Electrophilic aromatic substitutions meaning

The reason for this difference in selectivity of different electrophilic reagents between the 2- and 3-positions must be sought in the finer details of the mechanism of electrophilic aromatic substitution Melander and co-workers are studying this problem by means of isotope effects. 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

Nixon model, have been abandoned in all modern views of aromatic structure. Thus, given our present views of the structure of benzene, and the mechanism of electrophilic aromatic substitution, the Mills-Nixon hypothesis has no meaning. Nonetheless, the legacy of this hypothesis doggedly persists in our research discussions and should be laid to rest. ... 

For all practical purposes, electrophilic aromatic substitution is confined to the substitution of a ring hydrogen. Does this mean that an electrophile such as... 

Know the meaning of electrophilic aromatic substitution, halogenation, nitration, sulfonation, alkylation, acylation, Friedel-Crafts reaction. 

The carbocation intermediate of electrophilic aromatic substitution loses a proton to yield the aromatic product. In all cases, a base is involved with proton removal, but the nature of the base varies with the type of substitution reaction. Although this book shows the loss of the proton, it often doesn t show the base responsible for proton removal. This doesn t imply that the proton flies off, unassisted it just means that the base involved has not been identified in the problem. 

A theme of this section is making reactions work. Usually that means making reactions that already work quite well work much better. But we start with a reaction - electrophilic aromatic substitution on pyridines and related heterocycles - that, doesn t really work at all. We shall see why not and how the reaction can be successfully realised. This involves some serious mechanistic thinking and questions of regioselectivity are vital to the solution. This is new chemistry with a great future. 

At the low-temperature (0°) addition, rate-control is being observed. The o- and p-xylenes are formed faster. At the high-temperature (80°) addition, equilibrium-control is shown m-xylene is the most stable product. The methyl group of toluene activates the ring for electrophilic aromatic substitution and directs substituents to the ortho and para positions. Just as the methyl group favors alkylation at the ortho and para positions, it also favors dealkylation - via electrophilic attack by a proton - at these same positions. This means that while the ortho and para isomers are formed more rapidly, they are also dealkylated more rapidly as shown ... 

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