2. AROMATICITY
● In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to
help determine if a planar ring molecule would have aromatic properties.
● Property of an organic compound to be aromatic.
● Compounds with distinctive pleasant smells i.e benzene and toluene hence the
term aromaticity to distinguish compounds with the property.
● Useful to understand the structure, stability, spectroscopy, and chemical reactivity
of many compounds.
4. THERE ARE FOUR MAIN CONDITIONS FOR AROMATICITY:
• The compound must be cyclic.
A compound in which series of
atoms is connected and forms a
ring.
Examples:
5. ANTHRACENE
• The compound must be planar.
A compound in which all the atoms lie in a
same plane.
Example: Benzene, Anthracene,
Naphthalene etc.
6. • The molecule must have a fully
conjugated pi electron system.
Each element within the ring must have p-
orbital perpendicular to ring.
7. HUCKLE’S RULE:
The compound must have 4n+2 pi electrons. It
means number of pi electrons in the compound must
be equal to result of 4n+2.
Where ‘n’ is an integer 1,2,3,……
*Pi electrons* 2,6,10,14,18....
Examples:
Benzene:
Naphthalene:
Having 5 pi bonds means total of 10 pi electrons.
4n+2 = 4(2)+2 = 8+2 = 10
8. Huckle rule on heterocyclic compounds:
Furan
•Having 2 pi bonds and 2 pi electrons of Oxygen also take part in resonance
•So Furan has total of 6 pi electrons .
•4n+2 = 4(1)+2 = 6
10. Aromatic Anti aromatic Non aromatic
Cyclic Cyclic IT FAILS ANY OF THE
CRITERIA OF
AROMATICITY
Conjugated Conjugated May or may not cyclic
Planar Planar May or may not planar
Follow Huckels rule Don't follow Huckel's rule May or may not follow
Huckel's rule
Highly stable Unstable
Unreactive Reactive
Example: Benzene Cyclobutadiene Any straight/brached chain
Or cyclic(non conjugated)
16. WHY BENZENE SHOW PLANAR STRUCTURE?
It is planar because that is the only way that the
p orbitals can overlap sideways to give the
delocalised pi system.
WHY BENZENE SHOW CYCLIC
STRUCTURE?
Due to the comparable nature of all six
carbon atoms and their corresponding
p orbitals
17. WHY BENZENE ARE FULLY
CONJUCATED?
Conjugated molecules feature
numerous (double or triple) bonds that
alternate with single ones.
BENZENE SHOW HUCKEL
RULE
Benzene follow Huckel's
rule(4n+2 pi electrons).Benzene
has total 32 electrons. Total 3
double bonds (each containing 2
pi electrons)are present.So total 6
pi electrons are present.Now
apply Huckel's rule,
20. • Benzenoid compounds contain at least one benzene ring in the molecule
• Whereas non benzenoid compounds have no benzene rings.
• An aromatic compound is a cyclic, planar molecule with a ring of resonance
bonds. These structures are more stable than normal ring structures. Both
benzenoid and non benzenoid compounds are Aromatic compounds.
Therefore, all aromatic compounds are not necessarily benzenoid
compounds.
What is Benzenoid?
Benzenoid compounds are molecules that have at least
one benzene ring in its chemical structure. A benzene
ring is a cyclic structure having six carbon atoms as the
ring members. It has three pi bonds (double bonds) and
three sigma bonds arranged in an alternative pattern.
Therefore, we call this pattern a conjugated pi System.
21. AZULENE
• Since the molecule has double bonds due to benzene ring, the molecule is an unsaturated
compound with extra stability provided by the conjugated pi system
What is Non Benzenoid?
Non benzenoid compounds are aromatic molecules
that do not have benzene rings in its chemical
structure. Although a benzene ring is absent, these
molecules have a conjugated pi system. The ring
structures of these compounds have around 5-7 carbon
atoms..
The aromatic nature arises due to the presence of
conjugated pi system. This conjugated pi system
provides the molecule with extra stability. Some
common examples include azulenes,
Oxaazulanones, Pentafulvene, Tropones &
Tropolones, etc.
22. SIGNIFICANCE OF AROMATIC
COMPOUNDS
• Aromatic compounds are essential in industry.
• Key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and para-xylene.
• About 35 million tons of aromatic compounds are produced in the world every year.
• They are used to produce important chemicals and polymers, such as polyester, polystyrene and nylon.
• Aromatic compounds are vital to the biochemistry of all living things.
• Four of the twenty amino acids (histidine, phenylalanine, tryptophan, and tyrosine) used to form proteins are
aromatic compounds.
• All five of the nucleotides ((adenine, thymine, cytosine, guanine, and uracil)) that make up DNA and RNA sequences
are all aromatic compounds.
• Many vitamins such as vitamin B1, B2, B3, B6, B12, folic acid, vitamin E and vitamin K contain aromatic system.
• The molecule heme contains an aromatic system with 22 π-electrons.
• Chlorophyll also has a similar aromatic system.
• Aromatic compounds are integral part of many medicinal compounds such as Aspirin, Paracetamol, Ibuprofen,
Diclofenac etc.
