Ch. 17 - AromaticityWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

Hückel’s rule is a simple application of LCAO. It states that a fully conjugated cyclic molecule requires 4n+2 pi electrons, where n is any integer, to be to be considered an aromatic compound. That means the number of pi electrons in conjugation must be 2, 6, 10, 14, etc. 


How to determine n

Before we even try to find “n,” we should confirm that the other three rules of the Four Rules of Aromaticity are satisfied. The variable “n” stands for the number of pi electrons found in the would-be aromatic portion of a molecule. The pi electrons can be found in pi bonds (like those in alkenes and alkynes) and in lone pairs. 

Once the first three rules are satisfied, we can count the number of pi bonds. Determining if a molecule is cyclic is super easy: “Is it a ring? Yes? Then it’s cyclic.” Conjugation is pretty easy to determine once you’ve learned what to look for. Planarity is a little bit more nuanced, but you can generally assume planarity in molecules given by professors.  

Let’s look at benzene, which satisfies all four rules:

1.    Cyclic: √

2.    Fully conjugated: √

3.    Planar: √

4.    Huckel’s rule √

Benzene four rulesBenzene Four Rules

Benzene is cyclic, all carbons have sp2 hybridization, it’s a planar ring, and it has six pi electrons. Six is a Huckel’s rule number, so that makes it aromatic! Benzene is far from the only aromatic molecule, however; there are tons.

Aromatic vs antiaromatic

If a molecule satisfies rules 1-3 BUT has a 4n number (Breslow’s rule) of electrons, it’s what we call antiaromatic. Antiaromatic molecules are very unstable molecules that often decompose on their own. 

If a molecule doesn’t satisfy any of rules 1-3, it’s considered nonaromatic. 

Let’s count some pi-electrons in some practice problems and assume that each molecule is planar. Remember:

  • if the molecule has a Huckel’s rule number of pi electrons (4n+6) it’s aromatic
  • if it has 4n pi electrons, it’s antiaromatic
  • if it doesn’t satisfy rules 1, 2, or 3, it’s antiaromatic. 

Let’s determine whether the molecules below are aromatic, antiaromatic, or nonaromatic. 

Three-membered ringThree-membered Ring

  • The one on the left is nonaromatic because it breaks rule 2; it’s not fully conjugated. 
  • The one in the middle has a 4n number of pi electrons (where n=1), so it’s antiaromatic. The carbanion can resonate with the double bond, which means the molecule is fully conjugated. This is an antiaromatic ion.
  • The one on the right is fully conjugated and has a 4n+2 number of pi electrons (where n=0), so it’s aromatic. The carbocation has an empty orbital that the double bond’s electrons can resonate with. This is an aromatic ion.

That wasn’t so bad, right? Let’s look at a couple more:

Seven-membered ringSeven-membered Ring

  • The one on the left is antiaromatic because it has a 4n number of pi electrons ; it’s not fully conjugated. 
  • The one in the middle has a 4n number of pi electrons (where n=1), so it’s antiaromatic. The carbanion can resonate with the double bond, which means the molecule is fully conjugated. This is an antiaromatic ion.
  • The one on the right is fully conjugated and has a 4n+2 number of pi electrons (where n=0), so it’s aromatic. The carbocation has an empty orbital that the double bond’s electrons can resonate with. This is an aromatic ion.

Heteroatoms

Non-carbon atoms (heteroatoms) can “choose” to donate a lone pair (electrons in non-bonding orbitals) to complete conjugation and make a molecule aromatic. If, however, donating the lone pair would make the molecule antiaromatic the heteroatom would “choose” to not donate its lone pair. 

Let’s take a quick look at two classic heterocylic aromatics: pyridine and pyrrole. 

Pyridine and pyrrolePyridine and pyrrole

It’s easy to see that pyridine is aromatic because there are three pi bonds in alternation, but it’s a bit more difficult to tell that pyrrole is also aromatic. It’s donating its lone pair into the ring: Nitrogen donating its lone pairNitrogen donating its lone pair

On the left the molecule could be considered nonaromatic (assuming the lone pair is isolated from the ring. On the right the lone pair is participating as pi electrons in the ring, and that brings the number to six. Six is a 4n+2 number where n=1.


Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.