Resonance active substituents

Should the activating substituent be at position 2, further substitution will be almost exclusively at position 1 this follows from consideration of resonance structures, where the 2-substituent has minimal effect if attack occurs at position 4. Of course, this would equate to meta attack, which we know is unfavourable for an ortho and para director (see Section 8.4.3). 

If an activating substituent is present that is capable of interacting with the ring by resonance, the opposite holds true and the para isomer will be a stronger base than the meta isomer. This is because the crucial resonance structure mentioned above would have a negative charge immediately next to the ammonium ion and this would have a stabilising effect. 

The resonances at 8 6.68 ppm and 8 7.90 ppm are assignable to aromatic protons. The positions of aromatic protons are very strongly influenced by substituents. As a general rule, activating substituents lead to shielding, especially of the ortho and para protons, and therefore to chemical shifts at higher fields than 8 7.27 ppm (the chemical shift of... 

Br substituent in naphthalene) gives about 15 times the rate produced by internuclear s resonance activation (2-N02-6-Br- and l-N02-7-Br-naphthalene) this 2,3-resonance activation is still greater than both intra- and mter-nuclear s inductive activation but only by two orders of magntitude in the one comparison available. 

The 2,i-orientation of an azine-nitrogen and a leaving group is characterized by activation which is exceptionally poor compared to other resonance activations. The poor activation, which is often grossly underrated, is still substantial relative to the substituted naphthalene 10 -10 -fold increase in the rates of alkoxylation and of alkylamination. The properties of 2,3,-orientation come into play in ail 3-substituted naphthalenes or azanaphthalenes which bear an azine-nitrogen or activating substituent in the 2-position (Section IV, A, 2). This orientation is subject to such a decrease in activation due to the relatively poor stabilization of charge in the ortho,ortho-quinoid structure (352) that 3-substituted isoquinolines and 2-nitro-naphthalenes are less reactive than 2-substitnted pyridines and nitrobenzenes (Section IV, A, 2). Insertion of a 3-aza moiety into a... 

With arenes bearing a single resonance donor substituent (NR2, OMe, F), the addition is strongly preferred at the meta position, with small amounts of ortho substitution (0-10%). " The meta acylation of anisole, using a carbonyl anion equivalent as the nucleophile, illustrates the unique regioselectivity available with the Cr(CO)3 activation (equation 115). ... 

The reactivity of the hetero ring, with electron deficient 2(3)-positions, has been compared to glyoxal diimine." Quinoxalines are deactivated towards electrophilic substitutions, however, substitution is most likely to occur at the equivalent positions 5 and 8, where electron-localization calculations show the highest electron density to be. Electron-donor substituents in the benzenoid ring facilitate electrophilic substitution and, when activating substituents are present in the hetero ring, the site of reaction depends on the reaction conditions. In deprotonated alkyl-substituted quinoxalines an increased number of resonance possibilities exists and mildly basic conditions arc usually required for condensation reactions. 

The moderately activating substituents also donate electrons into the ring by resonance and withdraw electrons from the ring inductively. Because they are only moderately activating, we know that they donate electrons into the ring by resonance less effectively than do the strongly activating substituents. 

In summary, as shown in Table 16.1, the halogens and all the activating substituents are ortho-para directors. All substituents more deactivating than the halogens are meta directors. Put another way, all substituents that donate electrons into the ring inductively or by resonance are ortho-para directors, and all substituents that cannot donate electrons into the ring inductively or by resonance are meta directors. 

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