Aromatic ring reactivity trend

Substituted arylamines can be either more basic or less basic than aniline, depending on the substituent. Electron-donating substituents, such as — CH3, -NH2, and -OCH3, which increase the reactivity of an aromatic ring toward electrophilic substitution (Section 16.4), also increase the basicity of the corresponding arylamine. Electron-withdrawing substituents, such as —Cl, -NO2, and -CN, which decrease ring reactivity toward electrophilic substitution, also decrease arylamine basicity. Table 24.2 considers only -substituted anilines, but similar trends are observed for ortho and meta derivatives. 

The most common example of electrophilic substitution at a trigonal planar center is electrophilic aromatic substitution, which will serve as our archetype. Aromaticity was covered in Section 1.9, and the reactivity trends of aromatics will be covered in detail in Chapter 5. Chapter 8 has additional reaction examples. Aromatic rings are usually poor nucleophiles therefore excellent electrophiles are needed for the reaction to proceed. 

Rhodium carbenoids, especially the donor/acceptor carbenoids, act as very sterically demanding electrophiles. Hence, based on size alone, the favored order of reactivity of C-H bonds would be 1°>2°>3°, yet carbenoids are also very electrophilic, so they would prefer to react with more electron rich C-H bonds, thus 1°<2°<30. So, in practice, secondary C-H bonds tend to be the most active overall, because they possess the proper balance between these steric and electronic requirements [5], Furthermore, when the C-H bond is adjacent to an electron donating group such as a heteroatom or an aromatic ring, it becomes even further activated towards functionalization. Based upon these few general trends, a surprising level of control of reactivity can be achieved in carbenoid reactions with complex molecules, especially when their reactivity is attenuated with proper substituents on the carbenoid. 

When N-heterocycles bind to a transition metal complex, their reactivity may be enhanced with respect to the free molecules. The reactions depend on the electronic and geometric characteristics of the metal-containing fragment, as well as on the nature of the organonitrogen substrate however, no general trends relevant for HDN can be extracted from the accumulated literature. Nevertheless, two types of reactions merit further discussion, namely N-hetero-aromatic ring hydrogenation and the rare activation of C-N bonds by metal complexes. 

Taylor has collected the above and similar data and compared the ratio of reactivities of the ortho and para positions of compounds of type 19 (expressed as log/odog/p) with the ratio of reactivities of the equivalent positions, a and c, in compounds of type 20 and found that the latter ratio was lower, i.e., a relative increase in the reactivity of the para position (c) has occurred upon ring formation. This fall in the ratio log fa log fc increases along the series X = S < 0 (< NH < CHg) in 20. As this trend parallels the increase in strain in the fused bridging ring it was argued that ring strain was the primary cause of the reduction in ratio. Position a is a-aromatic and position c is j8-aromatic therefore the above concept represents an extension by Taylor of an earlier explanation of the Mills-Nixon effect in indane. Further substitution... 

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