Unit 15: Aromaticity
Why one particular ring of six carbons refuses to behave like an ordinary alkene, and the electron-counting rule that predicts which rings share its stability.
5 topics · 17 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 Structure of Benzene
- How it worksHow does each carbon's leftover p orbital overlap all the way around the ring to spread six electrons over every carbon equally?
- Worked exampleFind the length of every carbon-carbon bond in benzene relative to a typical single bond and a typical double bond.
- Common mistakeWhy isn't benzene actually flipping back and forth between its two Kekule structures the way early chemists first pictured it?
- Going furtherWhat happens to a ring's stability when its continuous overlapping p-orbital system is broken at even one carbon?
Your turn1 of 2
How does the length of every carbon-carbon bond in benzene compare to a typical single bond and a typical double bond?Criteria for Aromaticity
- How it worksHow do cyclic, planar, fully conjugated, and a specific electron count combine as four tests a ring must pass together?
- Worked exampleFind whether cyclooctatetraene satisfies every geometric requirement for aromaticity.
- Common mistakeWhy isn't having a ring of alternating double bonds by itself enough to make a molecule aromatic?
Your turn1 of 2
Does cyclooctatetraene satisfy every geometric requirement for aromaticity, being cyclic, fully conjugated, and planar?Huckel's Rule and the 4n+2 Count
- How it worksHow does completely filling every bonding molecular orbital, with none left half-empty, give the 4n+2 count its stability?
- Worked exampleFind whether the cyclopentadienyl anion, with 6 π electrons in a 5-membered ring, satisfies Huckel's rule.
- Common mistakeWhy doesn't a ring with 4 π electrons get partial credit toward aromaticity instead of becoming actively destabilized?
- Going furtherWhat happens to a ring's electron count, and its aromaticity, when cyclopentadiene loses a proton to form its anion?
Your turn1 of 2
For a ring to satisfy Huckel's rule for aromaticity with n = 3, how many π electrons must it contain?Antiaromatic and Nonaromatic Systems
- How it worksHow does a half-filled, non-bonding molecular orbital in a 4n system make antiaromatic compounds so unstable?
- Worked exampleFind whether cyclobutadiene is aromatic, antiaromatic, or nonaromatic based on its π electron count.
- Going furtherWhat happens to a molecule's classification when it satisfies every geometric rule for aromaticity except being flat?
Your turn1 of 2
Is cyclobutadiene aromatic, antiaromatic, or nonaromatic, based on its π electron count?Nomenclature of Substituted Benzenes
- How it worksHow do you decide whether two substituents on a benzene ring are ortho, meta, or para to each other?
- Worked exampleFind the correct name for a benzene ring carrying a bromo group and a nitro group in a 1,3 relationship.
- Going furtherWhat happens to a compound's name when its ring carries a substituent important enough to be its own parent name, like phenol?
Your turn1 of 2
A benzene ring carries a bromo group and a nitro group in a 1,3 relationship to each other. What is the correct name?