From addition of Cl+ (from Cl2 and FeCl3): Measure C-C distances |
From addition of methyl acylium (from CH3COCl and AlCl3): Measure C-C distances |
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Note the distinct difference in geometry at the reaction center: chlorination gives us a much more sp3-like carbon, whereas the acylium has a flatter geometry at carbon. The C-C distances in the ring reflect the loss of aromaticity. Now let's see what substituents do. We can put the substituent ortho, meta or para to the site of reaction (equivalent to reacting at the ortho-, meta- or para-position). We'll start with methoxy, a strong electron donor and o/p-director. | ||
| Erel = 0 | Erel = +20.5 kcal/mol | Erel = +3.6 kcal/mol |
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Show ESP surfaces. The ESP maps show the difference in how electrons are distributed in these intermediates: red is more electron density; blue is more positive charge. The meta isomer places more electron density on oxygen and more positive charge localized in the ring, particularly on the departing proton. The relative ratios of product reflect the energies of these intermediates (largely): 30-40% ortho, 0-2% meta, and 60-70% para. Now look at CF3, an electron-withdrawing group. | ||
| Erel = +0.60 kcal/mol | Erel = 0 kcal/mol | Erel = +1.42 kcal/mol |
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Show ESP maps. The ESP maps are more similar since there is no resonance stabilization of charge, but you can see that the meta isomer avoids placing it (blue color) right next to the electron-deficient CF3 group. Now, since the meta isomer of the sigma complex is the lowest energy, we see a stronger preference for it: 6% ortho, 91% meta, 3% para. The CF3 group is highly deactivating, so the sigma complex really needs to find the lower-energy isomer. |
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