You are watching: Ortho/para-directed substitution.
Ortho Meta Para Definitions
Let’s backtrack every the method to nomenclature from your very first (second?) thing in orgo 1. In IUPAC naming, every substituent is designated through a number.
Given a 5 carbon chain through a methyl ~ above carbon 4 and alcohol ~ above carbon 2, you obtain 4-methyl-2-pentanol.
But the wasn’t the only way to suggest the relationships of substituents, was it? Think earlier to cis and also trans or come E and Z.
When it involves a disubstituted benzene, using the state ‘ortho, meta and also para’ is simply another way to suggest the partnership of the substituents to each other, fairly than their as whole placement top top the ring.
For example, 1,2-dimethylbenzene tells us we have actually methyl teams both top top carbons 1 and 2.
Ortho-dimethylbenzene (ortho-xylene or o-xylene) tells us that the 2 groups are directly next come each various other on the ring.
Common failure Warning: in spite of their similar appearance, these relationship ONLY apply to benzene rings and NOT cyclohexane rings.Ortho
Ortho represents a 1,2 relationship, or 2 teams that are straight near each other on the benzene ring.Meta
Meta to represent a 1,3 relationship, or teams that room separated by one carbon ~ above the benzene ring.Para
Para to represent a 1,4 relationship, or teams that room opposite each other on the benzene ring.
Use the familiar OMP monster (explained in the video clip below) to aid you easily recognize these relationships.
Ok, so us have sophisticated names because that these relationships. Now what?
When you an initial learned EAS reactions, you focused on adding a straightforward super-electrophile to benzene. Use these videos to evaluation EAS reactions.
But necessary chemistry is never as simple as what you find out in lecture, is it?After mastering the basic reactions, you will do it be inquiry to finish EAS reactions on more complicated and substituted aromatic compounds.
For example, speak you’re request to complete a Friedel-Crafts Acylation ~ above a bromobenzene. Do you add it to the ortho, meta, or para position? Or, carry out you simply show all 3 alternatives for triple credit?
Not quite!WHERE you ar the just arrived electrophile is straight influenced by the chemical nature of the substituent(s) currently on the benzene ring. Initial substituents act together directors because that the just arrived groups.
Important Vocabulary because that OMP Directing Effects
Let’s first understand the language before jumping right into the why and how.
Directing / manager / Directing EffectA group already on benzene that will certainly dictate whereby the just arrive electrophile will certainly be added.
Activating GroupA group currently on benzene the activates the ring in the direction of Electrophilic aromatic Substitution reactions. A team that will speed up the EAS reaction.
Deactivating GroupA group already on benzene the deactivates the ring towards Electrophilic aromatic Substitution reactions. A group that will sluggish down (and sometimes fully halt) the EAS reaction.
Electron Donating groups – EDGA group already on benzene that puts negativity into the ring, either straight through resonance or indirectly through induction.
Electron Withdrawing groups – EWGA group currently on benzene the pulls negativity the end of the ring, either directly through resonance or indirectly through induction.
Now the we can speak gibberish, let’s analyze this to some useful applications.
Simply put, electron donating groups are activators and therefore act together ortho and also para directors. Electron withdrawing groups are deactivators and therefore act as meta directors. (Halogens room the annoying exception.)
On mine EAS Cheat Sheet, we look in ~ the most usual substituents and rank castle from strong to weak activators and deactivators.
And this is where most students will begin to studiously memorize the list, hoping to make sense of the information, however they eventually freak the end on test day.
But no you! due to the fact that we’re about to break this down to the suggest where friend will be able to evaluate any type of substituent and immediately and logically predict its EAS directing effects.First, a rapid review the the Electrophilic aromatic Substitution Mechanism:One pi link in benzene breaks open to attack a super (positively charged) electrophile.The attack carbon is currently bound come the electrophile, if the other former pi-bound carbon is currently positively charged due to having shed its fourth bond.Sigma-complex resonance help stabilize the intermediate.A weak base grabs hydrogen off the freshly substituted carbon atom, collapsing that electrons towards the carbocation.
Note: E+ to represent the super-electrophile, and also the attached E represents the electrophile having attached come the benzene ring.
Which intermediate? The positively fee sigma-complex.
Review these tutorials very first if you’re not comfortable v carbocation security or resonance.
Go back to the positively fee intermediate above. Notice how the carbocation is qualified of not one, however TWO added resonance forms?
Now think ago to orgo basics.What boosts the likelihood the a reaction acquisition place?Stability that the intermediate!
The an ext stable the intermediate, the much more likely the intermediate will form.The less stable the intermediate… well, molecules similar to people, want to protect against pain. And also if required to react, they will stall and also grumble every action of the way, hence slowing under the reaction.
First, an alert the position of the carbocation in the sigma-complex intermediate.
See exactly how it has 3 unique locations? try as you might, you cannot obtain the carbocation onto any other position.
This is key!
Pay fist to the pattern. Stop say we add a new electrophile come an currently substituted benzene ring. What 3 positions can hold the carbocation in each ortho, meta and also para addition?
Note: X represents the substituent currently present on the ring, and also E to represent the newly added electrophile.
Notice how ortho and para have actually the very same carbocation intermediate patterns, if meta is the exact opposite?
In the absence of additional resonance, we care most around the intermediate v a carbocation closest to the substituted carbon.
Which sigma-complex do you think is most stable? The one v an electron donating, or electron withdrawing group?
If you claimed the carbonyl is less stable, you space correct. The partially hopeful carbonyl carbon (EWG) would certainly be pissed to have a carbocation nearby.
The video clip below explains and demonstrates this in an ext detail – finish with step-by-step resonance structures.
Activating teams as Ortho/Para Directors
Instead that memorization, acknowledge activating teams as substituents that carry negativity (in a good way) towards the ring. This can be in the kind of a lone pair or very electronegative groups.
Since these an unfavorable groups room happy come sit straight near a carbocation, and since the carbocation creates AT their position just in ortho and para addition, activating groups force incoming electrophiles to add to the ortho or para position.
This provides EDGs ortho and also para directors.
Different reactions will certainly have different percentages because that ortho and para positions. This needs to be evaluate in a lab setup and so you will NOT be meant to differentiate. The one exception is to recognize that a bulky substituent will pressure para addition simply because of the steric hindrance the its bulk.
Deactivating groups as Meta Directors
Electron withdrawing teams like to traction negativity far from the ring. This renders a positive intermediate an extremely unstable, together it desire negativity quite than positive to stabilize it.
As such, electron withdrawing teams slow down, hence deactivating, the EAS reaction. And when they finally agree to reaction (slowly), these teams want to it is in as far away native the positive charge as possible.
A quick review that sigma-complex resonance shows that the meta enhancement pattern enables the carbocation to skip the substituent carbon.
And while not the most stable, this is the lesser the 2 evils and thus the desired pattern.
This is why deactivating electron withdrawing groups act together meta directors.
Halogens space the single exception come this trend.
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Yes, they are electron donating groups, yet they execute it so begrudgingly the they purposely slow down the reaction. This makes the halogen a deactivator, however still one ortho/para director.In Summary
Ortho, Meta and Para refer to the 1-2, 1-3, and also 1-4 relationships between benzene substituents. In Electrophilic fragrant Substitution reactions, O/M/P directing effects assist us figure out where to place the just arrive electrophile. Electron Donating teams activate the ring because that ortho and also para addition. Electron Withdrawing groups deactivate the ring because that meta addition. Halogens room the one exception.
I desire to hear from you!
Do you now feel prepared to overcome EAS exercise questions there is no relying ~ above memorized substituents?