Chapter 22 Flashcards
general mechanism for electrophilic aromatic substitution involves attack on the electrophile by the weakly nucleophilic … to form a resonance-stabilized cation intermediate on the ring that … to give a substituted arene
aromatic pi electrons; loses a proton
in a halogenation reaction, aromatic rings react with Cl2 in the presence of the … to give chloroarenes. this works with Br2 too
Lewis acid catalyst FeCl3
in a sulfonation reaction, aromatic rings react with … in the presence of … to yield arylsulfonic acids
SO3; sulfuric acid
in a nitration reaction, aromatic rings react with … in the presence of … to yield nitroarenes.
nitric acid; sulfuric acid
nitroarenes are useful synthetic intermediates because the nitro group can be reduced to an … group by reaction with … and a transition metal catalyst or, alternatively, by using …, ..,. or … in … followed by …
amino; H2; iron; zinc; tin; HCl; base
there is no electrophilic aromatic substitution rxn that introduces the … group directly onto the aromatic ring
amino
in a Friedel-crafts alkylation, aromatic rings react with … in the presence of a … such as AlCl3 to produce alkylbenzenes. … and … can be a problem
haloalkanes; Lewis acid; rearrangements; overalkylation
in a Friedel-crafts acylation, aromatic rings react with … in the presence of a … such as AlCl3 to produce acylbenzenes.
acid chlorides; lewis acid
the acylbenzene products of Friedel-Crafts acylation rxns can be reduced to the corresponding alkylbenzene using … or … reductions, providing a convenient method of producing alkylbenzenes that cannot be made in high yield using the Friedel-Crafts alkylation due to rearrangement or over-alkylation problems
Clemmensen (Zn(Hg), HCl); Wolff-Kishner (hydrazine + KOH)
reactions include using an … in the presence of a strong acid to give a carbocation that generates an alkylbenzene, using an .. in the presence of a … to generate an alkylbenzene, and using an … in the presence of strong acid to give a C+ that generates an alkylbenzene
alkene; alkene; Lewis acid; alcohol
mechanism for halogenation of benzene involves initial reaction between Cl2 and FeCl3 to generate a molecular complex that rearrange to give a …, … ion pair. The … reacts as a very strong electrophile with the aromatic pi cloud to form a resonance stabilized cation that loses a proton to give the product
Cl+, FeCl4-; Cl+
halogenation of an aromatic ring substituted by … does not requires a Lewis acid catalyst
strongly activating groups (e.g. -OH, -OR, -NH2)
the mech for nitration of benze involves an initial protonation of nitric acid (HNO3) by sulfuric acid followed by .. to yield the nitronium ion … this reacts as a strong electrophile
loss of water; NO2
Friedel-crafts acylation fails when there is one or more … groups on the ring. it is easy to stop the rxn after a single acylation bc the aromatic monoacylation product is less reactive than the starting material
strongly electron-withdrawing groups
alkyl groups all groups in which the atom bonded to the ring has an unshared pair of electrons are … direction, and most are electron releasing; therefore, they are … toward electrophilic aromatic substitution compared to benzene itself
ortho-para directing; activating
halogens are exceptions in that they are … directing but electron withdrawing; therefore, they are … toward electrophilic aromatic substitution compared to benzene itself
ortho-para directing; weakly deactivating
substituent directing and activation/deactivation effects are the result of two types of interactions operating on the cation intermediate:
an … effect in which the substituent withdraws more electron density out of or releases more e- density into the positively charged intermediate
an always activating … effect in which lone pairs create an extra contributing structure when they are ortho/para to the incoming electrophile. this stabilizes cation intermediate by further distributing its positive charge over the molecule
inductive; resonance
haloarenes react with … or … at … to yield products in which the halogen is replaced
very strong bases (eg. NaNH2); moderate bases (NaOH); high temperatures (3oo degrees C, 500 degrees C)
the base/nucleophile group ends up on the ring carbon atom that was originally bonded to the …, as well as positions … to it due to the benzyne intermediates
halogen; ortho
nucleophilic aromatic substitution involves an initial … rxn between the haloarene and strong base to give a … intermediate which undergoes addition at either sp hybridized carbon atom to give products
elimination; benzyne
addition-elimination of haloarenes involves a nucleophilic attack of the ring carbon containing the halogen to give a negatively charged Meisenheimer complex, followed by … to give the product. this does not occur unless there are electron-withdrawing groups ortho and/or para to the halogen, bc these groups activate the ring toward nucleophilic attack
loss of halogen
