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EWG's + EDG's

OCHEM II: Exam 3

QuestionAnswer
What can EWG's and EDG's determine? Play central role in determining stability, reactivity, and substitution patterns of aromatic systems
EWG's Pull electron density AWAY from conj. pi system or aromatic ring through resonance or inductive events - DECREASE electron density in ring - DESTABILIZE carbocation intermediates in EAS - DECREASE reactivity in EAS - Often STABILIZE negative charge
Examples of EWG's - Nitro group (NO2) - Carbonyl groups (C=O, including aldehydes, ketones, carboxylic acids, esters) - Cyano group (CN) - Sulfuric acid group (SO3H) - Halogens (F, Cl, Br, I) *Inductively withdrawing,Resonance donating, Deactivating but o/p directing
EDG's groups that push electron density INTO pi system or conj. ring through resonance or inductive effects - INCREASE electron density - STABILIZE carbocation intermediates in EAS - INCREASE reactivity in EAS - Often DESTABILIZE negative charge
Examples of EDG's - Hydroxyl group (-OH) - Alkyl group (-CH3, -R) - Amino group (-NH2, -NHR, -NR2) - Alkoxy group (-OCH3)
How is the preferred product in Electrophilic Aromatic Substitution (EAS) determined? It's determined by the stability of the rxn intermediate following the rate determine step - More stable intermediate = lower activation energy = forms FASTER than. other possible products and MORE product obtained
Rate DETERMINING step for Electrophilic Aromatic Substitution (EAS) rxns The nucleophilic attack of the aromatic
Rate LIMITING step for Electrophilic Aromatic Substitution (EAS) rxns The nucleophilic attack of the aromatic pi bond on the electrophile, as the formation of the carbocation disrupts aromaticity
How is stability of an Electrophilic Aromatic Substitution (EAS) rxn determined? Resonance Capabilities - attacks that result in a resonance structure with a more highly substituted (MORE stable) carbocation will be lower in energy; attacks that result in adjacent like charges will be LESS stable
Position(s) electron donating groups (EDG's) prefer Ortho/para positions (although PARA typically preferred) - create resonance structures with more substituted carbocations
Position(s) electron withdrawing groups (EWG's) prefer Meta -Meta position does not create unfavorable position of two like charges near each other (unlike o/p), so it's preferred
What do halogens do in Electrophilic Aromatic Substitution (EAS) rxns Halogens are the EXCEPTION - Highly electronegative and although technically EWG that deactivate benzene ring, they PREFER ORTHO/PARA positions
Relationship between electron density and rate of rxn MORE electron rich aromatic ring is = FASTER rxn
Activators in EAS A substituent that INCREASES rate of rxn relative to Hydrogen
Deactivators in EAS A substituent that DECREASES rate of rxn relative to Hydrogen
Exceptions of EAS relative to activating and deactivating groups - O-R and NR2 groups seem deactivating bc of electronegativity, but are actually ACTIVATING bc they can donate a lone pair of electrons into ring through resonance
Do EWG's increase or decrease acidity? And why? EWG's INCREASE acidity - pulls electrons AWAY from acidic proton through induction or resonance - this stabilizes the negative charge of the conjugate base, so the stronger the acid
Do EDG's increase or decrease acidity? And why? EDG's DECREASE acidity - pushes electrons TOWARD the acidic proton - this destabilizes the conjugate base bc of the extra negative charge - this makes the proton less likely to leave, therefore decreasing acidity
Created by: emily.zegarra
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