Electrophilic Aromatic Substitution Is

Question 1

What are the key learning objectives for Electrophilic Aromatic Substitution (EAS)?

Answer 1

Understand the concept of electrophilic aromatic substitution (EAS).
Analyze the mechanism of second substitution and influencing factors.
Differentiate between ortho, para, and meta directors.
Explore resonance stabilization of substitution products.
Examine variations in electrophilic substitution.

Question 2

What is the general mechanism for electrophilic substitution of benzene?

Answer 2

Step 1 (Slow):

Formation of a positively charged intermediate (arenium/benzenonium ion) by reaction with the electrophile (E⁺).
Electrophile examples: NO₂⁺, SO₃H⁺.
This step disrupts aromaticity temporarily.
Step 2 (Fast):

Loss of a proton (H⁺), restoring aromaticity and substituting H with E.

Question 3

How is the benzenonium ion stabilized in EAS reactions?

Answer 3

The benzenonium ion is stabilized via resonance, distributing the positive charge across the ring.

Question 4

What are the key mono-substitution reactions of benzene and their products?

Answer 4

Halogenation: Benzene + Cl₂/FeCl₃ → Chlorobenzene
Nitration: Benzene + HNO₃ (H₂SO₄ catalyst) → Nitrobenzene
Friedel-Crafts Alkylation: Benzene + RCl (AlCl₃ catalyst) → Alkylbenzene
Friedel-Crafts Acylation: Benzene + RCOCl (AlCl₃ catalyst) → Acylbenzene
Sulfonation: Benzene + SO₃ (H₂SO₄ catalyst) → Benzene Sulfonic Acid

Question 5

What key questions are considered in second substitution on benzene?

Answer 5

How does the rate of substitution compare to unsubstituted benzene?
Where does the substituent direct the incoming group (ortho, meta, para)?

Question 6

What are activating and deactivating groups in benzene substitution?

Answer 6

• Activating Groups (e.g., -NH₂, -OH, -OR): Increase reactivity, direct to ortho/para.
• Deactivating Groups (e.g., -NO₂, -COOH): Decrease reactivity, direct to meta.

Question 7

What are some examples of electron-donating (activating) and electron-withdrawing (deactivating) groups?

Answer 7

Ortho/Para Directors (Activating):

-NH₂, -NHR, -NR₂, -OH, -OR, -R, -C₆H₅
Meta Directors (Deactivating):

-NO₂, -SO₃H, -COOH, -CN, -NR₃⁺

Question 8

Why does aniline (-NH₂) act as an ortho/para director?

Answer 8

• The amino group is electron-donating.
• Ortho and para positions become negatively charged, making them favorable to electrophilic attack.
• This negative charge is delocalized via π-system conjugation.

Question 9

Compare the stability of intermediates in ortho, meta, and para attacks in EAS.

Answer 9

• Ortho Attack: Stabilized by resonance
• Para Attack: Also stabilized (4 resonance structures)
• Meta Attack: Least stable due to lack of resonance stabilization

Question 10

Why is nitrobenzene a meta director?

Answer 10

• The -NO₂ group is electron withdrawing.
• It pulls electrons away, making ortho and para positions positively charged.
• The positive charge is delocalized, deactivating the ring for electrophilic attack.
• Meta position remains less positive, so meta substitution is preferred.

Question 11

Why is meta substitution more favorable in nitrobenzene?

Answer 11

• In meta substitution, the positive charge is not adjacent to the NO₂ group.
• The intermediate is relatively more stable.
• Ortho/para intermediates are less stable due to adjacent positive charges and destabilization.