Reactions of Benzene

Step 1: Generation of the Electrophile: Because the benzene ring is so stable, a powerful electrophile ($E^+$) must be generated in situ using a catalyst. This electrophile is an electron-deficient species attracted to the high electron density of the $\pi$ ring.

Step 2: Electrophilic Attack: Two electrons from the delocalized $\pi$ system move to form a dative covalent bond with the electrophile. This creates a carbocation intermediate (often called a 'horseshoe' intermediate) where the delocalization is temporarily broken across one carbon, leaving a positive charge spread over the remaining five carbons.

Step 3: Regeneration of Aromaticity: To restore stability, the C-H bond at the site of substitution breaks heterolytically. The two electrons from this bond return to the $\pi$ system, and a hydrogen ion ($H^+$) is released, leaving the substituted benzene product.

Reagents and Conditions: Benzene reacts with a mixture of concentrated nitric acid ($HNO_3$) and concentrated sulfuric acid ($H_2SO_4$) at temperatures typically between $50^{\circ}C$ and $60^{\circ}C$.

Electrophile Generation: Sulfuric acid acts as a catalyst to produce the nitronium ion ($NO_2^+$). The reaction is: $HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + 2HSO_4^- + H_3O^+$

Product: The reaction produces nitrobenzene. If the temperature exceeds $60^{\circ}C$, further substitution may occur, leading to dinitrobenzene.

Halogen Carriers: Benzene does not react with halogens like $Cl_2$ or $Br_2$ under standard conditions. It requires a Lewis acid catalyst (halogen carrier) such as $AlCl_3$, $FeCl_3$, or $FeBr_3$.

Mechanism of Activation: The catalyst polarizes the halogen molecule, creating a strong electrophile ($Cl^+$ or $Br^+$). For example: $Cl_2 + AlCl_3 \rightarrow Cl^+ + [AlCl_4]^-$

Regeneration: After the substitution, the $H^+$ ion released from the ring reacts with the complex ion (e.g., $[AlCl_4]^-$) to reform the catalyst and produce a hydrogen halide ($HCl$ or $HBr$).

Alkylation: This process introduces an alkyl group (e.g., $-CH_3$) onto the benzene ring using a haloalkane and an $AlCl_3$ catalyst. It is a vital method for synthesizing ethylbenzene or methylbenzene.

Acylation: This process introduces an acyl group ($-COR$) using an acyl chloride and $AlCl_3$. The product is an aromatic ketone (e.g., phenylethanone).

Synthetic Importance: Friedel-Crafts reactions are essential 'building block' reactions in organic synthesis because they allow for the formation of new carbon-carbon bonds directly onto the stable aromatic ring.

The Horseshoe Rule: When drawing the mechanism, the positive charge must be inside the 'horseshoe' of the intermediate. The open end of the horseshoe MUST face the carbon atom where the substitution is happening.

Arrow Precision: Ensure curly arrows start from the ring (the $\pi$ system) to the electrophile, and from the C-H bond back into the ring to show the restoration of delocalization.

Catalyst Role: Always state the role of the catalyst in generating the electrophile. In exams, you are often asked to write the equation for the formation of the electrophile ($NO_2^+$, $Cl^+$, etc.).

Temperature Control: For nitration, remember that $50-60^{\circ}C$ is the standard range. Mentioning that higher temperatures lead to multiple substitutions is a common 'distinction' point in marking schemes.