Electrophilic aromatic substitution addition

Kinetic studies have been important in the characterization of varied electrophilic reactions, such as electrophilic aromatic substitutions, addition... 

Pertinent examples of zeolite-catalyzed reactions in organic synthesis include Friedel-Crafts alkylations and acylations and other electrophilic aromatic substitutions, additions and eliminations, cyclizations, rearrangements and isomeriza-tions, and condensations. 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Problems 12 2 12 3 and 12 7 offer additional examples of reactions in which only a sin gle product of electrophilic aromatic substitution is possible... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Detailed mechanistic studies by Fodor demonstrated the intermediacy of both imidoyl chlorides (6) and nitrilium salts (7) in Bischler-Napieralski reactions promoted by a variety of reagents such as PCI5, POCI3, and SOCh)/ For example, amide 1 reacts with POCI3 to afford imidoyl chloride 6. Upon heating, intermediate 6 is converted to nitrilium salt 7, which undergoes intramolecular electrophilic aromatic substitution to afford the dihydroisoquinoline 2. Fodor s studies showed that the imidoyl chloride and nitrilium salt intermediates could be generated under mild conditions and characterized spectroscopically. Fodor also found that the cyclization of the imidoyl chlorides is accelerated by the addition of Lewis acids (SnCU, ZnCh), which provides further evidence to support the intermediacy of nitrilium salts. ... 

Before seeing how electrophilic aromatic substitutions occur, let s briefly recall what we said in Chapler 6 about electrophilic alkene additions. When a reagent such as HCl adds to an alkene, the electrophilic hydrogen approaches the p orbitals of the double bond and forms a bond to one carbon, leaving a positive charge at the other carbon. This carbocation intermediate then reacts with the nucleophilic Cl- ion to yield the addition product. 

Diels-Alder reaction, 493 El reaction, 391-392 ElcB reaction, 393 E2 reaction, 386 Edman degradation, 1032 electrophilic addition reaction, 147-148. 188-189 electrophilic aromatic substitution, 548-549 enamine formation, 713 enol formation, 843-844 ester hydrolysis, 809-811 ester reduction, 812 FAD reactions. 1134-1135 fat catabolism, 1133-1136 fat hydrolysis, 1130-1132 Fischer esterification reaction, 796 Friedel-Crafts acylation reaction, 557-558... 

Substituent effect, additivity of, 570 electrophilic aromatic substitution and, 560-563 summary of. 569 Substitution reaction, 138 Substrate (enzyme), 1041 Succinic acid, structure of, 753 Sucralose, structure of. 1006 sweetness of, 1005 Sucrose, molecular model of. 999 specific rotation of, 296 structure of, 999 sweetness of, 1005 Sugar, complex, 974 d, 980 L, 980... 

Iron (III) chloride is a common catalyst used in electrophilic aromatic substitutions. In addition to those applications outlined above for the construction of aromatic C-C bonds, such salts have also been used for the introduction of heteroatom-based functional groups at the aromatic ring [47]. 

It has also been known for many years that sulfonyl groups, like carbonyl groups, promote the addition of nucleophilic reagents to carbon- carbon double bonds . The meta-directing properties of the S02Me group in electrophilic aromatic substitution were discovered in the mid-1920s . ... 

The mechanism for electrophilic aromatic substitution is addition-elimination. Using these working hypotheses, Mills and Nixon explained the regioselectivity of electrophilic substitution in 5-hydroxyindan versus 6-hydroxytetralin. 

More than just a few parameters have to be considered when modelling chemical reactivity in a broader perspective than for the well-defined but restricted reaction sets of the preceding section. Here, however, not enough statistically well-balanced, quantitative, experimental data are available to allow multilinear regression analysis (MLRA). An additional complicating factor derives from comparison of various reactions, where data of quite different types are encountered. For example, how can product distributions for electrophilic aromatic substitutions be compared with acidity constants of aliphatic carboxylic acids And on the side of the parameters how can the influence on chemical reactivity of both bond dissociation energies and bond polarities be simultaneously handled when only limited data are available ... 

A diverse group of organic reactions catalyzed by montmorillonite has been described and some reviews on this subject have been published.19 Examples of those transformations include addition reactions, such as Michael addition of thiols to y./bunsatu rated carbonyl compounds 20 electrophilic aromatic substitutions,19c nucleophilic substitution of alcohols,21 acetal synthesis196 22 and deprotection,23 cyclizations,19b c isomerizations, and rearrangements.196 24... 

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