Arenes and Aromaticity ( Benzene )
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Transcript of Arenes and Aromaticity ( Benzene )
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Arenes and Aromaticity(Benzene)
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140 pm 140 pm
140 pm 140 pm
140 pm140 pm
All C—C bond distances = 140 pm
Benzeneempirical formula = CH
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140 pm 140 pm
140 pm 140 pm
140 pm140 pm
146 pm
134 pm
All C—C bond distances = 140 pm
140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.
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Resonance & Benzene
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heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene"
H°= – 208 kJ
Thermochemical Measures of Stability
+ 3H2
Pt
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heat of hydrogenation = -208 kJ/mol (-49 kcal/mol)
heat of hydrogenation = -337 kJ/mol (-85.8 kcal/mol)
3H2
Pt
3H2
Pt
Cyclic conjugation versus noncyclic conjugation
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express the structure of benzene as a resonancehybrid of the two Lewis structures. Electrons arenot localized in alternating single and double bonds,but are delocalized over all six ring carbons.
Resonance & Benzene
H
H
H
HH
H
H
H
H
HH
H
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Circle-in-a-ring notation stands for resonance
description of benzene (hybrid of two resonance structures)
Resonance & Benzene
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Question
• Which of the following compounds has a double bond that is conjugated with the p system of the benzene ring?
• A) p-Benzyltoluene• B) 2-Phenyl-1-decene• C) 3-Phenylcyclohexene• D) 3-Phenyl-1,4-pentadiene• E) 2,4,6-trichloroanisole
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Question• Predict which of the following has the
smallest heat of combustion.
• A) B)
• C) D)
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The p Molecular Orbitalsof Benzene
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Orbital Hybridization Model of Bonding in Benzene
High electron density above and below plane of ring
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Frost's circle is a mnemonic that allows us to draw a diagram showing the relative energies ofthe p orbitals of a cyclic conjugated system. 1) Draw a circle.2) Inscribe a regular polygon inside the circle
so that one of its corners is at the bottom.3) Every point where a corner of the polygon
touches the circle corresponds to a p electron
energy level.4) The middle of the circle separates bonding
and antibonding orbitals.
Hückel's Rule
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Energy
Bondingorbitals
Antibondingorbitals
Benzene MOs
6 p AOs combine to give 6 p MOs3 MOs are bonding; 3 are
antibonding
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Energy
Bondingorbitals
Antibondingorbitals
Benzene MOs
All bonding MOs are filledNo electrons in antibonding
orbitals
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Benzene Molecular Orbitalsnodes
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Hückel's Rule:Annulenes
the additional factor that influences aromaticity is the number of p electrons
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Aromatic vs. “anti-aromatic”A. Deniz, et. al., Science, 5 November 1999
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Question
• How many p electrons does the compound shown have?
• A) 5• B) 8• C) 10• D) 12
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Among planar, monocyclic, completely conjugated polyenes, only those with 4N + 2 p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)
N 4N+20 21 62 103 144 18
Hückel's Rule
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Among planar, monocyclic, completely conjugated polyenes, only those with 4N + 2 p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)
N 4N+20 21 6 benzene!2 103 144 18
Hückel's Rule
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Question
• What is the value of N of Huckel's rule for the cyclopentadienyl cation (shown)?
• A) N=1/2• B) N=1/4• C) N=1• D) N=2
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Question
• Which is a true statement based on Huckel’s Rule.
• (Assume that both are planar and that vacant p-orbitals do not interupt conjugation.)
• A) I is aromatic and II is not.• B) II is aromatic and I is not.• C) I and II are aromatic.• D) I and II are not aromatic.
I II
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Hückel”s rule applies to: cyclic, planar,conjugated, polyenesthe p molecular orbitals ofthese compounds have a distinctive patternone p orbital is lowest in energy, another is highest in energy, and the others are arranged in pairs between the highestand the lowest
Hückel's Rule& molecular orbitals
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p-MOs of Benzene
Benzene
Antibonding
Bonding6 p orbitals give 6 p orbitals3 orbitals are bonding; 3 are antibonding
Non-bonding
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Benzene
Antibonding
Bonding6 p electrons fill all of the bonding orbitalsall p antibonding orbitals are empty
p-MOs of Benzene
Non-bonding
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Frost's Circle
p MOs of Benzene
Bonding
Antibonding
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FROST CIRCLES
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Question
• How many isomers of dibromophenol are aromatic?
• A) 3• B) 4• C) 6• D) 8• E) none
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Cyclo-butadiene
Antibonding
Bonding
4 p orbitals give 4p orbitals1 orbital is bonding, one is antibonding, and 2 are nonbonding
p-MOs of Cyclobutadiene(square planar)
Non-bonding
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Cyclo-butadiene
Antibonding
Bonding
4 p electrons; bonding orbital is filled; other 2p electrons singly occupy two nonbonding orbitals
p-MOs of Cyclobutadiene(square planar)
Non-bonding
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Antibonding
Bonding
8 p orbitals give 8 p orbitals3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding
p-MOs of Cyclooctatetraene(square planar)
Cyclo-octatetraene
Non-bonding
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Antibonding
Bonding
8 p electrons; 3 bonding orbitals are filled; 2nonbonding orbitals are each half-filled
p-MOs of Cyclooctatetraene(square planar)
Cyclo-octatetraene
Non-bonding
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p-Electron Requirement for Aromaticity
notaromatic
aromatic notaromatic
4 p electrons 6 p electrons 8 p electrons
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Aromatic
Anti-aromatic(planar)
Non-aromatic(non-planar)
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Completely Conjugated Polyenes
aromatic
6 p electrons;completely conjugated
notaromatic
6 p electrons;not completely
conjugated
H H
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Question
• The planar compound shown below is classified as
• A) non-aromatic• B) aromatic• C) anti-aromatic• D) none of the above
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predicted to be aromatic by Hückel's rule,but too much angle strain when planar and all double bonds are cis (therefore non-planar)
10-sided regular polygon has angles of 144°
[10]Annulene
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incorporating two trans double bonds intothe ring relieves angle strain but introducesvan der Waals strain into the structure andcauses the ring to be distorted from planarity
[10]Annulene
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incorporating two trans double bonds intothe ring relieves angle strain but also introducesvan der Waals strain into the structure andcauses the ring to be non-planar
[10]Annulene
van der Waalsstrain between
these two hydrogens
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14 p electrons satisfies Hückel's rule
van der Waals strain between hydrogens insidethe ring & thererfore non-planar
[14]Annulene
H H
H H
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16 p electrons does not satisfy Hückel's rule
alternating short (134 pm) and long (146 pm) bonds
not aromatic
[16]Annulene
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18 p electrons satisfies Hückel's rule
resonance energy = - 418 kJ/mol
[18]Annulene
H HH
HHH
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Buckball (Fullerenes) can also be made into tubes.
Single, double, and multi-walled carbon nanotubes have many applications:
Conductive Plastics, Energy
Storage, Conductive Adhesives, Molecular Electronics, Thermal Materials, Fibres and Fabrics, Catalyst Supports, Biomedical Applications
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Aromatic Ions
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Heterocyclic Aromatic Compounds
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Aromatic Heterocyclic Compounds
A heterocycleis a cyclic compound in whichone or more of the ring atoms is an atom other than
carbon.
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Pyrole & Furan are Aromatic
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Pyridine Is Aromatic
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aromatic
Acid-Base Chemistry & Aromaticity
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Acid-Base Chemistry & Aromaticity
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Question• Which of the following compounds is best
classified as an aromatic heterocycle?
• A) B)
• C) Aniline D) Pyridine
• E) All of them
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Hetero-bicylic-aromatic compounds
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Quinine
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Examples of Important NitrogenHetero-bicyclic Aromatic Compounds
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Aromatic Compounds & Cancer
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Benzene:
• Benzene is classified as a Group A, human carcinogen by the EPA.
• Increased incidence of leukemia has been observed in humans occupationally exposed to benzene.
• Chronic inhalation has caused various blood disorders, including reduced red blood cell count and aplastic anemia.
• Reproductive effects have been reported for women exposed to high levels by inhalation.
• Adverse effects on the developing fetus have been observed in animal tests.
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Human Metabolism
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Benzene: Toxic Metabolites
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Benzene: Toxic Metabolites
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Benzene: Toxicity
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Trace amounts of naphthalene are produced by magnolias, deer, termites, and have been found in meteorites.
It has been used as a common household fumigant which was referred to as “moth balls”. Humans, particularly children have developed hemolytic anemia after ingesting moth balls
Napthalene is regarded to be a possible human carcinogenic and has caused cancer in animal studies.
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Benzo[a]pyrene: Carcinogenic Mechanism
DNA-benzopyrene adduct
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Exposure / Toxicityhttp://cebp.aacrjournals.org/content/14/8/2030.long
cigarette smoke, ramen noodles, coffee, grilled red meat, waterhttp://oehha.ca.gov/water/phg/pdf/091610Benzopyrene.pdf
Toxicity/ Carcinogenicity: California Report
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DNA Repair / Cell Health