Selectivity between ortho and para positions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص483
2025-06-10
609
Phenols and anilines react in the ortho and/or para positions for electronic reasons. These are the most important effects in deciding where an electrophilic substitution will occur on a benzene ring. When it comes to choosing between ortho and para positions we need to consider steric effects as well. You will have noticed that we have seen one ortho selective reaction—the formation of salicylic acid from phenol—and several para selective reactions such as the bromination of an amide just discussed. If the reactions occurred merely statistically, we should expect twice as much ortho as para product because there are two ortho positions. However, we should also expect more steric hindrance in ortho substitution since the new substituent must sit closely beside the one already there. With large substituents, such as the amide, steric hindrance will be significant and it is not surprising that we get more para product. There is another effect that decreases the amount of ortho substitution, and that is the inductive electron-withdrawing effect of an electronegative substituent. As you’ve seen, oxygen and nitrogen, although they are electronegative, activate the ring towards attack by donating π electron density from their lone pairs. At the same time, the C–O or C–N σ bond is polarized back towards the O or N atom—in other words, they donate electron density to the π system but withdraw electron density from the σ framework. This is inductive electron with drawal—it affects the atoms nearest the O or N atom the most, and has the effect of decreasing the likelihood that attack will happen in the ortho positions.
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