You are watching: Ortho/para-directed substitution.
Ortho Meta Para Definitions
Let’s backtrack all the means to nomenclature native your first (second?) chapter in orgo 1. In IUPAC naming, every substituent is designated v a number.
Given a 5 carbon chain with a methyl on carbon 4 and also alcohol top top carbon 2, you gain 4-methyl-2-pentanol.
But the wasn’t the only way to indicate the relationship of substituents, to be it? Think ago to cis and trans or to E and Z.
When it comes to a disubstituted benzene, using the state ‘ortho, meta and para’ is just another way to show the partnership of the substituents to each other, fairly than their in its entirety placement on the ring.
For example, 1,2-dimethylbenzene tells united state we have actually methyl groups both top top carbons 1 and 2.
Ortho-dimethylbenzene (ortho-xylene or o-xylene) tells united state that the 2 groups are straight next come each various other on the ring.
Common mistake Warning: despite their similar appearance, these relationships ONLY apply to benzene rings and NOT cyclohexane rings.Ortho
Ortho represents a 1,2 relationship, or 2 groups that are directly near each various other on the benzene ring.Meta
Meta to represent a 1,3 relationship, or groups that space separated by one carbon top top the benzene ring.Para
Para represents a 1,4 relationship, or teams that room opposite each other on the benzene ring.
Use the friendly OMP monster (explained in the video below) to help you easily recognize these relationships.
Ok, so us have fancy names for these relationships. Currently what?
When you very first learned EAS reactions, you concentrated on adding a simple super-electrophile come benzene. Use these videos to review EAS reactions.
But essential chemistry is never as simple as what you discover in lecture, is it?After mastering the simple reactions, you will do it be asked to complete EAS reactions on more complicated and substituted aromatic compounds.
For example, speak you’re request to finish a Friedel-Crafts Acylation top top a bromobenzene. Carry out you include it to the ortho, meta, or para position? Or, do you simply show all 3 choices for triple credit?
Not quite!WHERE you place the just arrive electrophile is directly influenced through the chemical nature the the substituent(s) already on the benzene ring. Early substituents act together directors because that the just arrived groups.
Important Vocabulary for OMP Directing Effects
Let’s first understand the language before jumping right into the why and also how.
Directing / director / Directing EffectA group already on benzene that will dictate where the incoming electrophile will certainly be added.
Activating GroupA group currently on benzene that activates the ring in the direction of Electrophilic fragrant Substitution reactions. A group that will rate up the EAS reaction.
Deactivating GroupA group already on benzene that deactivates the ring towards Electrophilic aromatic Substitution reactions. A group that will slow down (and sometimes completely halt) the EAS reaction.
Electron Donating teams – EDGA group currently on benzene that puts negativity right into the ring, either straight through resonance or indirectly v induction.
Electron Withdrawing groups – EWGA group already on benzene that pulls negativity the end of the ring, either straight through resonance or indirectly through induction.
Now that we deserve to speak gibberish, let’s translate this come some handy applications.
Simply put, electron donating teams are activators and also therefore act as ortho and also para directors. Electron withdrawing teams are deactivators and also therefore act as meta directors. (Halogens space the annoying exception.)
On my EAS Cheat Sheet, us look in ~ the most typical substituents and rank them from strong to weak activators and also deactivators.
And this is where many students will begin to studiously memorize the list, hoping to make sense of the information, however they eventually freak out on test day.
But no you! since we’re about to rest this under to the allude where girlfriend will have the ability to evaluate any substituent and also immediately and also logically predict its EAS directing effects.First, a fast review the the Electrophilic fragrant Substitution Mechanism:One pi bond in benzene breaks open to strike a at sight (positively charged) electrophile.The attacking carbon is currently bound come the electrophile, while the other previous 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 newly substituted carbon atom, collapsing that electrons in the direction of the carbocation.
Note: E+ represents the super-electrophile, and the enclosed E to represent the electrophile having attached come the benzene ring.
Which intermediate? The positively charged sigma-complex.
Review this tutorials first if you’re no comfortable through carbocation stability or resonance.
Go back to the positively fee intermediate above. Notification how the carbocation is capable of no one, yet TWO additional resonance forms?
Now think ago to orgo basics.What increases the likelihood the a reaction taking place?Stability that the intermediate!
The more stable the intermediate, the more likely the intermediate will certainly form.The less stable the intermediate… well, molecules just like people, desire to avoid pain. And if forced to react, they will certainly stall and also grumble every action of the way, thus slowing under the reaction.
First, an alert the position of the carbocation in the sigma-complex intermediate.
See just how it has 3 distinctive locations? try as friend might, girlfriend cannot gain the carbocation onto any type of other position.
This is key!
Pay fist to the pattern. Stop say we add a new electrophile come an already substituted benzene ring. What 3 positions have the right to hold the carbocation in each ortho, meta and also para addition?
Note: X to represent the substituent currently present on the ring, and also E represents the newly included electrophile.
Notice exactly how ortho and para have the very same carbocation intermediate patterns, if meta is the specific opposite?
In the lack of additional resonance, we treatment most about the intermediate v a carbocation closest come the substituted carbon.
Which sigma-complex do you think is most stable? The one v an electron donating, or electron withdrawing group?
If you stated the carbonyl is less stable, you room correct. The partially hopeful carbonyl carbon (EWG) would be pissed to have actually a carbocation nearby.
The video below explains and demonstrates this in an ext detail – finish with step-by-step resonance structures.
Activating teams as Ortho/Para Directors
Instead of memorization, identify activating teams as substituents that bring negativity (in a great way) in the direction of the ring. This can be in the form of a lone pair or an extremely 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 pressure incoming electrophiles to include to the ortho or para position.
This renders EDGs ortho and also para directors.
Different reactions will certainly have different percentages for ortho and also para positions. This needs to be evaluated in a lab setting and therefore you will certainly NOT be supposed to differentiate. The one exemption is to recognize that a bulky substituent will force para enhancement simply due to the steric hindrance of its bulk.
Deactivating groups as Meta Directors
Electron withdrawing teams like to pull negativity away from the ring. This provides a positive intermediate very unstable, together it desire negativity rather than positive to stabilize it.
As such, electron withdrawing groups slow down, for this reason deactivating, the EAS reaction. And also when they lastly agree to react (slowly), these groups want to it is in as much away indigenous the confident charge as possible.
A fast review that sigma-complex resonance shows that the meta enhancement pattern permits the carbocation to skip the substituent carbon.
And while not the many stable, this is the lesser the 2 evils and also thus the desired pattern.
This is why deactivating electron withdrawing groups act as meta directors.
Halogens are the solitary exception to this trend.
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Yes, they room electron donating groups, however they do it therefore begrudgingly the they intentionally slow under the reaction. This renders the halogen a deactivator, however still an 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 help us figure out where to ar the just arrived electrophile. Electron Donating teams activate the ring for ortho and also para addition. Electron Withdrawing groups deactivate the ring because that meta addition. Halogens room the one exception.
I desire to hear indigenous you!
Do you currently feel ready to overcome EAS practice questions there is no relying on memorized substituents?