Lecture Outline: Electrophilic and Nucleophilic Aromatic Substitution

Lecture outline Ch 17 — Electrophilic Aromatic Substitution (EAS) and Nucleophilic Aromatic Substitution Electrophilic aromatic substitution (EAS) involves replacement of an H by an electrophile... (complete the resonance structures below, and add curvy arrows) H E + slow step — disrupts aromatic cycle E X (+ fast — regenerates aromatic cycle E H E H E H+ X–) X H E complete the resonance structures actual arenium ion intermediate: The positive charge must always be delocalized to what ring positions relative to the site of E+ attack? Recall that in many reactions of alkenes, initial addition of an electrophile (e.g. H+ or "Br ") is followed by addition of a nucleophile to the carbocation intermediate. Draw the product of addition below. Why is this pathway unfavorable in this case? + H E X Specific EAS reactions — 1. Halogenation — + X2 X FeX3 (+ HX) (catalyst) (X = Cl, Br) (AlX3 also works as a catalyst) mechanism of formation of the "X+" electrophile: (add the lone pairs and charges) Br Br + FeBr3 Br Br FeBr3 (Fe3+ is e –-deficient, ! a Lewis acid) Br Br FeBr3 actual: EAS mechanism: (complete the resonance structures, and add curved arrows) H Br Br Br FeBr3 Br H Br H FeBr3 H Br Br As mentioned in the text, when the ring is strongly activated by substituents halogenation can take place without catalyst present. X2 alone is electrophilic enough. 2. Nitration — + NO2 H2SO4 HNO3 Formation of the electrophile starts with an acid-base reaction between sulfuric acid and nitric "acid". In this context we should probably call it "nitric base" O HO S O O O H HO + N O HO SO3 + H2O O N – H2O O N O O EAS mech: (fill in the missing bits) O H O N O H N O HSO4– or H2O nitrobenzene etc (draw the resonance structures!) 3. Sulfonation — + SO3 H2SO4 SO3H benzenesulfonic acid (pKa ! –7) Draw the structure of the –SO3H group — how is it related to the structure of sulfuric acid, which also has pKa ≈ –7? Most aryl sulfonic acids, like PhSO3H, are easy-to-handle solids. This makes them convenient strong-acid catalysts for organic reactions. Sulfonation of compounds that have "activated" aromatic rings goes with sulfuric acid alone — this dissociates to water and the electrophile, sulfur trioxide... H2SO4 SO3 + H2 O ...but in most cases the reaction requires SO3 in H2SO4 (fuming sulfuric acid). The electrophile may be SO3, but more likely it is the conjugate acid, SO3H+ — this is a far better electrophile, so even a small equilibrium concentration would likely be sufficient. mechanism: H SO3 H + HO O S O H O HO S HSO4– HOSO2 or SO3 O etc (draw the resonance structures!) In contrast to the other EAS reactions, sulfonation is reversible in practice. "Desulfonation" can be carried out with dilute aqueous acid ("H3O+"). The excess water reacts with the SO3 produced to form sulfuric acid and drive the equilibrium in the desulfonation direction. Forward and reverse reactions go via the same mechanism! Write it! (this is the principle of microscopic reversibility) Sulfonation is an important reaction in the synthesis of the antibacterial sulfonamides, or sulfa drugs, which have the general structure O Ar S O ("Ar" is a generalized aromatic moiety) NR2 A compound in this class that is currently being marketed as a treatment for arthritis is Celecoxibp. N N H2N S O O CF3 Celecoxib (Celebrex)