Unit 16: Electrophilic Aromatic Substitution
Benzene keeps its aromaticity by swapping a hydrogen instead of adding across a double bond, and every substituent decides where the next one lands.
8 topics · 28 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The General Mechanism of Electrophilic Aromatic Substitution
- How it worksHow does an electrophile attack the ring to form a resonance-stabilized cation before a proton leaves to restore aromaticity?
- Worked exampleWalk me through the full arrow-pushing mechanism for benzene reacting with a generic electrophile E+ to give the product.
- Common mistakeWhy doesn't losing a proton at the end of this mechanism count as an elimination reaction the way E1 or E2 would?
- Going furtherWhat happens to the pathway if the intermediate cation tried to add a second electrophile before losing its proton?
Your turn1 of 2
Benzene reacts with a generic electrophile E+. Walk through the mechanism: what happens first, and how does the ring finish the reaction?Halogenation of Benzene
- How it worksHow does a Lewis acid like FeBr3 polarize the bromine molecule enough to generate an electrophile strong enough for the ring?
- Worked exampleFind the product formed when benzene reacts with bromine in the presence of FeBr3.
- Common mistakeWhy doesn't benzene undergo the same addition reaction with bromine that a simple alkene does?
Your turn1 of 2
Benzene reacts with bromine in the presence of FeBr3. What is the product?Nitration and Sulfonation
- How it worksHow does sulfuric acid protonate nitric acid to generate the nitronium ion, the actual electrophile in nitration?
- Worked exampleFind the product formed when benzene is treated with a mixture of concentrated nitric and sulfuric acid.
- Going furtherWhat happens to the reaction when sulfur trioxide replaces the nitronium ion, forming a sulfonic acid product instead?
Your turn1 of 2
Benzene is treated with a mixture of concentrated nitric acid and sulfuric acid. What is the product?Friedel-Crafts Alkylation
- How it worksHow does an aluminum chloride catalyst pull chloride off an alkyl halide to generate the carbocation the ring attacks?
- Worked exampleFind the product when benzene reacts with 2-chloropropane in the presence of AlCl3.
- Common mistakeWhy doesn't Friedel-Crafts alkylation with a primary alkyl halide give the straightforward product you would expect?
- Going furtherWhat happens to a Friedel-Crafts alkylation when the ring already carries a strongly deactivating group like a nitro group?
Your turn1 of 2
Benzene reacts with 2-chloropropane in the presence of AlCl3. What is the product?Friedel-Crafts Acylation
- How it worksHow does a resonance-stabilized acylium ion form as the electrophile without ever rearranging the way a carbocation would?
- Worked exampleFind the product when benzene reacts with acetyl chloride in the presence of AlCl3.
- Ties togetherHow does following a Friedel-Crafts acylation with a separate reduction step give the unrearranged alkylbenzene alkylation couldn't?
Your turn1 of 2
Benzene reacts with acetyl chloride in the presence of AlCl3. What is the product?Activating and Deactivating Groups
- How it worksHow does a lone pair on a substituent donate electron density into the ring by resonance to activate it toward electrophiles?
- Worked exampleFind whether anisole, carrying a methoxy group, reacts faster or slower than benzene toward electrophilic substitution.
- Common mistakeWhy doesn't every substituent that donates electron density by resonance also happen to donate it inductively?
- Going furtherWhat happens to a ring's reactivity when it carries a strongly electron-withdrawing group like a nitro group instead?
Your turn1 of 2
Does anisole, C6H5-OCH3, react faster or slower than benzene toward electrophilic aromatic substitution?Ortho/Para vs Meta Directing Effects
- How it worksHow does drawing out the resonance structures of the intermediate cation reveal which positions get extra stabilization?
- Worked exampleFind the major product when nitrobenzene undergoes a second nitration, predicting the position from its meta-directing group.
- Common mistakeWhy doesn't a halogen substituent direct the way the other electron-withdrawing groups it resembles otherwise do?
- Ties togetherHow does mapping resonance structures onto directing effects here reuse the exact same resonance skill from the bonding unit?
Your turn1 of 2
Nitrobenzene undergoes a second nitration. Where does the new nitro group go, given that the existing nitro group is meta-directing?Directing Effects on Disubstituted Rings
- How it worksHow does a stronger activating group override a weaker one when two substituents' directing effects would otherwise conflict?
- Worked exampleFind the major product when a third substituent is added to a ring already bearing a methyl group and a nitro group.
- Going furtherWhat happens when steric hindrance between two substituents blocks the position that electronic effects alone would favor?
Your turn1 of 2
A benzene ring already bears a methyl group and a nitro group. A third substituent is added. Which existing group's directing effect wins?