Named Reaction 1 — Friedel-Crafts Alkylation (1877, Friedel and Crafts)

$\text{ArH} + \text{RCl} \xrightarrow{\text{AlCl}_3} \text{ArR} + \text{HCl}$

Electrophile: $R^{+}$ (carbocation). Catalyst: $AlCl_{3}$ (Lewis acid). Key limitations: (a) Carbocation rearrangement — primary $R^{+}$ rearranges before attack; (b) Polyalkylation — product is more reactive than starting ArH. Not suitable for making 1° alkylbenzenes from 1° alkyl halides (rearrangement gives branched product). Toluene from benzene + $CH_{3}Cl$/$AlCl_{3}$ is the one exception (no rearrangement possible for $CH_{3}^{+}$).

Named Reaction 2 — Friedel-Crafts Acylation (1877, Friedel and Crafts)

$\text{ArH} + \text{RCOCl} \xrightarrow{\text{AlCl}_3} \text{ArCOR} + \text{HCl}$

Electrophile: R$CO^{+}$ (acylium ion, resonance stabilized). Catalyst: $AlCl_{3}$. Advantages over alkylation: (a) No rearrangement (acylium resonance: R-$C^{+}$=O ↔ R-C≡$O^{+}$); (b) No polyacylation (-COR deactivates ring). Limitation: fails on deactivated rings (e.g., $ArNO_{2}$). Example product: Acetophenone (CC(=O)c1ccccc1) from benzene + $CH_{3}COCl$/$AlCl_{3}$.

Nitration of Benzene

$\text{C}_6\text{H}_6 + \text{HNO}_3 \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2 + \text{H}_2\text{O}$

Electrophile generation: $\text{HNO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + \text{HSO}_4^- + \text{H}_2\text{O}$

$H_{2}SO_{4}$ is NOT consumed — it regenerates by capturing $H^{+}$ released in step 3 of EAS. Key product: Nitrobenzene ([O-][N+](=O)c1ccccc1).

Sulfonation of Benzene (Reversible)

$\text{C}_6\text{H}_6 + \text{SO}_3 \xrightarrow{\text{fuming H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{SO}_3\text{H}$

$\text{C}_6\text{H}_5\text{SO}_3\text{H} \xrightarrow{\text{dil. H}_2\text{SO}_4, \Delta} \text{C}_6\text{H}_6 + \text{H}_2\text{SO}_4$

The only reversible EAS. The $SO_{3}$ electrophile (neutral but electrophilic at S) attacks the ring. Reversal uses dilute $H_{2}SO_{4}$ at high temperature (desulfonation).