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Directive Effects of Substituents

Consider the reaction of benzene with an electrophile (E).
image\Ch 18 sec G, g1.png
Notice that the positive charge is spread only on secondary carbons, and thus all resonance structures are equally stable.
In aromatic substitution reactions, the groups already present in the benzene ring can significantly influence the place of electrophilic attack of the incoming substituent. In other words, the substitution is influenced by the groups that are already present in the benzene ring. There are two types of groups –activators and deactivators. The groups that are activators cause the ring to be increasingly reactive than benzene. The groups that are deactivators cause the ring to be decreasingly reactive than benzene.

Activating Groups

As mentioned above, the activators increase the reactivity of the ring than that of benzene. In addition, the substitution primarily occurs in the ortho and para positions of the ring relative to the activating group. For this reason, activators are also called ortho/para directors. Let us analyze this with an example. In the following reaction, phenol is nitrated. Notice that phenol contains a hydroxyl group (-OH) which is an ortho/para activator. So the substitution occurs at ortho and para positions with respect to the -OH group.

Sample reaction

image\Ch 18 sample 18-7.png

Now let's us consider an electrophilic attack on toluene.
image\Ch 18 page 251.png

Alkyl groups are electron-donating substituents, but it might seem counterintuitive to think at groups like -OH (hydroxyl) and -OCH3 (methoxy) are also ortho/para activators since oxygen is highly electronegative atom. This is because the oxygen atom that is bonded to the ring carbon has lone electrons or nonbonding electrons. As a result, the positive charge on the carbon atom is the intermediates formed from ortho and para attacks is stabilized through resonance. In other words, such groups can also donate electron density because of the presence of nonbonded electrons.

image\Ch 18 page 252, g1.png

Deactivating Groups

Deactivators make the ring less reactive than benzene. There are two types of deactivators –ortho/para and meta deactivators. 
  1. Ortho/para deactivators Some deactivators direct the incoming substituents primarily to the ortho and para positions. Halogens (Fluorine, Chlorine, Bromine, Iodine) are ortho/para deactivators. They are electron withdrawing substituents through inductive effect. Furthermore, they have nonbonded electrons that can donate electron density by resonance. Because of the high electronegativity of halogens, they pull the electron density away from the ring, making the ring less susceptible to electrophilic substitution (deactivating property). Halogen substituents have nonbonded electrons which can be donated to a positively charged carbon in the intermediates resulting from ortho and para attacks. Hence, halogens are ortho, para-directing, and deactivating substituents.

image\Ch 18 page 253 g1.png

  1. Meta deactivators The meta deactivating groups direct the incoming substituents to the meta position. These groups are strongly electron-withdrawing groups. Consider the following example.

image\Ch 18 page 253 g2.png


The substituent acyl group has a highly polarized carbon-oxygen double bond.


image\Ch 18 page 253 g3.png


The positively charged carbon withdraws electron density from the benzene ring inductively. This accounts for the deactivation (less reactivity than benzene) of the ring toward electrophilic substitution.

image\Ch 18, page 254, g1.png

image\Ch 18 page 254, g2.png

image\Ch 18 page 254, g3.png


Both ortho and para attacks result in an intermediate that has adjacent positively charged (polarized) atoms, making it a highly unstable (higher energy) form. On the other hand, meta substitution doesn't result in an intermediate with positively charged atoms adjacent to each other. Thus, meta substitution results in the most stable (lower energy) intermediates. This accounts for the predominance of meta substitution products if acyl or similar groups are present in the ring.

image\Ch 18 table 18-1.png


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