aromatic compounds ichemistry3.chemistry.sc.chula.ac.th/images/download/bsac/coursemat/...
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2302272 – Org Chem II – Part I
Lecture 1
Aromatic Compounds I
Instructor: Dr. Tanatorn Khotavivattana
E-mail: [email protected]
Recommended Textbook:
Chapter 16 in Organic Chemistry, 8th Edition, L. G. Wade, Jr., 2010,
Prentice Hall (Pearson Education)
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Course Outline
Quiz : from 9:00(or earlier) – 9:30
Open book, discussion allowed
10 marks out of 100 marks total
Homework : Hand in at the beginning of next class
A4 paper
5 marks out of 100 marks total
Final Exam : 35 marks out of 100 marks total
Short note allowed
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Organic Chemistry2
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Organic Chemistry3
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Organic Chemistry4
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Discovery of Benzene
• Isolated in 1825 by Michael Faraday
who determined C:H ratio to be 1:1
• Synthesized in 1834 by Eilhard
Mitscherlich who determined molecular
formula to be C6H6. He named it
benzin
• Other related compounds with low C:H
ratios had a pleasant smell, so they
were classified as aromatic
C6H6
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Kekulé Structure
• Proposed in 1866 by Friedrich Kekulé,
shortly after multiple bonds were
suggested
• Failed to explain existence of only one
isomer of 1,2-dichlorobenzene
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Resonance Structures of Benzene
• Benzene is actually a resonance hybrid between the two
Kekulé structures
• The C—C bond lengths in benzene are shorter than typical
single-bond lengths, yet longer than typical double-bond
lengths (bond order 1.5)
• Benzene's resonance can be represented by drawing a circle
inside the six-membered ring as a combined representation
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Structure of Benzene
• Each sp2 hybridized C in the ring has an unhybridized p
orbital perpendicular to the ring which overlaps around the
ring.
• The six pi electrons are delocalized over the six carbons.
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• Benzene is actually much more stable than we would
expect; For example, an alkene decolorizes potassium
permanganate by reacting to form a glycol. When
permanganate is added to benzene, however, no reaction
occurs
Unusual Reactivity of Benzene9
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Unusual Reactivity of Benzene
• When bromine adds to benzene, a catalyst such as FeBr3 is
needed.
• The reaction that occurs is the substitution of a hydrogen by
bromine (cf. addition of bromine across alkenes)
• Addition of Br2 to the double bond is not observed.
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Resonance Energy
• Predicted heat of hydrogenation of -359 kJ/mol; observed
value = -208 kJ/mol, a difference of 151 kJ (resonance
energy)
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Annulenes
• Hydrocarbons with alternating single and double bonds
• All annulenes were proposed to bearomatic (?)
• However!! cyclobutadiene is soreactive that it dimerizes before itcan be isolated
• Cyclooctatetraene adds Br2 readilyto the double bonds
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Aromaticity
• Aromatic structures are more stable
than their open-chain counterparts
• Antiaromatic structures are less stable than
their open-chain counterparts (delocalization
of pi electrons increases the electronic
energy!)
• A cyclic compound that does not have a continuous, overlapping ring of p orbitals
cannot be aromatic or antiaromatic. It is said to be nonaromatic, or aliphatic. Its
electronic energy is similar to that of its open-chain counterpart
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Aromaticity
Criteria:
• The molecule must be cyclic
• This cycle must be fully conjugated
• The cycle must be planar
• The electrons must be able to “circulate”
Hückel’s Rule: If the number of pi electrons in the cyclic system is:
• (4N + 2) = the system is aromatic
• (4N) = the system is antiaromatic
Cyclooctatetraene would be antiaromatic if
Hückel’s rule applied (4N; N = 2). Cyclooctatetraene
adopts a nonplanar “tub” conformation that avoids
most of the overlap between adjacent pi bonds;
becomes nonaromatic instead!
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Aromaticity – Molecular orbital
• Benzene:
The polygon ruleA
ntiaro
matic
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Aromaticity – Examples
Aromatic Compounds Antiaromatic Compounds
Antiaromatic (if planar)
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Deprotonation of Cyclopentadiene
• Cyclopentadiene is acidic because deprotonation will convert it to an aromatic
ion
• By deprotonating the sp3 carbon of cyclopentadiene, the electrons in the p orbitals
can be delocalized over all five carbon atoms and the compound would be
aromatic
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Cyclopentadienyl Cation
• Huckel’s rule predicts that the cyclopentadienyl cation, with four pi electrons, is
antiaromatic; therefore, the cyclopentadienyl cation is not easily formed
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Tropylium Ion
Cyclooctatetraene Dianion
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Which of the following is an aromatic compound?
Non-aromatic Aromatic
There is an sp3 carbon in
the ring, delocalization will
not be complete.
All carbons are sp2
hybridized and it obeys
Huckel’s rule.
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Problem #121
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Problem #222
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Polynuclear Aromatic Hydrocarbons (PAHs)
• Composed of two or more fused benzene rings. Fused rings share two carbon
atoms and the bond between them.
• Naphthalene is the simplest fused aromatic hydrocarbon.
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Polynuclear Aromatic Hydrocarbons (PAHs)
• As the number of fused aromatic rings increases, the resonance energy per
ring continues to decrease and the compounds become more reactive.
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Larger Polynuclear Aromatic Hydrocarbons
• Formed in combustion (tobacco smoke).
• Many are carcinogenic.
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Graphite
• Planar layered structure.
• Layer of fused benzene rings, bonds: 1.415 Å.
• Only van der Waals forces between layers.
• Conducts electrical current parallel to layers.
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Some New Allotropes
• Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure.
• Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings.
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Classification28
Organic
Compounds
Acyclic
(Open chain)
Cyclic
(Closed chain)
Carbocyclic Heterocyclic
Alicyclic Aromatic Non-aromatic Aromatic
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Applications of Heterocyclic Compounds
• Many synthetic (as well as natural) heterocyclic compounds are of extreme value
as medicinals, agrochemicals, plastics precursors, dyes, photographic
chemicals, and so on, and new structures are constantly being sought in
research in these areas
• Heterocyclic compounds can be synthesized in many ways
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Applications of Heterocyclic Compounds
Medicinal chemistry especially is associated intimately with heterocyclic
compounds; most of all chemicals used in medicine are based on heterocyclic
frameworks
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Pyridine
• Pyridine has six delocalized electrons in its pi system.
• The two non-bonding electrons on nitrogen are in an sp2 orbital, andthey do not interact with the pi electrons of the ring.
Heterocyclic Aromatic Compounds
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Pyridine
• Pyridine is basic, with a pair non-bonding electrons available to abstract a proton.
• The protonated pyridine (the pyridinium ion) is still aromatic.
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Pyrrole
• Pyrrole is a much weaker base than pyridine
• This difference is due to the structure of the protonated pyrrole
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Basic or Nonbasic?
Pyrimidine has two basic
nitrogens.
Imidazole has one basic
nitrogen and one nonbasic.
Only one of purine’s nitrogens
is not basic.N
N
N
N
H
N N H
NN
Not basic
Not basic
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Other Heterocyclics35
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Problem #336
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Common Names of Benzene Derivatives
Nomenclature
The following compounds are usually called by their historical common names,
and almost never by the systematic IUPAC names:
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Nomenclature - Disubstituted Benzenes
• named using the prefixes ortho-, meta-, and para- to specify the substitution
patterns (abbreviated o-, m-, and p-).
• Numbers can also be used for the IUPAC name.
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Nomenclature - Three or More Substituents
• Numbers are used to indicate their positions.
• Assign the numbers to give the lowest possible numbers to the substituents.
• The carbon atom bearing the functional group that defines the base name
(as in phenol or benzoic acid) is assumed to be C1.
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Nomenclature – Benzene as Substituents
• When the benzene ring is named as a substituent on another molecule, it is
called a phenyl group. (often abbreviated Ph)
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Nomenclature – Benzene as Substituents
• When the benzene ring is named as a substituent on another molecule, it is
called a phenyl group. (often abbreviated Ph)
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Problem #442
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Physical Properties of Aromatic Compounds
• Melting points: More symmetrical than corresponding alkane,pack better into crystals, so higher melting points.
MP = - 95 oC
BP = 69 oC
MP = 7 oC
BP = 81 oC
• Density: More dense than nonaromatics, less dense than water.
• Solubility: Generally insoluble in water.
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Physical Properties of Aromatic Compounds
• Boiling points: Intermolecular force• H-bonding (functional groups on aromatic)
• Dipole-dipole (dipole moment)
• London (molecular weight)
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Physical Properties of Aromatic Compounds45
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Homework46
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