Chapter 16 Aromatic Compounds and Reactions

Aromatic Coumpounds

Aromatic: pleasant aromas

Stable ring structure (ex: Benzene)

• Benzene

  • carbon bonds have the same bond order (1.5)

  • can be resonance stabilized (3 PI BONDS NOT DOUBLE BONDS)

    • pi bonds because the bonds aren’t fixed (it would be called double if it were fixed)

  • sp3 hybridized

Benzene Stability (Benzene Hydrogenation)

Not stabilized like a regular alkene (won’t be oxidized)

• BUT IT CAN BE HYDROGENIZED (with a strong catalyst)

  • breaks its double bonds

Annulenes

They are cyclic hydrocarbons that have non-fixed (alternating via resonance) single and double bonds

• labeled by the number of carbons they have (ex: benzene = [6]annulene)

Not all are aromatic (pleasant smell)

• aromaticity requirements: being cyclic & planar and follow 4n + 2pi rule

• ex: cyclobutadiene and cyclootatetraene aren’t aromatic

Aromaticity Requirements

Structure must be cyclic w/ conjugated pi bonds (bonds separated only be 1 C)

• each atom must have unhybridized p orbital

• all p orbitals must overlap continuously

• planar ring that is more stable than its open-chain counterpar

• Cyclic (forms a ring)

• Planar (flat, so p orbitals can overlap)

• Fully conjugated (every atom in the ring has a p orbital)

• Follows Hückel’s Rule: has 4n + 2 π electrons

Antiaromatic Compounds

Structures that are cyclic, conjugated, overlapping p orbitals BUT ITS OPEN-CHAIN COUNTERPART HAS GREATER ENERGY (MORE STABLE)

• follow rules or aromaticity but are unstable

Nonaromatic Compounds

Structures that are not a continuous ring, does not have overlapping p orbitals, and can be nonplanar

• do not follow aromaticity rules

Hückel’s Rule

Aromatic: A ring is aromatic if it has a

• continuous ring of overlapping p orbitals (fully conjugated)

• planar structure

• 4n + 2pi electrons

  • ex: 2, 6, 10

• They are very stable

Antiaromatic: A ring is antiaromatic if it has a

• continuous ring of overlapping p orbitals (fully conjugated)

• planar

• 4n pi elecetrons

  • ex: 4, 8

• They are very unstable (avoid total instability by usually becoming nonplanar)

Larger [N] annulenes aromoticity or antiaromoticity depends on if its ring can stay planar

Cyclopentadienyl Anion

Has a nonbonding pair of e- in a p orbital

• has 6 conjugated e-

• it is AROMATIC (forming the anion is favorable so it is much more acidic than typical alkenes)

6 pi —> aromoatic —> stable

Cyclopentadienyl Cation

Has an empty p orbital

• total 4 pi electrons (fits antiaromatic Hucks rule 4n)

• ANTIAROMATIC (very unstable so it is not easily formed)

4 pi —> antiaromatic —> very unstable

Cycloheptatrieneyl Ion

Tropylium Cation and Tropylium Anion

Tropylium Ion Cation

Has an empty p orbital

• has 6 pi e- (4n + 2pi)

• aromatic

Tropylium Ion Anion

Has a nonbonding pair of e- in a p orbital

• has 8 pi e

• anti-aromatic

Cyclooctatetraene Dianonion

Has 2 non bonding pairs of electrons

• has 10 pi electrons

• Aromatic

MAIN Heterocyclic Aromatic Coumpounds

Pyridine and Pyrrole

• also has Pyrimidine, Imidazole, Furan, and Thiophene

Pyridine

The nitrogen analog of benzene (benzene but nitrogen replaces one carbon)

• 6 pi e- —> aromatic —> stable

• nitrogen has lone pairs (don’t overlap with the aromatic pi system)

  

• if protonated —> becomes pyridinium ion that is still aromatic

  • PROTONATED STILL AROMATIC

Pyrrole

5 membered heterocycle ring (with nitrogen replacing a carbon, the N is conencted to an H)

• 6 pi e- —> aromatic —> stable

• nitrogen lone pairs does overlap with the aromatic pi system

Pyrimidine

6 membered ring with 2 nitrogens (benzene with 2 N’s instead of c’s)

• N’s lone pairs don’t overlap the aromatic pi system

• 6 conjugated pi e- = aromatic = stable

Imidazole

5-membered ring with 2 nitrogens (1 nitrogen is NH)

• one of the lone pairs of the Ns (the NH) DO OVERLAP with the aromatic pi system

  • the other N doesn’t

• 6 pi e’s —> aromatic —> stable

Furan

5-membered ring with OXYGEN

• 2 lone pairs do overlap with the aromatic pi system

• ONE OF THE PAIRS OVERLAP, the other doesn’t

• 6 conjugated pi e’s —> aromatic —> stable

Thiophene

5 membered ring with SULFUR

• 2 pair of lone pairs 

  • ONE OF THE PAIR OVERLAPS THE PI SYSTEM, the other doesn’t

• 6 conjugated pi e’s —> aromatic —> stable

  

Fused Heterocylic Compounds

When heterocyclic compounds are fused together, forming fused-ring heterocycles

• they are similar to the simple heterocycles

• THEY MUST FORM 10 π electrons (THEY ARE STILL AROMATIC)

  

Benzene Derivatives Common Names

Disubstituted Benzenes (Positions)

There is Ortho, Meta, and Para

Ortho: 1,2 pos

Meta: 1,3 pos

Para: 1,4 pos

Benzene with 3 or more substituents naming

Carbon with the root substituent (phenol or benzoic acid, OH or COOH) gets labeled pos 1

• the rest are numbered to give the lowest possible number to substituents

• if both phenol OH and benzoic acid COOH are the substituents, benzoic acid COOh takes 1st priority

Common Names for Disubstituted Benzenes

Phenyl and Benzyl Group Substitued

Phenyl Group: when the benzene ring is the substituent on another group

• benzene (on the ring) —> attaches to another molecule

Benzyl Group: when the benzene ring is attached to a methyl group (benzene + methyl that is attached to rest of another group)

• benzene + CH2 —> attaches to another molecule

DIFFERENCE IS THE PHENYL GROUP ONLY HAS BENZENE AS THE SUBSTITUTENT WHILE BENZYL GROUP HAS A BENZENE +CH2 AS THE SUBSTITUTENT