# Lecture 4: Substituent Effects in EAS
## Organic Chemistry II
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Classify substituents as activating or deactivating in electrophilic aromatic substitution
2. Predict the directing effects (ortho/para vs. meta) of common substituents
3. Explain substituent effects using resonance and inductive arguments
4. Predict the major product(s) of EAS on substituted benzenes
5. Apply directing effects to plan multi-step syntheses of polysubstituted benzenes
6. Analyze the reactivity of disubstituted benzenes with competing directing effects
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## Lecture Content
### I. Activating and Deactivating Groups
Substituents already present on a benzene ring affect both the rate of electrophilic aromatic substitution and the position at which the incoming electrophile attaches. Activating groups make the ring more reactive than unsubstituted benzene by donating electron density to the ring through resonance, induction, or both. This added electron density stabilizes the arenium ion intermediate more effectively, lowering the activation energy and accelerating the reaction. Deactivating groups have the opposite effect: they withdraw electron density from the ring, destabilize the arenium ion, and slow the reaction relative to benzene.
The full reactivity order, from most activated to most deactivated, runs as follows: -NH2, -NHR, -NR2 (most activating), then -OH, -OR, then -NHCOR, then -R, -Ar, then -H (benzene itself), then halogens (-X), then -CHO, -COR, then -SO3H, then -COOH, -COOR, then -CN, and finally -NO2 (most deactivating).
### II. Ortho/Para Directors
Electron-donating groups direct incoming electrophiles to the ortho and para positions of the ring. The most strongly activating ortho/para directors are groups with a lone pair on the atom directly attached to the ring, including amino groups (-NH2, -NHR, -NR2), hydroxyl (-OH), and alkoxy (-OR). These substituents donate electron density to the ring by resonance, placing negative charge at the ortho and para positions and thereby stabilizing the arenium ion when electrophilic attack occurs at those positions.
Moderately activating substituents such as amides (-NHCOR) also direct ortho/para, although the nitrogen lone pair is partially delocalized into the amide carbonyl, reducing the extent of donation into the ring. Weakly activating groups such as alkyl (-R) and aryl (-Ar) substituents lack lone pairs for direct resonance donation but nonetheless stabilize the arenium ion at ortho and para positions through inductive donation and hyperconjugation.
The underlying logic becomes clear when one draws the three resonance structures of the arenium ion for electrophilic attack at each possible position. Attack at the ortho or para positions places the positive charge on the carbon directly bearing the substituent, which allows direct resonance stabilization by the electron-donating group. Attack at the meta position never places the positive charge on the substituted carbon, so the substituent cannot stabilize the intermediate as effectively.
Three sets of arenium ion resonance structures for the nitration of anisole (methoxybenzene). Set 1: Electrophilic attack at the ortho position, showing three resonance structures. The fourth, especially stable structure shows the positive charge on the carbon bearing the -OCH3 group, with donation of the oxygen lone pair. Set 2: Attack at the para position with a similar especially stable resonance contributor. Set 3: Attack at the meta position showing that no resonance structure places the positive charge adjacent to the -OCH3 group. Labels indicate ortho and para are favored.
### III. Meta Directors
Electron-withdrawing groups direct incoming electrophiles to the meta position and are all deactivating (with the special exception of halogens, discussed below). Common meta directors include the nitro group (-NO2), cyano group (-CN), sulfonic acid (-SO3H), aldehydes and ketones (-CHO, -COR), carboxylic acids and esters (-COOH, -COOR), and trifluoromethyl (-CF3).
These groups withdraw electron density from the ring through resonance, induction, or both. When the electrophile attacks at the ortho or para positions, one of the resonance structures of the arenium ion places the positive charge directly on the carbon bearing the electron-withdrawing group. This creates an especially unstable resonance contributor, because the positive charge is adjacent to an atom that is already pulling electron density away. Attack at the meta position avoids placing the positive charge on the substituted carbon, making it the least destabilized position. Meta direction is therefore not the result of activation at the meta position; it simply reflects the fact that meta is the least unfavorable site on a deactivated ring.
### IV. Halogens: The Special Case
Halogens (-F, -Cl, -Br, -I) are unique among common substituents because they are deactivating yet ortho/para directing. This seemingly contradictory behavior arises from competing electronic effects. The inductive effect of halogens is deactivating: their high electronegativity pulls electron density away from the ring through the sigma bond, making the ring less electron-rich overall and slowing the reaction relative to benzene.
At the same time, halogens possess lone pairs that can donate electron density into the ring by resonance. This resonance donation is relatively inefficient, particularly for the larger halogens, because the 3p, 4p, or 5p orbitals of Cl, Br, and I overlap poorly with the 2p orbitals of carbon. Nevertheless, even this modest resonance donation is sufficient to preferentially stabilize the arenium ion when electrophilic attack occurs at the ortho and para positions. The net result is that the reaction proceeds more slowly than with benzene (deactivation dominates), but the products are primarily ortho and para (the directing effect of resonance donation prevails). Fluorine, with its 2p orbitals, is the best resonance donor among the halogens but also the strongest inductive withdrawer.
### V. Summary Table of Substituent Effects
The full classification of substituent effects organizes groups into six categories. Strongly activating ortho/para directors include -NH2, -NHR, -NR2, -OH, and -OR. Moderately activating ortho/para directors include -NHCOR and -OCOR. Weakly activating ortho/para directors include -R and -Ar. Weakly deactivating ortho/para directors are the halogens -F, -Cl, -Br, and -I. Moderately deactivating meta directors include -CHO, -COR, -COOH, -COOR, and -SO3H. Strongly deactivating meta directors include -NO2, -CN, -CF3, and -NR3+.
A comprehensive table organized as a gradient arrow from "strongly activating" at the top to "strongly deactivating" at the bottom. Each row shows the substituent, its classification (activating/deactivating), its directing effect (ortho-para or meta), and a small diagram of benzene showing the preferred positions of attack highlighted. Halogens are specially marked at the boundary between activating and deactivating with a note explaining the dual nature of their effects.
### VI. Predicting Products of Disubstituted Benzenes
When a benzene ring already bears two substituents, their directing effects may either cooperate or compete. In cooperative (reinforcing) cases, both substituents direct the incoming electrophile to the same position or positions, making product prediction straightforward. For example, in p-nitrotoluene, the methyl group directs ortho/para while the nitro group directs meta, and both effects point toward the same ring position between them.
When the two substituents direct to different positions, a competition arises, and the stronger activating group wins. The incoming electrophile preferentially attacks at the position dictated by the more strongly activating (or less strongly deactivating) substituent. Steric effects also play a role: when both ortho and para positions are available, bulky groups tend to reduce the amount of ortho substitution, making the para product predominate. A tert-butyl group, for instance, gives almost exclusively para substitution due to the steric congestion at the ortho positions.
### VII. Synthetic Strategy with EAS
The order in which substituents are introduced onto a benzene ring is critically important in multistep synthesis because each newly installed group alters the directing behavior for subsequent reactions. Consider the synthesis of p-bromonitrobenzene. If benzene is nitrated first, the nitro group (a meta director) is installed; subsequent bromination would then produce m-bromonitrobenzene, the wrong isomer. The correct approach is to brominate benzene first, installing the ortho/para-directing bromine, and then perform nitration to obtain p-bromonitrobenzene (along with some ortho isomer, which can be separated).
A blocking group strategy can further refine selectivity. Because sulfonation is reversible, a sulfonic acid group can be introduced to temporarily occupy a ring position, directing a subsequent EAS reaction to a different site. After the desired substitution has been accomplished, the sulfonic acid group is removed by desulfonation. Functional group interconversions after EAS also expand the synthetic toolkit: the nitro group can be reduced to an amino group, a benzylic methyl can be oxidized to a carboxylic acid with KMnO4, and an acyl group can be reduced to an alkyl group by Clemmensen or Wolff-Kishner reduction.
Flowchart showing two synthetic routes to para-nitrotoluene. Route A (incorrect order): benzene undergoes nitration to give nitrobenzene, then Friedel-Crafts alkylation with CH3Cl/AlCl3 gives meta-methylnitrobenzene (wrong product, X). Route B (correct order): benzene undergoes Friedel-Crafts alkylation to give toluene, then nitration with HNO3/H2SO4 gives a mixture of ortho- and para-nitrotoluene (correct products, checkmark), which can be separated. An arrow emphasizes "install the ortho/para director first."
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