Drill and practice with on-ring aromatic substitutions Class of substitution reaction most commonly occurring on a benzene Electrophilic aromatic substitution What key reactant species must be generated in all five of the classic aromatic substitutions on benzene? A very powerful (positively charged) Lewis acid A Lewis acid is…. An electron acceptor Another `affectionate’ name for the Lewis acid in aromatic substitutions is: electrophile Fyi…phile is Greek for lover

Br+ CH3+ Lewis acid responsible for aromatic bromination: Drill and practice with on-ring aromatic substitutions (cont.) Lewis acid responsible for aromatic bromination: Br+ Reagents & solvent used to make Br+ Fe or FeBr3 with Br2 in acetic acid Reaction leading to Br+ ? FeBr3 +Br2 FeBr4- +Br+ Lewis acid attacking benzene (Ar) to make methyl benzene: ArCH3 [Ar=aryl=> benzene group ] CH3+ Reagents & solvent (if any) used to make CH3+ AlCl3 with CH3Cl neat Reaction leading to CH3+ ? AlCl3 +CH3Cl AlCl4- +CH3+ Special name for ring alkylation reaction Friedel-Crafts alkylation

Lewis acid responsible for aromatic sulfonation: Drill and practice with on-ring aromatic substitutions (cont.) HSO3+ Lewis acid responsible for aromatic sulfonation: Reagents & solvent used to make HSO3+ SO3 in H2SO4 (fuming sulfuric acid) Reaction leading to HSO3+ ? H2SO4 +SO3 HSO4- + HSO3+ AlCl3 + neat What is the electrophile generated in the reaction above ? Reaction leading to ?

Special name for ring acylation reaction Friedel-Crafts acylation Drill and practice with on-ring aromatic substitutions (cont.) Special name for ring acylation reaction Friedel-Crafts acylation Conc. HNO3 + trace conc. H2SO4 NO2+ What is the electrophile generated in the reaction above ? Reaction leading to NO2+ ? HNO3+HNO3 NO3- + NO2+ + H2O NaOH 300 oC H+/H2O

All other deactivating groups direct… Drill and practice with on-ring aromatic substitutions (cont.) SnCl2 or Fe with H+ NaOH Activating groups direct… o and p (ortho and para) Halogens are de-activating and direct… o and p (ortho and para) All other deactivating groups direct… m (meta) Activating groups are electron…. Donors Dectivating groups are electron…. Acceptors

An Activating group on a ring Drill and practice with on-ring aromatic substitutions (cont.) An Activating group on a ring decreases increases the rate of substitution on the ring Too much Steric hindrance CH3 is activating (o,p directing) NO2 is de-activating (m directing) NH2 is activating Most likely species formed

C6H6 lately? He hasn’t benzene for a while….

Cats love all ringie thingies !

From rubric for Friedel-Crafts Theory (30 points total) Step 2: Initial electrophilic aromatic attack to form t-butylbenzene ___ /6 Clear explanation with pix for how Lewis acid disrupts the aromatic ring. Electron pushing protocol is strongly suggested.It is suggested that you use static Kekule resonance structures which provide specific images of how (+) charge migrates around disrupted ring. ___/4 Clear explanation with pix of how H is expelled and initial t-butyl benzene is formed. Again, electron pushing model is preferred

Step 4: o,p Directed attack to form 1,2 or 1,4-ditertbutyl benzene (o,p directing) ___ /5 explanation with pix of how activating t-butyl group on benzene produces o and p direction in interrupted ring (this is why the static Kekule structures are preferred above). Electron pushing should remain the mode of explanation ___ /3 explanation of why 1,4-ditertbutylbenzene is favored