In the previous write-up we presented ortho- ,para- and also meta- directors in electrophilic fragrant substitution. Previous to that we extended the system of electrophilic aromatic substitution, and also showed that the mechanism proceeds through a carbocation intermediate.

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Today, we’re going come tie those two principles together, and shot to show that whether a substituent is an ortho-, para- or meta- director counts on how the substituent affect the stability of the carbocation intermediate. 

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Clearly, the carbocation v an surrounding oxygen bearing a lone pair is far an ext stable than a carbocation surrounding to one electron-withdrawing group like CF3.

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

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

This leaves us through the somewhat tricky instance of halogens.

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

The answer must come together no surprised if you’ve been complying with along, but we’re walk to leaving this until the following post, due to the fact that this one is long enough already.

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

Notes

Note 1. It’s an ext correct to say that ortho- and para- products dominate since the transition states causing these products are reduced in energy, rather than the energies that the intermediates themselves. After ~ all, it’s the energy of the change states which determines the activation barrier, and also therefore the reaction rate.

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