Meta attack

Attack at the meta position leads to a more stable intermediate than attack at either the ortho or the para position and so meta substitution predominates Even the inter mediate corresponding to meta attack however is very unstable and is formed with dif ficulty The trifluoromethyl group is only one bond farther removed from the positive charge here than it is m the ortho and para intermediates and so still exerts a significant although somewhat diminished destabilizing inductive effect... 

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). 

Now we can also understand why meta attack is preferred in a deactivated ring. Only if attack is at that position do none of the resonance structures of the transition state have a positive charge on that carbon that bears the electron-withdrawing group. 

The electron-withdrawing effects also destabilises the reaction intermediate and makes the reaction more difficult. This destabilisation is more pronounced in the intermediates arising from ortho/para attack and so meta attack is favoured. 

The circled resonance forms are unfavorable, because they place two positive charges adjacent to each other. The intermediate from meta attack is thus favored. 

Resonance structures explain that bromination occurs in the ortho and para positions of the rings. The positively charged intermediate formed from ortho or para attack can be stabilized by resonance contributions from the second ring of biphenyl, but this stabilization is not possible for meta attack. 

Again, because of photoexcitation, the important frontier orbital of the electrophile (the LUMO) is able to interact productively with the LUMO of the benzene ring which was not productive in the ground state of any drop in energy. Certainly i/ 5 has an electron distribution ideal for explaining ortho/meta attack in anisole, and, as we saw in Chapter 4, the LUMOs of nitrobenzene do lead to reactivity at the para position. Now that photoexcitation has placed an electron in these orbitals, an electrophile can take advantage of this electron distribution, whereas, in the ground state, only a nucleophile could. 

A similar situation arises with species 9 associated with attack at the 4-position and this carbocation intermediate is therefore also additionally stabilized by 10. However, no such structure can be drawn following meta attack and so the cation derived from this mode of attack is not additionally stabilized. 

While attack at the 3-position is still much slower than for benzene, no canonical form places positive charges on adjacent atoms and so the intermediate is less destabilized than those arising from ortho and para attack. Hence meta attack is the preferred reaction, as illustrated in Figure 2.3. For example, nitration of nitrobenzene gives 88% of 1,3-dinitrobenzene and only 8% and 1% of the 1,2- and 1,4-isomers, respectively. The reaction occurs at a relative rate of 6 x 10 to that of benzene. 

Conclusion With the NO2 group (and all meta directors), meta attack occurs because attack at the ortho or para position gives a destabilized carbocation intermediate. 

Next, let us compare the carbonium ions formed by attack at the para and meta positions of nitrobenzene, a compound that contains a deactivating group. Each of these is a hybrid of three structures, X-XII for para attack, Xllf -XV for meta attack. In one of the six structures, XI, the positive charge is located on the... 

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