Reactions of Aromatic Molecules
Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
Last updated: September 27th, 2022 |
It’s one thing to learn about electrophilic aromatic substitution reactions of benzene itself.
But once you move toward substituted benzenes, that’s when things start getting really interesting.
Today we’ll describe the two main patterns by which various substituents “direct” electrophilic aromatic substitution.
- In one pattern, substituents direct the reaction to give either the “ortho” (1,2) or “para” product, with a slight preference for “para” (1,4).
- In the second pattern, a different family of substituents direct the reaction to give primarily the “meta” (1,3) product.
Table of Contents
- ortho-, para- Directors
- meta- Directors
- How Well Do “ortho-, para” And “meta“- Directors Correlate With “Activating” and “Deactivating” Groups?
- The Key To Understanding ortho-, para- Directors And meta- Directors Is To Understand The Stability of The Carbocation Intermediate
Here’s a fascinating observation.
Start with a monosubstituted benzene. Then perform some kind of electrophilic aromatic substitution (nitration, halogenation, sulfonation – turns out it doesn’t matter).
Two important reaction patterns are observed.
It’s important to note that these two patterns are wholly a function of the substituent and not the reaction itself.
Here’s a concrete example: the nitration of methoxybenzene (also known as anisole).
We call the substituents which lead to this result “meta- directors”. Examples of meta– directors include nitriles, carbonyl compounds (such as aldehydes, ketones, and esters), sulfones, electron-deficient alkyl groups, nitro groups, and alkylammoniums.
Specific example: nitration of trifluoromethylbenzene gives the meta product in about 90% yield. (Compare that to the case of anisole, above, where nitration resulted in a <5% yield of the meta product. )
3. How Well Do “ortho-, para” And “meta“- Directors Correlate With “Activating” and “Deactivating” Groups
One aspect we’ve covered previously is the concept of “activating” and “deactivating” groups.
We said that
- Activating groups increase the rate of electrophilic aromatic substitution, relative to hydrogen.
- Deactivating groups decrease the rate of electrophilic aromatic substitution, relative to hydrogen.
Likewise, the list of meta- directors (nitro, CF3, cyano) is like a who’s who of deactivating groups.
If you’re a real nerd, you could even make a 2 × 2 matrix, like this:
What do we notice?
- First: no activating groups are meta directors.
- Second: what’s up with the halogens?
4. The Key To Understanding ortho-, para- Directors And meta- Directors Is To Understand The Stability of The Carbocation Intermediate
There’s no quick and thorough answer to these questions, and it’s worth its own separate article for that reason.
However, the first place to start is that it has to do with the stability of the carbocation intermediate in electrophilic aromatic substitution reactions. [See this previous post on the mechanism of electrophilic aromatic substitution]. More specifically, how does each substituent affect the stability of that intermediate?
It might be worth going back and revisiting some of the factors that affect the stability of carbocations.
And also, if you prefer to look at it from the opposite side of the coin, here are some of the factors which make carbocations more unstable.