Directing Effects

Core Concepts

Electrophilic aromatic substitution adds substituents to aromatic molecules. When a monosubstituted aromatic ring gets another substituent, the new substituent bonds to a specific carbon atom. Directing effects determine which carbon atom receives the new substituent. Examples of directing effects include whether the existing substituent is an electron donating or withdrawing group, an activating or deactivating group, and the extent of steric effects.

Introduction to Directing Effects: The Basics of EAS

**Figure 1.** Overview of EAS reaction. The product is a substituted aromatic ring, where the substituent is the electrophile used in the reaction.

Electrophilic aromatic substitution (EAS) is the reaction by which an aromatic ring gains a substituent (group). Before the reaction, the aromatic ring is already in a relatively stable, low-energy state because its electrons are delocalized. Delocalization distributes the negative charge more equally across the molecule.

EAS involves a catalyst, a Lewis base, and an aromatic ring. The catalyst bonds with the Lewis base, forming an electrophile. The electrophile is a cation, so the ring’s electrons readily attack it. This shifts electrons onto the electrophile, leaving one of the ring’s carbon atoms with a partial positive charge (Figure 2). The ring is no longer aromatic.

Electrons within the ring shift toward that partially-positive carbon to redistribute the charge throughout the structure. In turn, the partial positive charge shifts to other carbon atoms in the ring. Multiple resonance structures exist simultaneously as the charge shifts to different carbons (Figure 2).

**Figure 2.** In the first step of EAS, an electron pair from the ring attacks the positively-charged electrophile, giving one carbon atom on the ring a partial positive charge. The ring’s remaining electrons shift around the ring, shifting the partial positive charge in turn.

The catalyst’s electrons remove a hydrogen atom from the carbon that’s adjacent to the partially-positive carbon, leaving the ring with a lone electron pair where the hydrogen used to be. That lone pair forms a pi bond with the other orbitals in the ring, restoring the ring’s aromaticity. The end result of the EAS is that the electrophile has bonded to one of the ring’s carbons as a substituent (Figure 3).

**Figure 3.** In the second step of EAS, the Lewis base returns to remove a hydrogen atom from one of the ring’s resonance structures. The electron pair that formerly bonded this hydrogen to the ring now attacks the ring’s partial positive charge, re-forming a pi bond and restoring the ring’s aromaticity.

Regioselectivity explains how this chemical reaction preferentially occurs at a specific carbon atom on the ring, as opposed to any other carbon atom.

What are directing effects in organic chemistry?

Substituting an aromatic ring that already has at least one substituent is more complicated than if it had none. The properties, or directing effects, of the existing substituent(s) impact the ring’s next chemical reaction.

There are four types of directing effects: electron donating, electron withdrawing, activation, and deactivation. A substituent is either electron donating or electron withdrawing, and either activating or deactivating.

Electron Donating Groups and Electron Withdrawing Groups

To determine where on the aromatic ring an incoming substituent will bond, first we have to consider the existing substituent. Specifically, we consider the substituent’s atom that’s directly bonded to the ring, how that atom impacts the ring, and how resonance makes it all happen.

If this particular atom has a partial negative charge (such as having a lone pair), then it can donate electron density to the aromatic ring, and is an electron donating group (EDG). Examples of EDGs are hydroxy groups (R-OH), amine groups (R-NH2), and aryl groups (an R group substituted with an aromatic ring). If this atom has a partial positive charge, then the substituent draws electron density toward itself (away from the ring), so it’s an electron withdrawing group (EWG). EWG examples include carboxyl groups (R-COOH), nitro groups (R-NO2), and cyanide groups (R-CN).

Activating Groups and Deactivating Groups

The existing substituent also determines how readily the monosubstituted aromatic ring undergoes another EAS. EDGs contribute partial negative charge to the ring, so the ring will more readily bond with a positively charged incoming electrophile in another EAS. This ring is said to be activated, and the EDG serves as an activating group.

However, EWGs give the monosubstituted aromatic ring partial positive charge, so the ring bonds less readily with an incoming electrophile. In this scenario, the ring is deactivated, and the EWG is a deactivating group.

Ortho, Meta, and Para Positions

Ortho, meta, and para are terms used when discussing an aromatic ring that has multiple substituents. These terms describe the spatial relationship between two substituents on the same aromatic ring.

When the two substituents are on adjacent carbon atoms (the substituents have a 1,2 relationship), we say that the substituents are ortho to one another (Figure 4). When there is one carbon between the substituents (the substituents have a 1,3 relationship), they are meta to each other (Figure 5). Substituents bonded to carbon atoms that are directly opposite each other on the ring (the substituents have a 1,4 relationship) are said to be para to each other (Figure 6).

**Figure 4.** In each of these molecules, substituents R and R’ are *ortho* to each other, as characterized by their 1,2 relationship.

**Figure 5.** In each of these molecules, substituents R and R’ are *meta* to each other. Note their 1,3 relationship.

**Figure 6.** In this molecule, substituents R and R’ are *para* to each other, as seen in their 1,4 relationship.

How do directing effects direct the substitution of an aromatic ring?

The existing substituent influences which of the aromatic ring’s other carbons bonds with the incoming substituent. It can also determine how readily the ring undergoes the next reaction.

Directing Effects: Ortho/Para Directors

When an EDG donates its electron density to the ring, the ring is left with a lone electron pair. The ring undergoes resonance to alleviate this burdensome partial negative charge and redistribute the charge across the ring. Essentially, the lone pair’s negative charge shifts among all carbon atoms within the ring, until the carbon adjacent to (ortho) or directly opposite (para) the EDG ends up with a partial negative charge.

Since one of these carbons has a partial negative charge due to the EDG donating its electron density, an incoming electrophile will attack this carbon. The result is that an incoming electrophile, the new substituent, will bond in the aromatic ring’s ortho or para position (Figure 7). Therefore, we say that EDGs are ortho/para directors.

**Figure 7.** In this example, the aromatic ring’s existing substituent (a hydroxy group) is an EDG, an *ortho*/*para* director. When the ring undergoes an EAS, the incoming electrophile (a nitro group) bonds to the ring at the *ortho* or *para* position relative to the hydroxy group.

Directing Effects: Meta Directors

Alternatively, when an EWG withdraws electron density from the ring, the ring has a partial positive charge on one carbon atom. Resonance shifts the electrons’ negative charge among all of the ring’s carbon atoms, until the carbon in the meta position has the partial negative charge. Drawn to that carbon’s partial negative charge, an incoming substituent will bond there, so EWGs are meta directors (Figure 8).

**Figure 8.** In this example, the aromatic ring’s existing substituent (a nitro group) is an EWG, a *meta* director. When the ring undergoes an EAS, the incoming electrophile (a chlorine atom) bonds to the ring at the *meta* position relative to the nitro group.

Through this process, we see that an aromatic ring’s existing substituent guides the incoming substituent to attach at the ring’s ortho, meta, or para position. We also see how electron donation or withdrawal leads to resonance within the ring.

Halogens and Alkyl Chains as Substituents

Halogens, which are very electronegative, draw electron density toward themselves through the inductive effect. If the existing substituent is a halogen, it draws electron density out of the aromatic ring. Left with a partial positive charge, the ring is less likely to undergo another EAS. So halogens act as slightly deactivating groups, despite being ortho/para directors.

If the existing substituent is an alkyl chain, another ortho/para director, the aromatic ring is slightly activated. This is because the alkyl chain’s constant rotation leads to transient moments of hyperconjugation with the adjacent carbon’s C–H bonds. Hyperconjugation helps maintain the molecule’s stability.

The Impact of Steric Effects

Steric effects can also serve as directing effects. For example, even if the existing substituent is an ortho/para director, steric effects could hinder the incoming substituent bonding to the ortho position. This is especially plausible if one or both substituents are large in size. Bonding to the ortho position would place the two substituents adjacent to each other, which is sterically unfavorable.

Importantly, this sterically-hindered EAS reaction still happens; it’s simply less likely. When a monosubstituted aromatic ring undergoes the next EAS, all three possible products (ortho-, meta-, and para-substituted) form. They undergo resonance with each other, but the most abundant product in the mixture is the most energetically favorable one. Often, this is the product with the least steric hindrance.

Conclusion

Through EAS, an aromatic ring gains a substituent which, in preparation for the next EAS reaction, donates or withdraws electrons from and activates or deactivates the ring. These directing effects, and any steric effects present, dictate whether the incoming electrophile will bond at the ring’s ortho, meta, or para position. Ongoing resonance underscores the electron density throughout the ring and the extent of activation.