The document summarizes aromaticity and related topics for chemistry students. It discusses:
- Benzenoid and non-benzenoid aromatic compounds, including their properties and reactions.
- Resonance structures of benzene and how it follows Huckel's rule for aromaticity.
- Classification of compounds based on aromaticity and examples of antiaromatic compounds.
- Aromatic ions and heterocyclic aromatic compounds like pyrrole, furan and pyridine.
THE PERICYCLIC REACTION THE MOST COMMON TOPIC INCLUDE THE SYLLABUS OF MANY SCIENCE STUDY INCLUDING BSC, MSC , PHARMA STUDY, AND MORE HENCE WE ARE COVERED ALL THE DATA OF IT HOPE THIS WILL MAKE READER EASY.
Crown ethers
NOMENCLATURE
GENERAL SYNTHESIS OF CROWN ETHER
AZA CROWN
CRYPTAND
APPLICATIONS
1. SYNTHETIC APPLICTION
Esterification
Saponification
Anhydride formation
Potassium permanganate oxidation
Aromatic substitution reactions
Elimination reactions
Displacement reaction
Generation of carbenes
Superoxide anion
Alkylations – 1. o-alkylations
2. c-alkylations
3. n-alkylations
2. ANALYTICAL APPLICATION
Determination of gold in geological samples
Super critical fluid extraction of trace metal from solid and liquid materials
Application of ionic liquids in analytical chemistry
Oxidation and determination of aldehydes
Crown ethers are used in the laboratory as phase transfer catalyst
OTHER APPLICATION
It is used in photocynation
Resolution of racemic mixture
Benzoin condensation
Hetrocyclisation
Synthesis of furanones
Acetylation of secondary amines in presence of primary amine
The presentation is prepared for lecture for the M. Sc Chemistry students studying under University of Madras (MER3A: Unit III). It is dealing with Aromaticity and Organic Photochemistry
more chemistry contents are available
1. pdf file on Termmate: https://www.termmate.com/rabia.aziz
2. YouTube: https://www.youtube.com/channel/UCKxWnNdskGHnZFS0h1QRTEA
3. Facebook: https://web.facebook.com/Chemist.Rabia.Aziz/
4. Blogger: https://chemistry-academy.blogspot.com/
photo redox reactions
General Chemistry of AROMATIC COMPOUNDS
CONTENTS:
-Aromatic compounds and aromaticity.
- Characteristic properties of aromatic compound.
- Nomenclature of derivatives of benzene.
- Benzene.
- Kekule’s structure of Benzene.
X-ray study of the structure of Benzene.
Molecular orbital description of benzene.
- Hückel 4n+2 Rule.
- Resonance of Benzene.
- Preparations of Benzene.
- Reactions of aromatic compound.
- Orientation in electrophilic substitution reactions.
THE PERICYCLIC REACTION THE MOST COMMON TOPIC INCLUDE THE SYLLABUS OF MANY SCIENCE STUDY INCLUDING BSC, MSC , PHARMA STUDY, AND MORE HENCE WE ARE COVERED ALL THE DATA OF IT HOPE THIS WILL MAKE READER EASY.
Crown ethers
NOMENCLATURE
GENERAL SYNTHESIS OF CROWN ETHER
AZA CROWN
CRYPTAND
APPLICATIONS
1. SYNTHETIC APPLICTION
Esterification
Saponification
Anhydride formation
Potassium permanganate oxidation
Aromatic substitution reactions
Elimination reactions
Displacement reaction
Generation of carbenes
Superoxide anion
Alkylations – 1. o-alkylations
2. c-alkylations
3. n-alkylations
2. ANALYTICAL APPLICATION
Determination of gold in geological samples
Super critical fluid extraction of trace metal from solid and liquid materials
Application of ionic liquids in analytical chemistry
Oxidation and determination of aldehydes
Crown ethers are used in the laboratory as phase transfer catalyst
OTHER APPLICATION
It is used in photocynation
Resolution of racemic mixture
Benzoin condensation
Hetrocyclisation
Synthesis of furanones
Acetylation of secondary amines in presence of primary amine
The presentation is prepared for lecture for the M. Sc Chemistry students studying under University of Madras (MER3A: Unit III). It is dealing with Aromaticity and Organic Photochemistry
more chemistry contents are available
1. pdf file on Termmate: https://www.termmate.com/rabia.aziz
2. YouTube: https://www.youtube.com/channel/UCKxWnNdskGHnZFS0h1QRTEA
3. Facebook: https://web.facebook.com/Chemist.Rabia.Aziz/
4. Blogger: https://chemistry-academy.blogspot.com/
photo redox reactions
General Chemistry of AROMATIC COMPOUNDS
CONTENTS:
-Aromatic compounds and aromaticity.
- Characteristic properties of aromatic compound.
- Nomenclature of derivatives of benzene.
- Benzene.
- Kekule’s structure of Benzene.
X-ray study of the structure of Benzene.
Molecular orbital description of benzene.
- Hückel 4n+2 Rule.
- Resonance of Benzene.
- Preparations of Benzene.
- Reactions of aromatic compound.
- Orientation in electrophilic substitution reactions.
Benzene has 6π electrons delocalized in 6p orbitals that overlap above and below the plane of the ring. Because benzene’s six pie electrons satisfy Huckel’s rule, benzene is especially stable. Reaction that keep the aromatic ring intact are therefore favoured
Organic chemistry has two main divisions. One division deals with aliphatic (fatty) compounds, the first compounds you encountered in Organic Chemistry I. The second division includes the aromatic (fragrant) compounds, of which benzene is a typical example.
1. Introduction
2. History of benzene
3. Nomenclature
4. Orbital structure
5. Kekule structure
6. Resonance structure
7. Resonance energy and stability 8. Structural evidence
9. Synthetic evidence
10. Analytical evidence
11. Aromaticity and huckle rule
12. Method of preparation of benzene
13. Electrophilic substitution of benzene
14. Classification of substituent 15. Directive effect : Ortho and para director, meta director
16. Reaction of monosubstituted benzene
17. Effect of Substituents on reactivity and orientation of monosubstituted benzene towards electrophilic substitution 18. Structure and uses of BHC,DDT, Saccharine and chloramine
Benzene is an organic chemical compound with the molecular formula C6H6. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point
Bonding and Antibonding interactions; Idea about σ, σ*, π, π *, n – MOs; HOMO, LUMO and SOMO; Energy levels of π MOs of different conjugated acyclic and cyclic systems; Hückel’s rules for aromaticity; Frost diagram
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This is a presentation by Dada Robert in a Your Skill Boost masterclass organised by the Excellence Foundation for South Sudan (EFSS) on Saturday, the 25th and Sunday, the 26th of May 2024.
He discussed the concept of quality improvement, emphasizing its applicability to various aspects of life, including personal, project, and program improvements. He defined quality as doing the right thing at the right time in the right way to achieve the best possible results and discussed the concept of the "gap" between what we know and what we do, and how this gap represents the areas we need to improve. He explained the scientific approach to quality improvement, which involves systematic performance analysis, testing and learning, and implementing change ideas. He also highlighted the importance of client focus and a team approach to quality improvement.
The Roman Empire A Historical Colossus.pdfkaushalkr1407
The Roman Empire, a vast and enduring power, stands as one of history's most remarkable civilizations, leaving an indelible imprint on the world. It emerged from the Roman Republic, transitioning into an imperial powerhouse under the leadership of Augustus Caesar in 27 BCE. This transformation marked the beginning of an era defined by unprecedented territorial expansion, architectural marvels, and profound cultural influence.
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Under Augustus, the empire experienced the Pax Romana, a 200-year period of relative peace and stability. Augustus reformed the military, established efficient administrative systems, and initiated grand construction projects. The empire's borders expanded, encompassing territories from Britain to Egypt and from Spain to the Euphrates. Roman legions, renowned for their discipline and engineering prowess, secured and maintained these vast territories, building roads, fortifications, and cities that facilitated control and integration.
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2. Benzenoid and non-benzenoid aromatic compounds
• Aromatic compounds are those compounds which are planar, cyclic conjugated
and follow Huckel’s aromaticity rule (4n+2) π electrons where n = 0, 1, 2…….
• They give substitution reactions easily rather than addition reactions and retained
their aromaticity.
• They are stable due to resonance and have high resonance energy.
• They show ring current effect in NMR spectrum, chemical shift values obtained at
highly deshield area δ>6.0 ppm.
• Give specific feature in UV, IR and 13C-NMR (δ value: 145-170 ppm) spectroscopy
also.
• Three absorption bands observed in UV-Visible spectroscopy:
• λmax = 184 nm(intense), 203.5 nm(K band), 254 nm(B band)
2Dr. Meenal Gupta, Vikram University, Ujjain
3. Resonating structures of benzene
According to Huckel’s rule (4n+2π) e: n= 1 6 π electrons in benzene, gives
nitration, Friedel craft reaction etc, easily.
Conc.HNO3
Conc. H2SO4
CH3COCl
An.AlCl3
Nitration
Acetylation
NO2
COCH3
Nitro benzene
acetophenone
First three are Dewar structures and after that are Kekule structures and last is resonance
hybrid. All carbon-carbon bond lengths are equal 1.39 Ao in between double and single
bond, indicates resonance of it .
3Dr. Meenal Gupta, Vikram University, Ujjain
4. Distribution of pi-molecular orbital in benzene can be shown as:
Naphthalene
According to Huckel’s rule (4n+2π) electrons: n = 2 10 π electrons, ring is
cyclic, coplanar, having conjugated pi- electrons and stabilized by resonance, shows
ring current effect, therefore aromatic.
4Dr. Meenal Gupta, Vikram University, Ujjain
5. Possibility of easy aromatization in dihydro derivatives - dihydropyridine is easily
oxidized to more stable pyridine.
Stable
5Dr. Meenal Gupta, Vikram University, Ujjain
7. [14] annulene
[14] annulene is aromatic, monocyclic
ring is cyclic coplanar having conjugated double
bond which delocalized and follows Huckel's rule
of aromaticity.
H
H H
H
H
H
[10] annulene
It is cyclic, pi-bonds are conjugated and follows
Huckel's rule having 10 pi electrons but due to
junction of trans hydrogens having non-bonded
repulsion more therefore it is highly unstable, hence
it it not aromatic.
When interacting hydrogens of the ring is replaced by bridging
methylene(-CH2) group, having 10 pi-electrons and shows
resonance,highly stable and it is aromatic.
[10] annulene
[10] annulene
7Dr. Meenal Gupta, Vikram University, Ujjain
8. O
Tropone
Tropone is aromatic due to electron withdrawing
nature of carbonyl group and system will be
conjugated with positive charge of carbonyl
carbon with 6 pi-electrons.
O
O O O
Resonating structure of tropone
Cyclopentadiene
It is cyclic coplanar having 4-pi electrons in the ring and it is not conjugated because
one carbon is SP3 hybridized therefore it is nonaromatic.
But it behaves as acid and form salt with reaction of base to form potassium salt of
cyclopentadienide which is aromatic and negative charge is involved in resonance
due to conjugation.
Follows Huckel’s rule of aromaticity where 4n+2 pi electrons (n=1 has 6 pi electrons).
8Dr. Meenal Gupta, Vikram University, Ujjain
9. Distribution of pi-electron density can be shown by resonating structure of pyridine
N N
N N
It is cyclic, coplanar, due to 6π electrons are conjugated in pyridine therefore it is
aromatic like benzene.
The lone pair of electron is free not involve in conjugation therefore can be easily
donate to an acid, hence it acts as Lewis base and form pyridinium salt, retained their
aromaticity.
N N
H Cl
HCl
Pyridine
Pyridinium chloride
9Dr. Meenal Gupta, Vikram University, Ujjain
10. Resonating structures of five membered heterocycles
Five membered rings are coplanar, cyclic having 4π electrons but due to presence of lone
pair of electron on hetero atom of the ring which is conjugated with pi- bonds, they are
aromatic.
Lone pair is donated to the ring and showing resonance, follow Huckel’s aromaticity rule,
where total 6 π- electrons are involved in resonance.
X X X X X
Resonating structures in five membered aromatic heterocycles
(X=NH, O or S) Pyrrole,furan or thiophene, respectively
Like benzene they give electrophilic substitution reactions easily.
Benzofused heterocycles like –Benzofuran, benzopyrrole Indole are also aromatic
compounds.
N
S
N
H
Benzo-Pyrrole
O
Benzo[b]furan
10Dr. Meenal Gupta, Vikram University, Ujjain
11. A monocyclic ring having conjugated double bonds is alled annulene.
Simplest annulene is Benzene; named as [6] annulene and 1,3,5,7-cyclooctatetraene
named as [8]annulene respectively.
The number of pi-electrons is shown as prefix in square bracket and then written
annulene.
For aromaticity , annulene must obey Huckel’s (4n+2) pi electrons where n=o.,1,2……along
with they must coplanar, conjugated system,
showing ring current effect in nmr spectrum and show resonance. They are highly stable.
Annulenes may be antiaromatic, they follow Huckel’s 4nPi electrons rule where n = 1,2,3,
….. and ring must be coplanar and conjugated.
They are highly unstable.
As shown in the following diagram, 1,3,5,7,9-cyclodecapentaene fails to adopt a planar
conformation, either in the all cis-configuration or
in its 1,5-trans-isomeric form. The transannular hydrogen crowding that destabilizes
the latter may be eliminated by replacing the interior
hydrogens with a bond or a short bridge (colored magenta in the diagram).
As expected, the resulting 10 π-electron annulene derivatives
exhibit aromatic stability and reactivity as well as characteristic ring current anisotropy
in the nmr.
11Dr. Meenal Gupta, Vikram University, Ujjain
12. Naphthalene and azulene are [10]annulene analogs stabilized by a transannular bond.
Although
the CH2 bridged structure to the right of naphthalene in the diagram is not exactly planar,
the conjugated 10 π-electron ring is sufficiently close to planarity to achieve aromatic
stabilization. The bridged [14]annulene compound on the far right, also has aromatic
properties.
12Dr. Meenal Gupta, Vikram University, Ujjain
13. Antiaromaticity
Conjugated ring systems having 4n π-electrons (e.g. 4, 8, 12 etc. electrons) not only fail to show any aromatic properties, but appear to be
less
stable and more reactive than expected. As noted above, 1,3,5,7-cyclooctatetraene is non-planar and adopts a tub-shaped conformation.
The compound is readily prepared, and undergoes addition reactions typical of alkenes. Catalytic hydrogenation of this tetraene produces
cyclooctane. Planar bridged annulenes having 4n π-electrons have proven to be relatively unstable. Examples of 8 and 12-π-electron
systems
are shown below, together with a similar 10 π-electron aromatic compound.
Planar (antiaromatic) anti-aromatic aromatic anti-aromatic Non-planar (non-aromatic)
The simple C8H6 hydrocarbon pentalene does not exist as a stable compound, and its hexaphenyl derivative is air sensitive.
The 12-π-electron
analog heptalene has been prepared, but is also extremely reactive (more so than cyclooctatetraene). On the other hand,
azulene is a stable
10-π-electron hydrocarbon that incorporates structural features of both pentalene and heptalene. Azulene is a stable blue
crystalline solid that
undergoes a number of typical aromatic substitution reactions. The unexpected instability of 4n π-electron annulenes has
been termed
"antiaromaticity".
Other examples may be cited. Thus, all attempts to isolate 1,3-cyclobutadiene have yielded its dimer, or products from
reactions with
other compounds introduced into the reaction system. Similarly, cyclopentadienyl cation (4 π-electrons) and cycloheptatrienyl
anion (8 π-electrons)
show very high reactivity when forced to form. 13Dr. Meenal Gupta, Vikram University, Ujjain
14. Aromatic Ions
Carbanions and carbocations may also show aromatic stabilization. Some
examples are:
The three-membered ring cation has 2 π-electrons and is surprisingly stable,
considering its ring strain.
Cyclopentadiene is as acidic as ethanol, reflecting the stability of its 6 π-electron
conjugate base.
Salts of cycloheptatrienyl cation (tropylium ion) are stable in water solution, again
reflecting
the stability of this 6 π-electron cation.
14Dr. Meenal Gupta, Vikram University, Ujjain
15. 1. To select order of basic strength in pyrrole, pyridine and piperidine.
2. Account for aromaticity observed in following:
(i) 1,3-cyclopentadienyl anion but not 1,3-cyclopentadiene.
(ii) 1,3,5,-cycloheptatrienyl cation but not for 1,3,5-cycloheptatriene.
(iii) Cyclophenyl cation.
(iv) The heterocycles pyrrole, furan and pyridine.
3. Why cyclooctatetraene decolorizes dil KMnO4 and Br2 in CCl4 both.
4. Deduce structure when a red compound formed by reaction of 2 mol of AgBF4 with 1
mol
of 1,2,3,4-teraphenyl-3,4-dibromocyclobut-1-ene.
5. A stable compound from the reaction of 2 mol of K with 1 mol of 1,3,5,7-cycloocta-
tetraene with no liberation of H2.
6. Account that [18] annulene is aromatic but [16] and [20] annulenes are not.
7. Draw Pi molecular energy orbitals of cyclooctatetraene and cyclo butadiene and
benzene.
8. The obs. Heat of combustion of benzene is -3301.5 KJ/mol. Experimentally obtained
theoretical value from other compound are (Kj/mol) C=C (-491.5); C-C (-206.5); C-H (-
225.8).
By use these data to calculate heat of combustion for benzene and the difference between
this and experimental value.
9. Exp. Determined heat of combustion of cyclooctatetraene is -4580 KJ/mol, use above
data
calculate the resonance energy of it. 15Dr. Meenal Gupta, Vikram University, Ujjain