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IITM MURTHALB.Sc 2nd sem
Organic Chemistry Notes
UNIT – 2 (ARENES AND AROMATICITY)
NOMENCLATURE OF BENZENE DERIVATIVES
Benzene is a hydrocarbon with the chemical formula C 6H6. It has 6 carbon atoms joined in a
ring and has 1 hydrogen atom attached to each of the carbon atoms. By replacing one or more
of the hydrogen atoms with some functional group, we get several benzene compounds. While
naming the substituted benzene compounds, we prefix the name of the substituent to the word
benzene.
Mono-substituted Benzene Compounds
For the benzene compounds consisting of a single substituent, we simply prefix the name of the
substituent to benzene. Some examples along with their common names are listed below.
Methylbenzene or Toluene
Toluene
Hydroxybenzene or Phenol
Phenol
Aminobenzene or Aniline
Aniline
, IITM MURTHAL
B.Sc 2nd sem
Organic Chemistry Notes
UNIT – 2 (ARENES AND AROMATICITY)
Ethylbenzene
Di-substituted Benzene Compounds
When there are two substituents present in the compound, we number each of the carbon
atoms in such a manner that the substituents are attached to the lowest possible
numbered carbon atom.
1,3-dinitrobenzene is the name of the following compound. Naming it as 1,5-
dinitrobenzene is incorrect because the carbon atom would not be numbered lowest.
1,3-dichlorobenzene
Moreover, when there is more than one substituent, we also name their positions as ortho- (o),
meta- (m) and para- (p). They refer to the positions 1,2-;1,3- and 1,4- respectively. Thus, we can
name the compound 1,3-dichlorobenzene as m-dichlorobenzene.
Poly-substituted Benzene Compounds
In case of poly-substituted compounds, if there is a base compound present, we assign it as the
position 1. We then choose the next compound for numbering such that it gets the lowest
number. However, if there is no special or base group present, we list them in the alphabetical
order, giving them the lowest numbers.
, IITM MURTHAL
B.Sc 2nd sem
Organic Chemistry Notes
UNIT – 2 (ARENES AND AROMATICITY)
1-Bromo-2,4-dinitrobenzene
For the following compound, aniline is the base compound. So, we name it as 3-Chloro-
2-nitroaniline.
What is Huckel’s Rule?
Huckel’s Rule is used in order to estimate the aromatic qualities of any planar ring-shaped
molecule in the field of organic chemistry. The supporting quantum mechanics required for the
formulation of this rule was solved first by the German physical chemist and physicist Erich
Armand Arthur Joseph Huckel in the year 1931.
The Huckel 4n + 2 Pi Electron Rule
A ring-shaped cyclic molecule is said to follow the Huckel rule when the total number of
pi electrons belonging to the molecule can be equated to the formula ‘4n + 2’ where n can be
any integer with a positive value (including zero).
Examples of molecules following Huckel’s rule have only been established for values of ‘n’
ranging from zero to six. The total number of pi electrons in the benzene molecule depicted
below can be found to be 6, obeying the 4n+2 𝛑 electron rule where n=1.
, IITM MURTHAL
B.Sc 2nd sem
Organic Chemistry Notes
UNIT – 2 (ARENES AND AROMATICITY)
Thus, the aromaticity of the benzene molecule is established since it obeys the Huckel rule.
Generally, aromatic compounds are quite stable due to the resonance energy or the delocalized
electron cloud. For a molecule to exhibit aromatic qualities, the following conditions must be met
by it:
There must be 4n + 2 𝛑 electrons present in a system of connected p orbitals (where the
electrons are delocalized) belonging to the molecule.
In order to meet the first condition, the molecule must have an approximately planar
structure wherein the p orbitals are more or less parallel and have the ability to interact
with each other.
The molecule must have a cyclic structure and must have a ring of p orbitals which
doesn’t have any sp3 hybridized atoms.
Other examples of aromatic compounds that comply with Huckel’s Rule include
pyrrole, pyridine, and furan. All three of these examples have 6 pi electrons each, so the value
of n for them would be one.
AROMATIC IONS
Aromatic molecules need not be neutral molecules. Ions that satisfy Huckel's rule of 4n + 2 π-
electrons in a planar, cyclic, conjugated molecule are considered to be aromatic ions. For
example, the cyclopentadienyl anion and the cycloheptatrienylium cation are both considered to
be aromatic ions, and the azulene molecule can be approximated as a combination of both.
In order to convert the atom from sp 3 to sp2, a carbocation, carbanion, or carbon radical must be
formed. These leave sp2-hybridized carbons that can partake in the π system of an aromatic
molecule. Like neutral aromatic compounds, these compounds are stable and form easily. The
cyclopentadienyl anion is formed very easily and thus 1,3-cyclopentadiene is a very acidic
hydrocarbon with a pKa of 16. Other examples of aromatic ions include
the cyclopropenium cation (2 π-electrons) and cyclooctatetraenyl dianion (10 π electrons).
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