Chemistry/Chapter 8: Hydrocarbons
8.4 Electrophilic aromatic substitution in substituted arenes
The main idea is that the substituent already attached to the benzene ring directs the incoming electrophile to specific positions (ortho, m
Explanation level
In the previous section, we learned that benzene undergoes electrophilic aromatic substitution to form mono-substituted benzenes. But what happens when a second substituent is introduced? The presence of the first substituent on the benzene ring influences both the rate of the second substitution and the position (regiochemistry) where the new group attaches. This is a crucial concept in organic synthesis, as it allows chemists to predict and control the outcome of reactions on aromatic rings.
Main Idea
The main idea is that the substituent already attached to the benzene ring directs the incoming electrophile to specific positions (ortho, meta, or para) and can either activate (speed up) or deactivate (slow down) the ring towards further substitution. This is known as the directing effect of a substituent. Substituents are classified as either ortho/para-directing (usually activating) or meta-directing (usually deactivating).
Core Concept
The core concept is that the stability of the arenium ion (sigma complex) intermediate determines where the second electrophile will attack. The substituent already on the ring can stabilize or destabilize the positive charge in the arenium ion, depending on its electronic effects (inductive and resonance).
- Electron-donating groups (EDGs) stabilize the arenium ion, making the ring more reactive (activating) and directing the electrophile to the ortho and para positions (where the positive charge can be delocalized onto the substituent).
- Electron-withdrawing groups (EWGs) destabilize the arenium ion, making the ring less reactive (deactivating) and directing the electrophile to the meta position (where the positive charge is not delocalized onto the already positive substituent).
Detailed Explanation
The Reaction of Methylbenzene (Toluene) with Nitric Acid
- 2-nitromethylbenzene (ortho): 58%
- 3-nitromethylbenzene (meta): 4%
- 4-nitromethylbenzene (para): 38%
The textbook notes that if the reaction were carried out in a random way, the distribution would be 40:40:20 (because there are two ortho positions, two meta positions, and one para position). The fact that the actual distribution is 58:4:38 indicates that the methyl group directs the incoming electrophile preferentially to the ortho and para positions. The methyl group is an ortho/para-directing group.
Directing Effects of Substituents
Substituents can be classified into two main categories based on their directing effects.
(a) Ring-Activating or Ortho/Para-Directing Substituents
Electron-donating substituents activate the benzene ring and direct the incoming electrophile to the ortho (position 2) and para (position 4) positions.
- How it works: The substituent has a lone pair of electrons on the atom directly attached to the ring (or is an alkyl group that donates electron density through inductive effect). This electron density can be delocalized into the arenium ion intermediate, stabilizing it. The stabilization is greatest when the electrophile attacks the ortho or para position, because the positive charge in the arenium ion can be delocalized onto the substituent.
- Why ortho/para? In both ortho and para attack, a resonance structure is formed possessing a positive charge on the carbon directly connected to the substituent. This resonance structure is the most stable one among all because its positive charge is stabilized more than that resulting from meta attack.
- Examples: Alkyl group (-R), hydroxyl group (-OH), amino group (-NH2), alkoxy group (-OR), and halo group (-X).
- Key Information: All halo groups (-X) are ortho and para directing due to resonance effect but deactivating due to inductive effect.
(b) Ring-Deactivating or Meta-Directing Substituents
Electron-withdrawing substituents deactivate the benzene ring and direct the incoming electrophile to the meta (position 3) position.
- How it works: These substituents withdraw electrons from the benzene ring through the inductive effect, making the overall benzene ring electron-deficient and less reactive. Such substituents possess a partial or full positive charge on the atom directly connected to the benzene ring. This destabilizes the arenium ion intermediate.
- Why meta? When the electrophile attacks the meta position, the positive charge in the arenium ion is not delocalized onto the carbon atom directly connected to the electron-withdrawing substituent. This avoids placing the positive charge adjacent to an already positive (or partially positive) atom, which would be highly destabilizing.
- Examples: Nitro group (-NO2), aldehyde group (-CHO), carboxylic acid group (-COOH), ketone group (-COR), ester group (-COOR), and cyano group (-CN).
Summary of Directing Effects
| Substituent | Effect on Ring | Directing Effect |
|---|---|---|
| Alkyl (-R) | Activating | Ortho/para |
| Hydroxyl (-OH) | Activating | Ortho/para |
| Amino (-NH2) | Activating | Ortho/para |
| Alkoxy (-OR) | Activating | Ortho/para |
| Halo (-X) | Deactivating | Ortho/para |
| Nitro (-NO2) | Deactivating | Meta |
| Carbonyl (-CHO, -COR) | Deactivating | Meta |
| Carboxyl (-COOH) | Deactivating | Meta |
| Cyano (-CN) | Deactivating | Meta |
The Two Roles of Aluminium Chloride in Friedel-Craft Alkylation
- It acts as a Lewis acid to generate the electrophile (the carbocation).
- It acts as a halogen carrier, carrying the halogen with itself in the reaction.
Intermediate Level
The directing effect of a substituent on a benzene ring is determined by its ability to stabilize or destabilize the arenium ion intermediate. Electron-donating groups (EDGs) such as alkyl, hydroxyl, and amino groups stabilize the arenium ion by delocalizing the positive charge onto the substituent. This stabilization is greatest for ortho and para attack, so these groups are ortho/para-directing and activating. Electron-withdrawing groups (EWGs) such as nitro, carbonyl, and cyano groups destabilize the arenium ion by withdrawing electron density. For meta attack, the positive charge in the arenium ion is not placed on the carbon bearing the EWG, so the meta attack is less destabilized than ortho or para attack. Therefore, EWGs are meta-directing and deactivating. Halogens are a special case: they are deactivating due to their inductive electron withdrawal, but they are ortho/para-directing because they can donate electrons through resonance.
Important Points
- - Directing effect: The substituent on the benzene ring determines where the incoming electrophile attacks.
- - Activating groups: Increase the rate of electrophilic substitution.
- - Deactivating groups: Decrease the rate of electrophilic substitution.
- - Ortho/para-directing: Electron-donating groups (e.g., -OH, -NH2, -CH3) and halogens.
- - Meta-directing: Electron-withdrawing groups (e.g., -NO2, -CHO, -COOH, -CN).
- - Reason for directing effect: Stabilization or destabilization of the arenium ion intermediate.
- - Halogens: Deactivating but ortho/para-directing.
- - Alkyl groups: Activating and ortho/para-directing.
Remember
- - EDG = Activating, ortho/para-directing.
- - EWG = Deactivating, meta-directing.
- - Halogens = Deactivating, ortho/para-directing.
- - Reason: Stability of the arenium ion intermediate.
- - Ortho/para: Positive charge can be delocalized onto the substituent.
- - Meta: Positive charge avoids the substituent.
- - Example: Toluene → ortho/para products.
- - Example: Nitrobenzene → meta product.
Common Mistakes
- - Thinking that all activating groups are ortho/para-directing. This is generally true, but halogens are an exception (deactivating but ortho/para-directing).
- - Assuming that all deactivating groups are meta-directing. This is also generally true, but halogens are an exception.
- - Forgetting the reason for the directing effect. The key is the stabilization of the arenium ion intermediate.
- - Confusing the terms "activating" and "directing." Activating/deactivating refers to the rate of reaction; directing refers to the regiochemistry.
- - Not considering the number of positions. There are two ortho positions, two meta positions, and one para position. This affects the statistical distribution, but the directing effect is more significant.
Quick Revision
The presence of a substituent on a benzene ring affects both the rate and the position of further electrophilic aromatic substitution. Electron-donating groups (EDGs) such as alkyl, hydroxyl, and amino groups are activating and direct the incoming electrophile to the ortho and para positions because they stabilize the arenium ion intermediate through resonance and inductive effects. Electron-withdrawing groups (EWGs) such as nitro, carbonyl, and cyano groups are deactivating and direct the electrophile to the meta position because the positive charge in the arenium ion is less destabilized when it avoids the substituted carbon. Halogens are a special case: they are deactivating due to their strong inductive electron withdrawal, but they are ortho/para-directing because they can donate electrons through resonance. The directing effect is determined by the relative stability of the arenium ion intermediates formed from attack at different positions.
60-Second Revision
The substituent on a benzene ring determines where the next electrophile will attack. Electron-donating groups (EDGs) are activating and direct to ortho/para positions. Electron-withdrawing groups (EWGs) are deactivating and direct to the meta position. Halogens are deactivating but ortho/para-directing. The reason is the stability of the arenium ion intermediate: ortho/para attack allows the positive charge to be stabilized by EDGs or destabilized by EWGs, while meta attack avoids the substituent.
Frequently Asked Questions (FAQs)
Q1. What is the directing effect of a substituent? Answer: The directing effect refers to the ability of a substituent already attached to a benzene ring to influence the position (ortho, meta, or para) where an incoming electrophile will attack during a further substitution reaction. Substituents can be classified as ortho/para-directing or meta-directing.
Q2. Why is the methyl group in toluene an ortho/para-directing group? Answer: The methyl group is an electron-donating group (through inductive effect). It stabilizes the arenium ion intermediate that forms when the electrophile attacks the ortho or para positions, because the positive charge can be delocalized onto the carbon bearing the methyl group. This makes ortho and para attack more favorable, so toluene directs the incoming electrophile to the ortho and para positions.
Q3. Why is the nitro group in nitrobenzene a meta-directing group? Answer: The nitro group is a strong electron-withdrawing group (through both inductive and resonance effects). When the electrophile attacks the ortho or para positions, the positive charge in the arenium ion is placed on the carbon bearing the nitro group, which is highly destabilizing. The meta attack avoids this, so the meta product is favored.
Q4. Why are halogens deactivating but ortho/para-directing? Answer: Halogens are deactivating because they are strongly electronegative and withdraw electron density from the ring through the inductive effect. However, they are ortho/para-directing because they have lone pairs of electrons that can be donated to the ring through resonance. The resonance donation stabilizes the arenium ion intermediate for ortho/para attack, even though the overall reaction is slower due to the inductive withdrawal.
Q5. What is the difference between an activating group and a deactivating group? Answer: An activating group increases the electron density of the benzene ring, making it more reactive towards electrophiles and increasing the rate of substitution. A deactivating group decreases the electron density, making the ring less reactive and slowing the reaction.
Q6. Why is ortho attack not the only product for toluene, even though there are two ortho positions? Answer: Ortho attack can be sterically hindered by the methyl group, making it less favorable than para attack in some cases. The distribution of products depends on both the electronic directing effect and steric hindrance. For toluene nitration, the distribution is 58% ortho, 4% meta, and 38% para, reflecting the combination of electronic and steric factors.
Q7. What are the two roles of aluminium chloride in Friedel-Craft alkylation? Answer: AlCl_3 acts as a Lewis acid to generate the electrophile (a carbocation) from the alkyl halide. It also acts as a halogen carrier, taking the halogen from the alkyl halide to form the reactive electrophile.
Q8. How does the directing effect relate to the stability of the arenium ion? Answer: The stability of the arenium ion intermediate determines the preferred position of attack. For ortho/para attack, the positive charge can be delocalized onto the carbon bearing the substituent. If the substituent is electron-donating, it stabilizes this positive charge, making ortho/para attack favorable. If the substituent is electron-withdrawing, it destabilizes the positive charge, making ortho/para attack unfavorable. Meta attack avoids placing the positive charge on the substituted carbon, so it is favored by electron-withdrawing groups.
- Ortho/para-directing: The hydroxyl group (-OH) in phenol is an activating, ortho/para-directing group.
- Meta-directing: The nitro group (-NO2) in nitrobenzene is a deactivating, meta-directing group.
Multiple Choice Questions (MCQs)
Q1. Which of the following is an ortho/para-directing group? A. -NO2 B. -COOH C. -OH D. -CN Correct Answer: C Explanation: The hydroxyl group (-OH) is an electron-donating group that activates the ring and directs the incoming electrophile to the ortho and para positions.
- Q2. Which of the following is a meta-directing group?
- A. -CH3
- B. -NH2
- C. -NO2
- D. -OH
- Correct Answer: C
- Explanation: The nitro group (-NO2) is an electron-withdrawing group that deactivates the ring and directs the incoming electrophile to the meta position.
- Q3. Why is the methyl group in toluene an ortho/para-directing group?
- A. It is electron-withdrawing.
- B. It is electron-donating.
- C. It is neutral.
- D. It is a halogen.
- Correct Answer: B
- Explanation: The methyl group donates electron density through inductive effect, stabilizing the arenium ion for ortho/para attack.
- Q4. Which of the following is a deactivating but ortho/para-directing group?
- A. -OH
- B. -NH2
- C. -Cl
- D. -NO2
- Correct Answer: C
- Explanation: Halogens like chlorine are deactivating due to inductive withdrawal but ortho/para-directing due to resonance donation.
- Q5. What is the product distribution when toluene is nitrated?
- A. 100% ortho
- B. 100% para
- C. 58% ortho, 4% meta, 38% para
- D. 40% ortho, 40% meta, 20% para
- Correct Answer: C
- Explanation: The textbook states that the distribution is 58% ortho, 4% meta, and 38% para.
- Q6. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid to generate the electrophile and as a halogen carrier.
- Q7. Why are halogens ortho/para-directing despite being deactivating?
- A. Because they are electronegative.
- B. Because they have lone pairs that can be donated through resonance.
- C. Because they are small in size.
- D. Because they are strong bases.
- Correct Answer: B
- Explanation: Halogens have lone pairs that can stabilize the arenium ion through resonance for ortho/para attack.
- Q8. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -CH3
- D. -COOH
- Correct Answer: C
- Explanation: The methyl group (-CH3) is an activating group that donates electron density to the ring.
- Q9. What is the directing effect of the -COOH group?
- A. Ortho/para
- B. Meta
- C. Both ortho and meta
- D. No directing effect
- Correct Answer: B
- Explanation: The carboxylic acid group (-COOH) is an electron-withdrawing group that directs to the meta position.
- Q10. Why is the arenium ion more stable for ortho/para attack with an electron-donating group?
- A. Because the group withdraws electrons.
- B. Because the positive charge can be delocalized onto the substituent.
- C. Because the group is bulky.
- D. Because the group is a halogen.
- Correct Answer: B
- Explanation: The positive charge in the arenium ion can be delocalized onto the electron-donating substituent, stabilizing the intermediate.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is an ortho/para-directing group?
- A. -NO2
- B. -COOH
- C. -OH
- D. -CN
- Correct Answer: C
- Explanation: The hydroxyl group (-OH) is an ortho/para-directing group.
- Q2. Which of the following is a meta-directing group?
- A. -CH3
- B. -NH2
- C. -NO2
- D. -OH
- Correct Answer: C
- Explanation: The nitro group (-NO2) is a meta-directing group.
- Q3. Why is the methyl group in toluene an ortho/para-directing group?
- A. It is electron-withdrawing.
- B. It is electron-donating.
- C. It is neutral.
- D. It is a halogen.
- Correct Answer: B
- Explanation: The methyl group donates electron density.
- Q4. Which of the following is a deactivating but ortho/para-directing group?
- A. -OH
- B. -NH2
- C. -Cl
- D. -NO2
- Correct Answer: C
- Explanation: Halogens are deactivating but ortho/para-directing.
- Q5. What is the product distribution when toluene is nitrated?
- A. 100% ortho
- B. 100% para
- C. 58% ortho, 4% meta, 38% para
- D. 40% ortho, 40% meta, 20% para
- Correct Answer: C
- Explanation: The distribution is 58% ortho, 4% meta, and 38% para.
- Q6. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid and halogen carrier.
- Q7. Why are halogens ortho/para-directing despite being deactivating?
- A. Because they are electronegative.
- B. Because they have lone pairs that can be donated through resonance.
- C. Because they are small in size.
- D. Because they are strong bases.
- Correct Answer: B
- Explanation: Halogens have lone pairs that stabilize the arenium ion through resonance.
- Q8. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -CH3
- D. -COOH
- Correct Answer: C
- Explanation: The methyl group is an activating group.
- Q9. What is the directing effect of the -COOH group?
- A. Ortho/para
- B. Meta
- C. Both ortho and meta
- D. No directing effect
- Correct Answer: B
- Explanation: The -COOH group is meta-directing.
- Q10. Why is the arenium ion more stable for ortho/para attack with an electron-donating group?
- A. Because the group withdraws electrons.
- B. Because the positive charge can be delocalized onto the substituent.
- C. Because the group is bulky.
- D. Because the group is a halogen.
- Correct Answer: B
- Explanation: The positive charge can be delocalized onto the electron-donating substituent.
Multiple Choice Questions (MCQs)
- Q1. Which of the following groups is meta-directing?
- A. -OH
- B. -NH2
- C. -CHO
- D. -CH3
- Correct Answer: C
- Explanation: The aldehyde group (-CHO) is meta-directing.
- Q2. Which of the following groups is ortho/para-directing?
- A. -NO2
- B. -CN
- C. -OCH3
- D. -COOH
- Correct Answer: C
- Explanation: The methoxy group (-OCH3) is ortho/para-directing.
- Q3. What is the directing effect of the -NH2 group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: A
- Explanation: The amino group (-NH2) is ortho/para-directing.
- Q4. What is the directing effect of the -CN group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: B
- Explanation: The cyano group (-CN) is meta-directing.
- Q5. Which of the following is a deactivating group?
- A. -OH
- B. -NH2
- C. -NO2
- D. -CH3
- Correct Answer: C
- Explanation: The nitro group (-NO2) is deactivating.
- Q6. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -OH
- D. -COOH
- Correct Answer: C
- Explanation: The hydroxyl group (-OH) is activating.
- Q7. Why does the nitro group direct meta?
- A. Because it donates electrons.
- B. Because it withdraws electrons, destabilizing ortho/para intermediates.
- C. Because it is a halogen.
- D. Because it is bulky.
- Correct Answer: B
- Explanation: The nitro group withdraws electrons, destabilizing the arenium ion for ortho/para attack.
- Q8. Which of the following is a correct statement about the directing effect of halogens?
- A. They are activating and ortho/para-directing.
- B. They are deactivating and meta-directing.
- C. They are deactivating and ortho/para-directing.
- D. They are activating and meta-directing.
- Correct Answer: C
- Explanation: Halogens are deactivating but ortho/para-directing.
- Q9. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid and halogen carrier.
- Q10. Which of the following is the major product when toluene is nitrated?
- A. 2-nitromethylbenzene
- B. 3-nitromethylbenzene
- C. 4-nitromethylbenzene
- D. All are equal
- Correct Answer: A
- Explanation: The major product is 2-nitromethylbenzene (ortho), with 58% yield.
Multiple Choice Questions (MCQs)
- Q1. Which of the following groups is meta-directing?
- A. -OH
- B. -NH2
- C. -CHO
- D. -CH3
- Correct Answer: C
- Explanation: The aldehyde group (-CHO) is meta-directing.
- Q2. Which of the following groups is ortho/para-directing?
- A. -NO2
- B. -CN
- C. -OCH3
- D. -COOH
- Correct Answer: C
- Explanation: The methoxy group (-OCH3) is ortho/para-directing.
- Q3. What is the directing effect of the -NH2 group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: A
- Explanation: The amino group (-NH2) is ortho/para-directing.
- Q4. What is the directing effect of the -CN group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: B
- Explanation: The cyano group (-CN) is meta-directing.
- Q5. Which of the following is a deactivating group?
- A. -OH
- B. -NH2
- C. -NO2
- D. -CH3
- Correct Answer: C
- Explanation: The nitro group (-NO2) is deactivating.
- Q6. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -OH
- D. -COOH
- Correct Answer: C
- Explanation: The hydroxyl group (-OH) is activating.
- Q7. Why does the nitro group direct meta?
- A. Because it donates electrons.
- B. Because it withdraws electrons, destabilizing ortho/para intermediates.
- C. Because it is a halogen.
- D. Because it is bulky.
- Correct Answer: B
- Explanation: The nitro group withdraws electrons, destabilizing the arenium ion for ortho/para attack.
- Q8. Which of the following is a correct statement about the directing effect of halogens?
- A. They are activating and ortho/para-directing.
- B. They are deactivating and meta-directing.
- C. They are deactivating and ortho/para-directing.
- D. They are activating and meta-directing.
- Correct Answer: C
- Explanation: Halogens are deactivating but ortho/para-directing.
- Q9. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid and halogen carrier.
- Q10. Which of the following is the major product when toluene is nitrated?
- A. 2-nitromethylbenzene
- B. 3-nitromethylbenzene
- C. 4-nitromethylbenzene
- D. All are equal
- Correct Answer: A
- Explanation: The major product is 2-nitromethylbenzene (ortho), with 58% yield.
Multiple Choice Questions (MCQs)
- Q1. Which of the following groups is meta-directing?
- A. -OH
- B. -NH2
- C. -CHO
- D. -CH3
- Correct Answer: C
- Explanation: The aldehyde group (-CHO) is meta-directing.
- Q2. Which of the following groups is ortho/para-directing?
- A. -NO2
- B. -CN
- C. -OCH3
- D. -COOH
- Correct Answer: C
- Explanation: The methoxy group (-OCH3) is ortho/para-directing.
- Q3. What is the directing effect of the -NH2 group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: A
- Explanation: The amino group (-NH2) is ortho/para-directing.
- Q4. What is the directing effect of the -CN group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: B
- Explanation: The cyano group (-CN) is meta-directing.
- Q5. Which of the following is a deactivating group?
- A. -OH
- B. -NH2
- C. -NO2
- D. -CH3
- Correct Answer: C
- Explanation: The nitro group (-NO2) is deactivating.
- Q6. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -OH
- D. -COOH
- Correct Answer: C
- Explanation: The hydroxyl group (-OH) is activating.
- Q7. Why does the nitro group direct meta?
- A. Because it donates electrons.
- B. Because it withdraws electrons, destabilizing ortho/para intermediates.
- C. Because it is a halogen.
- D. Because it is bulky.
- Correct Answer: B
- Explanation: The nitro group withdraws electrons, destabilizing the arenium ion for ortho/para attack.
- Q8. Which of the following is a correct statement about the directing effect of halogens?
- A. They are activating and ortho/para-directing.
- B. They are deactivating and meta-directing.
- C. They are deactivating and ortho/para-directing.
- D. They are activating and meta-directing.
- Correct Answer: C
- Explanation: Halogens are deactivating but ortho/para-directing.
- Q9. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid and halogen carrier.
- Q10. Which of the following is the major product when toluene is nitrated?
- A. 2-nitromethylbenzene
- B. 3-nitromethylbenzene
- C. 4-nitromethylbenzene
- D. All are equal
- Correct Answer: A
- Explanation: The major product is 2-nitromethylbenzene (ortho), with 58% yield.
Multiple Choice Questions (MCQs)
- Q1. Which of the following groups is meta-directing?
- A. -OH
- B. -NH2
- C. -CHO
- D. -CH3
- Correct Answer: C
- Explanation: The aldehyde group (-CHO) is meta-directing.
- Q2. Which of the following groups is ortho/para-directing?
- A. -NO2
- B. -CN
- C. -OCH3
- D. -COOH
- Correct Answer: C
- Explanation: The methoxy group (-OCH3) is ortho/para-directing.
- Q3. What is the directing effect of the -NH2 group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: A
- Explanation: The amino group (-NH2) is ortho/para-directing.
- Q4. What is the directing effect of the -CN group?
- A. Ortho/para
- B. Meta
- C. Both
- D. None
- Correct Answer: B
- Explanation: The cyano group (-CN) is meta-directing.
- Q5. Which of the following is a deactivating group?
- A. -OH
- B. -NH2
- C. -NO2
- D. -CH3
- Correct Answer: C
- Explanation: The nitro group (-NO2) is deactivating.
- Q6. Which of the following is an activating group?
- A. -NO2
- B. -CN
- C. -OH
- D. -COOH
- Correct Answer: C
- Explanation: The hydroxyl group (-OH) is activating.
- Q7. Why does the nitro group direct meta?
- A. Because it donates electrons.
- B. Because it withdraws electrons, destabilizing ortho/para intermediates.
- C. Because it is a halogen.
- D. Because it is bulky.
- Correct Answer: B
- Explanation: The nitro group withdraws electrons, destabilizing the arenium ion for ortho/para attack.
- Q8. Which of the following is a correct statement about the directing effect of halogens?
- A. They are activating and ortho/para-directing.
- B. They are deactivating and meta-directing.
- C. They are deactivating and ortho/para-directing.
- D. They are activating and meta-directing.
- Correct Answer: C
- Explanation: Halogens are deactivating but ortho/para-directing.
- Q9. What is the role of aluminium chloride in Friedel-Craft alkylation?
- A. It acts as a nucleophile.
- B. It acts as a Lewis acid and halogen carrier.
- C. It acts as a base.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: AlCl_3 acts as a Lewis acid and halogen carrier.
- Q10. Which of the following is the major product when toluene is nitrated?
- A. 2-nitromethylbenzene
- B. 3-nitromethylbenzene
- C. 4-nitromethylbenzene
- D. All are equal
- Correct Answer: A
- Explanation: The major product is 2-nitromethylbenzene (ortho), with 58% yield.
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8.4 Electrophilic aromatic substitution in substituted arenes
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Key Point 1
- EDG: Activating, ortho/para-directing.
Key Point 2
- EWG: Deactivating, meta-directing.
Key Point 3
- Halogens: Deactivating, ortho/para-directing.
Key Point 4
- Reason: Stability of the arenium ion intermediate.
Key Point 5
- Ortho/para: Positive charge delocalized onto substituent.
Key Point 6
- Meta: Positive charge avoids substituent.
Key Point 7
- Example: Toluene gives ortho (58%) and para (38%) products.
Key Point 8
- Example: Nitrobenzene gives meta product.
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