3.3.10.2 Electrophilic Substitution Reactions

• In an electrophilic substitution reaction, the electrophile attacks the electron-rich benzene ring.


  True
• The delocalization of pi electrons in benzene creates a continuous cloud of electron density above and below the ring.


  True
• In nitration, concentrated nitric acid reacts with concentrated sulfuric acid to form the nitronium ion.
• The carbocation intermediate formed in electrophilic substitution is called the sigma complex.


  True
• Steps in the mechanism of electrophilic substitution on benzene

  1️⃣ Formation of the electrophile

  2️⃣ Attack on benzene

  3️⃣ Loss of proton
• The sigma complex loses a proton to reform the aromatic ring.


  True
• Electrophiles in electrophilic substitution reactions are generated with the aid of catalysts
• The final step in electrophilic substitution involves the loss of a proton
• What are the two main effects of substituents on electrophilic substitution reactions?

  Rate and regioselectivity
• What type of substituents increase the likelihood of substitution at the ortho and para positions?

  Ortho/para-directing
• In nitration, the nitronium ion (NO₂⁺) replaces a hydrogen atom with a nitro (-NO₂) group
• In nitration, concentrated nitric acid reacts with concentrated sulfuric acid to form the nitronium ion (NO₂⁺)
• During electrophilic substitution, the electrophile attacks the benzene ring, forming an intermediate called the sigma complex
• The aromatic stability of benzene is due to the delocalization of pi electrons.


  True
• What is the first step in the mechanism of electrophilic substitution on benzene?

  Formation of the electrophile
• Electrophiles are electron-rich species.

  False
• The loss of a proton reforms the aromatic benzene ring.


  True
• What type of intermediate is formed when the electrophile attacks benzene?

  Carbocation intermediate
• Deactivating groups direct substitution to the meta position.
• The nitronium ion in nitration is generated from HNO₃ and H₂SO₄.

  True
• Electrophilic substitution reactions allow for the introduction of functional groups onto aromatic compounds like benzene.
• What intermediate is formed after the electrophile attacks the benzene ring in an electrophilic substitution reaction?

  Sigma complex
• Why is the delocalization of pi electrons in benzene important for its stability?

  Creates aromatic stability
• Steps of the electrophilic substitution mechanism:

  1️⃣ Formation of the electrophile

  2️⃣ Attack on benzene, forming the sigma complex

  3️⃣ Loss of a proton to reform the aromatic ring
• The electrophile in nitration is the nitronium ion, which is written as NO₂⁺
• What type of compound is typically attacked in electrophilic substitution reactions?

  Aromatic compound
• What type of bond is present in benzene that contributes to its aromatic stability?

  Delocalized pi bond
• Match the electrophile with its role in electrophilic substitution:

  Nitronium ion (NO₂⁺) ↔️ Nitration

  Halogens (Cl⁺, Br⁺) ↔️ Halogenation

  Acyl cations (RCO⁺) ↔️ Acylation
• The sigma complex in electrophilic substitution has a positive charge on the carbocation.

  True
• What effect do activating groups have on the rate of electrophilic substitution reactions?

  Increase the rate
• Activating groups in electrophilic substitution reactions are ortho and para directing
• What is the key electronic effect of activating groups on the benzene ring?

  Increase electron density
• Halogenation requires a Lewis acid catalyst to form halogen electrophiles.


  True
• All electrophilic substitution reactions proceed via a common mechanism involving a sigma complex.


  True
• Steps in the mechanism of electrophilic substitution reactions

  1️⃣ Electrophile attacks benzene ring

  2️⃣ Sigma complex is formed

  3️⃣ Proton is lost to reform aromatic ring
• What two acids are used to generate the nitronium ion (NO₂⁺) in nitration?

  Concentrated HNO₃ and H₂SO₄
• What electrophile is used in nitration reactions?

  Nitronium ion (NO₂⁺)
• What acids react to form the nitronium ion?

  HNO₃ and H₂SO₄
• Steps in the electrophilic substitution mechanism on benzene:

  1️⃣ Formation of the Electrophile

  2️⃣ Attack on Benzene

  3️⃣ Loss of Proton
• Activating groups are meta-directing.

  False