Electrophilic Substitution
Aromatic Stability: The delocalization of six electrons in benzene creates a state of lower energy (resonance energy) compared to localized structures. This stability makes the ring resistant to reactions that would permanently break the delocalization.
High Electron Density: The electron clouds above and below the ring act as a nucleophile, attracting electrophiles. However, the density is not high enough to polarize non-polar molecules like without assistance.
Thermodynamic Driving Force: Substitution is preferred over addition because the final product retains the aromaticity of the starting material. The energy required to break the aromatic system in the intermediate step is recovered when the system is restored.
Step 1: Generation of the Electrophile: Most electrophiles are too reactive to exist independently and must be produced in the reaction mixture using a catalyst or specific reagents (e.g., using a Lewis acid to polarize a halogen).
Step 2: Electrophilic Attack: Two electrons from the delocalized system move to form a covalent bond with the electrophile. This creates a carbocation intermediate (often called a Wheland intermediate or sigma complex) where the aromaticity is temporarily lost and the positive charge is delocalized over the remaining five carbon atoms.
Step 3: Regeneration of Aromaticity: A base in the reaction mixture (often the conjugate base of the catalyst) removes a proton () from the carbon atom that was attacked. The two electrons from the bond return to the system, restoring the stable aromatic ring.
| Feature | Electrophilic Addition | Electrophilic Substitution |
|---|---|---|
| Typical Substrate | Alkenes () | Arenes (Benzene ring) |
| Aromaticity | N/A | Preserved in the final product |
| Intermediate | Cyclic bromonium or carbocation | Delocalized carbocation (horseshoe) |
| Outcome | Saturation of the bond | Replacement of a hydrogen atom |
| Catalyst | Usually not required for halogens | Required (Lewis Acid) for halogens |
Curly Arrow Precision: Always start the curly arrow from the delocalized ring (the circle) or a specific bond, and point it directly to the electrophile ().
The Intermediate Horseshoe: Ensure the 'horseshoe' shape in the carbocation intermediate covers five carbon atoms and is open toward the carbon atom bonded to the electrophile. The positive charge must be drawn inside the horseshoe, not on a specific atom.
Catalyst Regeneration: In mechanisms involving Lewis acids (like ), always show the final step where the catalyst is regenerated (e.g., ).
Directing Effects: Remember that substituents already on the ring influence where the next group enters. For example, the group in phenol directs incoming electrophiles to the 2, 4, and 6 positions.
Addition Confusion: A common mistake is drawing an addition product where both the electrophile and a halogen atom add to the ring. Remember that the atom is lost to restore stability.
Incorrect Intermediate Charge: Students often forget to include the positive charge in the intermediate or place it incorrectly. The charge represents the loss of two electrons from the delocalized system.
Reagent Specifics: Forgetting that nitration requires concentrated sulfuric acid as a catalyst, or that halogenation of benzene requires a halogen carrier (like or ), whereas alkenes do not.