Aromatic Compounds - 1 Summary of Class Discussion

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4 Aromatic CompoundsThe following reagents react to form the actual active electrophile. These reactions are illustrated below. Complete the mechanisms using the activeelectrophile.Reaction e+ONOHOO+ HSO 4 ++HSOH 2 SO 4N4 N+ H 2 O O + H 2 OO + H 2 OHHNO 3 + H 2 SO 4As explained in the question and illustrated in the equations and on p. 476 of CGW, the mixture of HNO 3 and H 2 SO 4 reactto form the electrophile, NO 2+ , water and hydrogen sulfate anion. The after the initial electrophilic addition step, the HEEon HSO 4- remove the proton from the intermediate complex forming the nitro aromatic and reforming sulfuric acid.Reaction fFor 2˚ or 3˚ alkyl halides, the aluminum trihalide complex dissociates to form the free carbocation, which acts as theelectrophile. (See reactions b & c above). (See also CGWW p. 553)CHCH 3H3dd++ C + AlBrCH 43CH 3CHCH 333˚ cation has sufficient e - density that itcan form without a large energy increase.C Br AlBr 3HCH 3CCH 3CH 3CH 3+HBrHCCH 3CH 3AlBr 4
Aromatic Compounds 5Acyl halides (Rxn g) interact with the catalysts to form acylium ions that act as carbon electrophiles toward the aromatic ring.Reaction h.+CH 3COClAlCl 3An Acylium IonCH 3 C OH++ AlCl 4OHCHC3+ AlCl 4CH 3CO+ H-Cl(f.) What problems did you encounter in applying your general mechanism to all of the reactions? Could you modify your mechanismto work for all of the reactions? How or why not? Explain.Explanations above.(g.) Based upon your experience in (2) (a.) -> (f.), your understanding of alkene reactions, and the structure and energy of aromaticcompounds, propose a reason why it is reasonable that aromatic compounds might undergo substitution rather than addition asalkenes do. You should consider (write out) the expected mechanism for addition and compare the energies of the transitionstates for substitution vs. addition in each case.The mechanism proposed in d. above still seems to be appropriate and can account for all of the products in the data set.H+ E Nu!HENu !1 2H!E + NuHENu !E+HNuaromtic partially aromtic non-aromtic partially aromticaromticThe second step differs from that in electrophilic addition to alkenes because the partial aromaticity developing in thesubstitution (loss of the proton makes the substitution lower in energy than the addition (Br adding to the cation)which lacks aromaticity.
Page 1 and 2: (a.)(b.)(c.)(d.)(e.)Chemistry 212 S
Page 3: Aromatic Compounds 3Reactions dOH+O
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Aromatic Compounds - 1 Summary of Class Discussion

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