Directive Influence of Functional Group in Monosubstituted Benzene — Revision Notes

⚡ 30-Second Revision

Ortho-Para Directors (Activating): — $-\text{NH}_2$, $-\text{NHR}$, $-\text{NR}_2$, $-\text{OH}$, $-\text{OR}$, $-\text{NHCOCH}_3$, $-\text{R}$ (alkyl), $-\text{C}_6\text{H}_5$. Increase reactivity.
Ortho-Para Directors (Deactivating): — Halogens ($-\text{F}$, $-\text{Cl}$, $-\text{Br}$, $-\text{I}$). Decrease reactivity.
Meta Directors (Deactivating): — $-\text{NO}_2$, $-\text{CN}$, $-\text{SO}_3\text{H}$, $-\text{CHO}$, $-\text{COOH}$, $-\text{COOR}$, $-\text{COR}$, $-\text{NR}_3^+$. Decrease reactivity.
Reactivity Order: — Strongly Activating > Moderately Activating > Weakly Activating > Benzene > Halogens > Weakly Deactivating (meta) > Moderately Deactivating (meta) > Strongly Deactivating (meta).
Key Effects: — Inductive ($I$) and Resonance ($M$) effects determine directive influence and reactivity.

2-Minute Revision

The directive influence of a functional group on a monosubstituted benzene dictates where an incoming electrophile will attach during electrophilic aromatic substitution (EAS). This is governed by the electronic effects (inductive and resonance) of the existing substituent. Groups are broadly classified as ortho-para directing or meta directing.

Ortho-para directors typically activate the benzene ring, making it more reactive than benzene. These are usually electron-donating groups like $-\text{OH}$, $-\text{NH}_2$, alkyl groups ($-\text{CH}_3$), which increase electron density at ortho and para positions. The major product is often para due to less steric hindrance. A crucial exception is halogens ($-\text{Cl}$, $-\text{Br}$), which are deactivating (due to strong $-I$ effect) but still ortho-para directing (due to $+M$ effect).

Meta directors typically deactivate the benzene ring, making it less reactive than benzene. These are usually electron-withdrawing groups like $-\text{NO}_2$, $-\text{COOH}$, $-\text{CHO}$, which deplete electron density from the ortho and para positions, leaving the meta position as the relatively less electron-deficient site for electrophilic attack. Understanding these classifications and the underlying electronic reasons is key for predicting reaction outcomes in NEET.

5-Minute Revision

The directive influence of a functional group in monosubstituted benzene is a fundamental concept for predicting the regioselectivity and reactivity in electrophilic aromatic substitution (EAS) reactions. The existing substituent modifies the electron density of the benzene ring through its inductive ($I$) and resonance ($M$) effects, thereby influencing the stability of the arenium ion intermediate formed during electrophilic attack.

1. Ortho-Para Directing Groups: These groups direct the incoming electrophile to the ortho and para positions. Most are ring-activating, meaning they increase the rate of EAS compared to benzene. They achieve this by donating electron density to the ring, primarily at the ortho and para positions, stabilizing the arenium ion.

Examples include: * Strongly Activating: $-\text{NH}_2$, $-\text{NHR}$, $-\text{NR}_2$, $-\text{OH}$, $-\text{OR}$ (e.g., Aniline, Phenol, Anisole). * Moderately Activating: $-\text{NHCOCH}_3$, $-\text{OCOR}$.

* Weakly Activating: Alkyl groups ($-\text{CH}_3$, $-\text{C}_2\text{H}_5$), Phenyl ($-\text{C}_6\text{H}_5$). * Exception (Deactivating but o,p-directing): Halogens ($-\text{F}$, $-\text{Cl}$, $-\text{Br}$, $-\text{I}$).

They are deactivating due to strong electron-withdrawing inductive effect ($-I$), but their electron-donating resonance effect ($+M$) is strong enough to direct to ortho and para positions. Para product is usually major due to steric hindrance.

2. Meta Directing Groups: These groups direct the incoming electrophile to the meta position. They are generally ring-deactivating, meaning they decrease the rate of EAS compared to benzene. They achieve this by withdrawing electron density from the ring, particularly from the ortho and para positions, making these positions highly electron-deficient and thus unfavorable for electrophilic attack.

The meta position, while still deactivated, is relatively less electron-deficient. Examples include: * Strongly Deactivating: $-\text{NO}_2$, $-\text{CN}$, $-\text{SO}_3\text{H}$, $-\text{CHO}$, $-\text{COOH}$, $-\text{COOR}$, $-\text{COR}$, $-\text{NR}_3^+$.

Reactivity Order: The overall reactivity follows a general trend: Strongly Activating > Moderately Activating > Weakly Activating > Benzene > Halogens > Weakly Deactivating (meta) > Moderately Deactivating (meta) > Strongly Deactivating (meta).

Example: Nitration of Toluene (o,p-director) yields o-nitrotoluene and p-nitrotoluene. Nitration of Nitrobenzene (m-director) yields m-dinitrotoluene. This understanding is critical for predicting products and relative reaction rates in NEET.

Prelims Revision Notes

Directive Influence in Monosubstituted Benzene (NEET Revision)

I. Core Concept: An existing functional group on a benzene ring dictates the position (ortho, meta, para) of an incoming electrophile in Electrophilic Aromatic Substitution (EAS) and affects the ring's overall reactivity.

II. Electronic Effects:

* **Inductive Effect ($I$):** Electron withdrawal ($-I$) or donation ($+I$) through $sigma$-bonds. Decreases with distance. * **Resonance Effect ($M$):** Electron withdrawal ($-M$) or donation ($+M$) through $pi$-electron delocalization. Stronger than inductive effect for most groups.

III. Classification of Functional Groups:

A. Ortho-Para Directing Groups:

1. Activating (Increase Reactivity): Electron-donating groups (via $+M$ or $+I$). Stabilize arenium ion. * Strongly Activating: $-\text{NH}_2$, $-\text{NHR}$, $-\text{NR}_2$, $-\text{OH}$, $-\text{OR}$ (e.

g., Aniline, Phenol, Anisole). Dominated by strong $+M$. * Moderately Activating: $-\text{NHCOCH}_3$, $-\text{OCOR}$. * Weakly Activating: Alkyl groups ($-\text{CH}_3$, $-\text{C}_2\text{H}_5$), Phenyl ($-\text{C}_6\text{H}_5$).

Primarily $+I$ and hyperconjugation. 2. Deactivating (Decrease Reactivity) but Ortho-Para Directing: * Halogens: $-\text{F}$, $-\text{Cl}$, $-\text{Br}$, $-\text{I}$. Strong $-I$ effect (deactivating) > $+M$ effect (o,p-directing).

Overall deactivating, but regioselectivity is o,p.

B. Meta Directing Groups:

1. Deactivating (Decrease Reactivity): Electron-withdrawing groups (via $-M$ or $-I$). Destabilize arenium ion, especially at o,p positions. * Strongly Deactivating: $-\text{NO}_2$, $-\text{CN}$, $-\text{SO}_3\text{H}$, $-\text{CHO}$, $-\text{COOH}$, $-\text{COOR}$, $-\text{COR}$, $-\text{NR}_3^+$. * These groups make ortho and para positions highly electron-deficient, disfavoring electrophilic attack. Meta position is relatively less deactivated.

IV. Reactivity Order (General Trend):

* Strongly Activating > Moderately Activating > Weakly Activating > Benzene > Halogens > Weakly Deactivating (meta) > Moderately Deactivating (meta) > Strongly Deactivating (meta).

V. Key Points for NEET:

* Steric Hindrance: For o,p-directors, para product is usually major due to less steric hindrance. * Halogen Anomaly: Crucial exception to remember (deactivating but o,p-directing). * Predicting Products: Identify substituent, its directive nature, and then the electrophile's attack position.

* Reagents for EAS: Know common reagents (e.g., $ext{HNO}_3/\text{H}_2\text{SO}_4$ for nitration, $ext{Br}_2/\text{FeBr}_3$ for bromination, $ext{RCl}/\text{AlCl}_3$ for Friedel-Crafts alkylation).

VI. Example:

* Toluene + Nitration: $-\text{CH}_3$ is weakly activating, o,p-directing. Products: o-nitrotoluene, p-nitrotoluene (major). * Nitrobenzene + Bromination: $-\text{NO}_2$ is strongly deactivating, m-directing. Product: m-bromonitrobenzene.

Master this table and the underlying electronic principles for quick and accurate answers.

Vyyuha Quick Recall

To remember common meta-directing groups (which are mostly deactivating), think of a 'Carboxylic Acid, Nitrile, Aldehyde, Ketone, Sulfonic Acid, Nitro' group as being 'CAN ASK NO' for ortho/para positions.

Carboxylic Acid ($-\text{COOH}$) Aldehyde ($-\text{CHO}$) Nitrile ($-\text{CN}$) Alkylammonium ($-\text{NR}_3^+$) Sulfonic Acid ($-\text{SO}_3\text{H}$) Ketone ($-\text{COR}$) Nitro ($-\text{NO}_2$)

These 'CAN ASK NO' groups are meta-directing. All others (except halogens) are generally ortho-para directing.