitution? Explain.

Answer

Toluene is more reactive; the trifluoromethyl group is electron-withdrawing.

Key Terms

Make certain that you can define, and use in context, the key terms below.

• inductive effect
• resonance effect

Study Notes

On reading Objective 2 students may exclaim “How am I ever going to memorize all of this!”—or words to that effect. The answer is that if you are trying to memorize such things, you are taking the wrong approach to organic chemistry. What you should be doing is trying to understand the factors that determine whether a given substituent will activate or deactivate a benzene ring with respect to electrophilic substitution.

You may wish to review earlier material on to the inductive effect. If so, refer to Sections 2.1, 7.9 (paying particular attention to the “Study Notes”) and 14.5.

Note that one argument sometimes used to explain the ability of alkyl groups to donate electrons inductively to an aromatic ring is that sp²‑hybridized carbon atoms are more electronegative than sp³‑hybridized carbon atoms. Thus, a sigma bond between sp²- and sp³‑carbon is slightly polarized, as follows: