Which of the following is most likely to be the first step in the general mechanism for aromatic electrophilic substitution reactions?

Question

BlackTom AI · Accepted Answer

Electrophilic attack of the aromatic pi bond on the electrophile to give the sigma (arenium) ion is the first and rate‑determining step in typical aromatic electrophilic substitution (EAS), so option a is correct.

1) Relevant concepts
- Aromatic rings are nucleophilic at the pi system and resist reactions that destroy aromaticity.  
- EAS proceeds in two main stages: (i) attack of the aromatic π electrons on an electrophile to form a non‑aromatic σ (arenium) ion (also called the Wheland intermediate or arenium complex), and (ii) deprotonation of that σ complex to re‑establish aromaticity.  
- The formation of the σ complex is the slow, energy‑requiring (rate‑determining) step because aromaticity is temporarily lost; resonance delocalization of the positive charge stabilizes the intermediate.

2) Step‑by‑step reasoning (arrow pushing)
- Step 1 (correct first step): A pair of π electrons from the benzene ring attacks E+, forming a new C–E bond at one carbon. That carbon becomes sp3 and bears E and H; the adjacent carbons and that ring carbon together bear a delocalized positive charge (arenium ion). This is depicted in option a. Resonance structures place the positive charge at positions ortho and para to the new substituent, showing delocalization and partial stabilization.
- Step 2 (subsequent step): A base (often the conjugate base in the reaction medium) removes the proton from the sp3 carbon, restoring the sp2 hybridization and re‑forming the aromatic π system, yielding substituted benzene + H+ (or the protonated base).

3) Calculations
- No numerical calculations are required; the reasoning is mechanistic and qualitative.

4) Why option a is correct
- Option a explicitly shows the π electrons attacking E+ to give the σ (arenium) ion intermediate, matching the standard EAS mechanism and identifying the formation of the non‑aromatic cation as the first step.

5) Why the other options are incorrect
- b/e imply proton loss or complete substitution occurring simultaneously as the first step (skipping formation of the σ complex); deprotonation occurs after σ complex formation, not before.  
- c shows formation of a benzene radical cation plus HE (single‑electron transfer pathway), which is not the common two‑electron EAS pathway.  
- d appears to show the arrow direction or intermediate incorrectly (it does not present the correct σ complex formation as the initial step).

6) Conclusion
- The correct first mechanistic step is formation of the σ (arenium) ion by attack of the benzene π electrons on the electrophile; option a is therefore confirmed as the correct answer.

Which of the following is most likely to be the first step in the general mechanism for aromatic electrophilic substitution reactions?单项选择题

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