Reactions of Aromatic Molecules
Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution
Last updated: February 3rd, 2023 |
Two Important Reaction Patterns: Ortho- , Para- Directors and Meta- Directors
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
- Quiz Yourself!
1. ortho-, para- Directors
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.
In one pattern, ortho- and para– products dominate, and the meta- product is an extremely minor byproduct.
Substituents which lead to this result are called, “ortho-, para- directors”. Examples of ortho-, para– directors are hydroxyl groups, ethers, amines, alkyl groups, thiols, and halogens.
Here’s a concrete example: the nitration of methoxybenzene (also known as anisole).
ortho- and para- products dominate, while meta– products comprise less than 3%.
2. meta- Directors
In the second pattern, the meta– product dominates, and the ortho- and para– products are minor.
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
What factors could be in play here? How do ortho-, para- and meta– directors differ, and how could this difference affect the product distribution?
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.
If you look through the list of ortho- , para- directors, you might recognize that many of them are also activating groups.
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?
Yes indeed. What is up with the halogens, and how is it that they can be deactivating (i.e. slow down the reaction rate) and yet lead to ortho-, para- products?
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.
In our next post, we’ll explain the reasons for both ortho-, para- and meta- direction, and try to show why halogens fit in the former category but not the latter.
Next Post: Understanding Ortho, Meta, and Para Directors
45 thoughts on “Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution”
Explain why the cyano group is a moderately deactivating Meta Director??
Let’s say you did 4 different electrophilic aromatic substitution reactions (e.g. nitration) with PhCl, PhNO2, PhCF3, and PhCN. You measure the rates. All all slower than benzene. Of the four, one is fastest and one is slowest. You’d probably call the one that is the fastest, “weakly deactivating” and the one that is the slowest, “strongly deactivating”, and the ones in the middle, “moderately deactivating”.
Is allyl group an ortho para director
Yes, but it would be difficult to conduct an EAS reaction in the presence of the double bond, which tends to be a better nucleophile.
How to determine where to add when there are two ortho para directing group for eg in
3 methyl phenol nitration->???
The strongest activating group wins. In this case, OH is a stronger activator than CH3, so in the case of p-methylphenol, the result would be 4-methyl-2-nitrophenol.
So what is -OTs group
Ring activating or deactivating ?
And ortho-para or meta directing ?
There are lone pairs on the oxygen, it is ortho-para for sure. It will be activating, but less so than OCH3 or other O-alkyl groups.
Very good website
Hey, I had a doubt. When we have phenol substituted with no2 which is an electron withdrawing group. Which one will be more acidic, ortho nitro phenol or para nitro phenol?
Draw the resonance forms of the conjugate base. In which is the negative charge the most stable?
that is really a wonderful article but something confuses me that what is the trend of halogens when they behave as deactivating ortho para directors which halogen is most de activating and which is least next i want to ask that which is more activating OR or OH
OH is more activating than OR. It is even more activating when deprotonated to give O(-) .
This is the kinda answer I was looking for .
I understood completely what is meant by Ortho para director and deactivating group.
Hope you’re well.
i wanted to ask that if we are given several ortho , para or either meta grps , howw would we know which ortho para will activate the benzene ring most and which meta will deactivate the most ?
e.g -N(CH3)2 , -NH2 , -OH , OCH3 , -Cl , Br- , I- (ORTHO , PARA)
e.g -NR3 , CYANIDE ION , -COOH , -CHO , COR- ,
KINDLY ARRANGE THEM ACCORDING TO THE QS .
GoD Bless you
What about sulphonation of m-ethylphenol?major HSO3 will be directed in Ortho or para? Why?
How does the pathway if we want to turning -CHO as a meta director into ortho and para director? For example if we want to substitute a hydroxyl group to the position of para or ortho on cinnamaldehyde. thans for ur answer.
Substitution reactions to install hydroxy groups are very difficult. In theory you could nitrate, reduce to NH2, convert to diazonium, and then KOH, but in practice, you could start with benzaldehyde and install a temporary directing group for directed ortho lithiation followed by an oxygen electrophile
Why is a meta-product major and ortho-para-products minor? May be I did not get the point clearly
See the next post.
Interesting,though the principles that guide ortho,meta and para directors does not come out clearly
Thanks for this…. I have my board exams from feb or march. Do you also have a page for conversions?
Is -CH₂OH an Ortho Para director or Meta Director?
CH2OH is not a great example (it will easily ionize with lewis acids to form the benzyl cation) but CH2OCH3 gives a ratio of 51% ortho, 42% para, and 7% meta in nitration. Regerence: B. Moodie, J.R. Penton, and K. Schofield, “Nitration and Aromatic Reactivity” Cambridge University Press, Cambridge 1971. https://www.cambridge.org/vi/academic/subjects/chemistry/chemistry-general-interest/nitration-and-aromatic-reactivity?format=PB
This article is quite detailed. But how does this ortho para affect the difference in boiling points and melting points when it comes to isomerism.
That’s not really key to the discussion.
Hi James, thank you for the brilliant article?
What if we have a trityl as a substituent? I believe it should be a meta director since the carbon would be quite electrophilic due to the -I effect of the phenyl group. Thoughts?
Why would you choose to have trityl as a substituent? You have three phenyl groups which could interfere with your desired EAS. I think you should comment with a pic of what you’re proposing.
What is the resonating structure of -CHO??
It’s an aldehyde. Meta director.
Excellent I learnt a lot from this tq
What are alkynes considered? Ortho/para directors or meta directors??
“In the second pattern, the meta– product dominates, and the ortho- and meta– products are minor.”
Just FYI, I think you meant to say “ortho- and para- products are minor”
Ah yes. Fixed. Thank you.
Which one would be more acidic
Shouldn’t it be the 2nd one as it is Me3n+ is more e- withdrawing than no2?
Think about the resonance forms of the conjugate base.
I know it’s very specific, but definitely interesting how aniline (and maybe even phenol), direct meta when they are nitrated or sulfonated, mainly because the lone pair on the nitrogen of the aniline gets protonated by the H2SO4, making it a meta director. It’s a very small point but it would have saved me a few points on a test!
That is a great point. I wish I’d put that in too.
Great article! But, what if you have a Ph group as an substituent? Is it considered as a alkyl group as far a being a o,p, director?
Excellent question. It would be considered an electron donating group, yes – an ortho,para director.