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The ratio between numbers of ortho and para molecules is about 3:1 at standard temperature where many rotational energy levels are populated, favoring the ortho form as a result of thermal energy. However, at low temperatures only the J = 0 level is appreciably populated, so that the para form dominates at low temperatures (approximately 99.8% ...
For nitration, for example, fluorine directs strongly to the para position because the ortho position is inductively deactivated (86% para, 13% ortho, 0.6% meta). On the other hand, iodine directs to ortho and para positions comparably (54% para and 45% ortho, 1.3% meta). [12]
Fractional crystallisation can be used to obtain pure para product, relying on the principle that it is less soluble than the ortho and thus will crystallise first. Care must be taken to avoid cocrystallisation of the ortho isomer. [2] Many nitro compounds' ortho and para isomers have quite different boiling points. These isomers can often be ...
Diagram showing the ortho, meta and para positions relative to a substituent X on a benzene ring. Both the regioselectivity—the diverse arene substitution patterns—and the speed of an electrophilic aromatic substitution are affected by the substituents already attached to the benzene ring.
Ortho-, meta-and para-positions of a phenyl group, with respect to a substituent group "G". Made with the free software program ChemTool , and fine-tuned editing by hand with Vim (text editor) . Source
Consider para-disubstituted benzene p-F-C 6 H 4-Z, where Z is a substituent such as NH 2, NO 2, etc. The fluorine atom is para with respect to the substituent Z in the benzene ring. The image on the right shows four distinguished ring carbon atoms, C1, C2, C3, C4 in p-F-C 6 H 4-Z molecule. The carbon with Z is defined as C1(ipso) and ...
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The ortho effect also occurs when a meta-directing group is positioned in a meta arrangement relative to an ortho–para-directing group, a new substituent introduced into the molecule tends to preferentially occupy the ortho position relative to the meta-directing group rather than the para position.