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Arenes and Aromaticity ( Benzene )

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Arenes and Aromaticity ( Benzene ). All C—C bond distances = 140 pm. 140 pm. 140 pm. 140 pm. 140 pm. 140 pm. 140 pm. Benzene empirical formula = CH. All C—C bond distances = 140 pm. 146 pm. 140 pm. 140 pm. - PowerPoint PPT Presentation
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Arenes and Aromaticity (Benzene)
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Page 1: Arenes and Aromaticity ( Benzene )

Arenes and Aromaticity(Benzene)

Page 2: Arenes and Aromaticity ( Benzene )

140 pm 140 pm

140 pm 140 pm

140 pm140 pm

All C—C bond distances = 140 pm

Benzeneempirical formula = CH

Page 3: Arenes and Aromaticity ( Benzene )

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.

Page 4: Arenes and Aromaticity ( Benzene )
Page 5: Arenes and Aromaticity ( Benzene )

Resonance & Benzene

Page 6: Arenes and Aromaticity ( Benzene )

heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene"

H°= – 208 kJ

Thermochemical Measures of Stability

+ 3H2

Pt

Page 7: Arenes and Aromaticity ( Benzene )

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

Page 8: Arenes and Aromaticity ( Benzene )
Page 9: Arenes and Aromaticity ( Benzene )

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

Page 10: Arenes and Aromaticity ( Benzene )

Circle-in-a-ring notation stands for resonance

description of benzene (hybrid of two resonance structures)

Resonance & Benzene

Page 11: Arenes and Aromaticity ( Benzene )

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

Page 12: Arenes and Aromaticity ( Benzene )

Question• Predict which of the following has the

smallest heat of combustion.

• A) B)

• C) D)

Page 13: Arenes and Aromaticity ( Benzene )

The p Molecular Orbitalsof Benzene

Page 14: Arenes and Aromaticity ( Benzene )

Orbital Hybridization Model of Bonding in Benzene

High electron density above and below plane of ring

Page 15: Arenes and Aromaticity ( Benzene )
Page 16: Arenes and Aromaticity ( Benzene )

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

Page 17: Arenes and Aromaticity ( Benzene )

Energy

Bondingorbitals

Antibondingorbitals

Benzene MOs

6 p AOs combine to give 6 p MOs3 MOs are bonding; 3 are

antibonding

Page 18: Arenes and Aromaticity ( Benzene )

Energy

Bondingorbitals

Antibondingorbitals

Benzene MOs

All bonding MOs are filledNo electrons in antibonding

orbitals

Page 19: Arenes and Aromaticity ( Benzene )

Benzene Molecular Orbitalsnodes

Page 20: Arenes and Aromaticity ( Benzene )
Page 21: Arenes and Aromaticity ( Benzene )

Hückel's Rule:Annulenes

the additional factor that influences aromaticity is the number of p electrons

Page 22: Arenes and Aromaticity ( Benzene )
Page 23: Arenes and Aromaticity ( Benzene )

Aromatic vs. “anti-aromatic”A. Deniz, et. al., Science, 5 November 1999

Page 24: Arenes and Aromaticity ( Benzene )

Question

• How many p electrons does the compound shown have?

• A) 5• B) 8• C) 10• D) 12

Page 25: Arenes and Aromaticity ( Benzene )

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

Page 26: Arenes and Aromaticity ( Benzene )

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

Page 27: Arenes and Aromaticity ( Benzene )

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

Page 28: Arenes and Aromaticity ( Benzene )

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

Page 29: Arenes and Aromaticity ( Benzene )

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

Page 30: Arenes and Aromaticity ( Benzene )

p-MOs of Benzene

Benzene

Antibonding

Bonding6 p orbitals give 6 p orbitals3 orbitals are bonding; 3 are antibonding

Non-bonding

Page 31: Arenes and Aromaticity ( Benzene )

Benzene

Antibonding

Bonding6 p electrons fill all of the bonding orbitalsall p antibonding orbitals are empty

p-MOs of Benzene

Non-bonding

Page 32: Arenes and Aromaticity ( Benzene )

Frost's Circle

p MOs of Benzene

Bonding

Antibonding

Page 33: Arenes and Aromaticity ( Benzene )

FROST CIRCLES

Page 34: Arenes and Aromaticity ( Benzene )

Question

• How many isomers of dibromophenol are aromatic?

• A) 3• B) 4• C) 6• D) 8• E) none

Page 35: Arenes and Aromaticity ( Benzene )

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

Page 36: Arenes and Aromaticity ( Benzene )

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

Page 37: Arenes and Aromaticity ( Benzene )

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

Page 38: Arenes and Aromaticity ( Benzene )

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

Page 39: Arenes and Aromaticity ( Benzene )

p-Electron Requirement for Aromaticity

notaromatic

aromatic notaromatic

4 p electrons 6 p electrons 8 p electrons

Page 40: Arenes and Aromaticity ( Benzene )

Aromatic

Anti-aromatic(planar)

Non-aromatic(non-planar)

Page 41: Arenes and Aromaticity ( Benzene )

Completely Conjugated Polyenes

aromatic

6 p electrons;completely conjugated

notaromatic

6 p electrons;not completely

conjugated

H H

Page 42: Arenes and Aromaticity ( Benzene )

Question

• The planar compound shown below is classified as

• A) non-aromatic• B) aromatic• C) anti-aromatic• D) none of the above

Page 43: Arenes and Aromaticity ( Benzene )

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

Page 44: Arenes and Aromaticity ( Benzene )

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

Page 45: Arenes and Aromaticity ( Benzene )

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

Page 46: Arenes and Aromaticity ( Benzene )

14 p electrons satisfies Hückel's rule

van der Waals strain between hydrogens insidethe ring & thererfore non-planar

[14]Annulene

H H

H H

Page 47: Arenes and Aromaticity ( Benzene )

16 p electrons does not satisfy Hückel's rule

alternating short (134 pm) and long (146 pm) bonds

not aromatic

[16]Annulene

Page 48: Arenes and Aromaticity ( Benzene )

18 p electrons satisfies Hückel's rule

resonance energy = - 418 kJ/mol

[18]Annulene

H HH

HHH

Page 49: Arenes and Aromaticity ( Benzene )
Page 50: Arenes and Aromaticity ( Benzene )

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

Page 51: Arenes and Aromaticity ( Benzene )

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.

Page 57: Arenes and Aromaticity ( Benzene )
Page 58: Arenes and Aromaticity ( Benzene )

Pyrole & Furan are Aromatic

Page 59: Arenes and Aromaticity ( Benzene )

Pyridine Is Aromatic

Page 60: Arenes and Aromaticity ( Benzene )

aromatic

Acid-Base Chemistry & Aromaticity

Page 61: Arenes and Aromaticity ( Benzene )

Acid-Base Chemistry & Aromaticity

Page 62: Arenes and Aromaticity ( Benzene )

Question• Which of the following compounds is best

classified as an aromatic heterocycle?

• A) B)

• C) Aniline D) Pyridine

• E) All of them

Page 63: Arenes and Aromaticity ( Benzene )

Hetero-bicylic-aromatic compounds

Page 64: Arenes and Aromaticity ( Benzene )
Page 65: Arenes and Aromaticity ( Benzene )

Quinine

Page 66: Arenes and Aromaticity ( Benzene )
Page 67: Arenes and Aromaticity ( Benzene )

Examples of Important NitrogenHetero-bicyclic Aromatic Compounds

Page 68: Arenes and Aromaticity ( Benzene )
Page 69: Arenes and Aromaticity ( Benzene )

Aromatic Compounds & Cancer

Page 70: Arenes and Aromaticity ( Benzene )

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.

Page 71: Arenes and Aromaticity ( Benzene )

Human Metabolism

Page 72: Arenes and Aromaticity ( Benzene )

Benzene: Toxic Metabolites

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Benzene: Toxic Metabolites

Page 74: Arenes and Aromaticity ( Benzene )

Benzene: Toxicity

Page 75: Arenes and Aromaticity ( Benzene )

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.

Page 76: Arenes and Aromaticity ( Benzene )
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Benzo[a]pyrene: Carcinogenic Mechanism

DNA-benzopyrene adduct

Page 78: Arenes and Aromaticity ( Benzene )

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

Page 79: Arenes and Aromaticity ( Benzene )

DNA Repair / Cell Health


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