In the previous short article we presented ortho- ,para- and also meta- directors in electrophilic fragrant substitution. Ahead to that we extended the mechanism of electrophilic aromatic substitution, and showed the the system proceeds v a carbocation intermediate.

You are watching: Why is para more stable than ortho

Today, we’re going to tie those two principles together, and try to present that whether a substituent is an ortho-, para- or meta- director counts on how the substituent affects the security of the carbocation intermediate. 

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Clearly, the carbocation with an nearby oxygen bearing a lone pair is far more stable than a carbocation nearby to one electron-withdrawing team like CF3.

And that comprises the difference in between an ortho-, para- director prefer OCH3 and also a meta- director prefer CF3.

It’s worth noting that most alkyl teams (such together CH3 ) absence a lone pair, but are still ortho-, para- directors. This is regular with everything we’ve learned before about how alkyl groups are typically stabilizing because that carbocations- recall that carbocation stability typically increases with substitution

This pipeline us through the rather tricky instance of halogens.

Why space fluorine, chlorine, bromine, and also iodine ortho-, para- directors even though they space deactivating groups?

The answer need to come together no surprised if you’ve been following along, yet we’re going to leave this until the next post, because this one is long enough already.

Next post: Why are halogens ortho-, para- directors?

Notes

Note 1. It’s more correct come say the ortho- and para- products dominate due to the fact that the transition states leading to these assets are lower in energy, quite than the energies that the intermediates themselves. ~ all, it’s the energy of the change states which identify the activation barrier, and also therefore the reaction rate.

See more: What Did Chargaff Discover While Studying Bases In The Dna Of Organisms?

Note 2. Or hyperconjugation, which most textbooks (with the notable exemption of Maitland Jones) typically avoid.