aromatic hydrocarbons introduction kekule proposed the structure of benzene resonance theory the...
TRANSCRIPT
Aromatic Hydrocarbons
IntroductionIntroduction
Kekule proposed the structure of benzeneKekule proposed the structure of benzene
Resonance TheoryResonance Theory
The Stability of BenzeneThe Stability of Benzene
The Criteria for Aromaticity—Hückel’s RuleThe Criteria for Aromaticity—Hückel’s Rule
Dr. Manal Fawzy Abou TalebDr. Manal Fawzy Abou Taleb
Introduction
Benzene
Benzene (C6H6) is the simplest aromatic hydrocarbon
• Highly unsaturated
• Six-membered ring compound with alternative single and double bonds between adjacent carbon atoms
• Chemically unreactive compared to alkenes
In 1865, Kekule proposed the structure of benzene:
31.3 The Stability of Benzene (SB p.151)
KekuléKekulé suggested that benzene was suggested that benzene was......
Six-membered ring compound with alternative single and double bonds between adjacent carbon atoms
6 carbon ring with a hydrogen bonded to each carbon It is planar. one electron from each carbon is free to participate in a
double bond
31.3 The Stability of Benzene (SB p.151)
According to the Kekulé structure, there should be two different 1,2-dibromobenzenes:
Only one 1,2-dibromobenzene has been found!!
The Stability of Benzene
According to the Kekulé structure, benzene should
• undergo addition reactions readily
• it gave substitution reaction products rather than addition reaction products
Kekulé structure cannot explain the behaviour of benzene
•Experiments show that the Kekulé structure is not correct.
• All C-C bonds are identical
•A correct description is given by resonance theory or by orbital models – valence bond or molecular orbital.
Resonance Theory1.Resonance forms are imaginary
The resonance description of benzene consists of two equivalent Lewis structures, each with three double bonds that alternate with three single bonds.
benzene has a single hybrid structure which combines the characteristics of both resonance forms
Resonance forms
Hybrid structure
Molecular Orbital
* electron cloud delocalized all over the ring
* the resonance picture this helps to explain lack of reactivity of benzene
* great stability (substitution not addition )
Benzene - Resonance Energy
one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene.
heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated)
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• Consider the heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene and benzene, all of which give cyclohexane when treated with excess hydrogen in the presence of a metal catalyst.
Stability of Benzene
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• The low heat of hydrogenation of benzene means that benzene is especially stable—even more so than conjugated polyenes. This unusual stability is characteristic of aromatic compounds.
• Benzene’s unusual behavior is not limited to hydrogenation. Benzene does not undergo addition reactions typical of other highly unsaturated compounds, including conjugated dienes.
• Benzene does not react with Br2 to yield an addition product. Instead, in the presence of a Lewis acid, bromine substitutes for a hydrogen atom, yielding a product that retains the benzene ring.
Stability of Benzene
The Stability of Benzene
• Benzene is more stable than Kekulé structure
• The energy difference for the stabilization of benzene is called resonance energy of benzene
The Stability of Benzene
From X-ray crystallography,
In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å).
The Resonance Explanation of the Structure of Benzene
The Resonance Explanation of the Structure of Benzene
The Stability of Benzene
All carbon atoms in benzene are sp2-hybridized
The side-way overlap of unhybridized 2p orbitals on both sides gives a delocalized electron cloud above and below the plane of the ring
31.3 The Stability of Benzene (SB p.154)
The delocalization of electrons gives benzene extra
stability and determines the chemical properties of
benzene
The Stability of Benzene
Structural formula of benzene:
The circle represents the six electrons that are delocalized
about the six carbon atoms of the benzene ring
Molecules for which you can write resonance structures have an greater stability due to the electron delocalization.
Aromaticity: cyclic conjugated organic compounds such as benzene, exhibit special stability .Explain Whydue to resonance delocalization of -electrons.
31.3 The Stability of Benzene (SB p.155)
• All C atoms in the ring is sp2-hybridized
• The C atom in the methyl group is sp3-hybridized
• The delocalized electron clouds give rise to extra stability
Structure of MethylbenzeneStructure of Methylbenzene
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Four structural criteria must be satisfied for a compound to be aromatic.
The Criteria for Aromaticity—Hückel’s Rule
[1] A molecule must be cyclic.
To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.
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All adjacent p orbitals must be aligned so that the electron density can be delocalized.
Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes.
[2] A molecule must be planar.
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Aromatic compounds must have a p orbital on every atom.
[3] A molecule must be completely conjugated.
[4] A molecule must satisfy Hückel’s rule, and contain
a particular number of electrons.
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Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons.
Hückel's rule:
[4] A molecule must satisfy Hückel’s rule, and contain
a particular number of electrons.
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Note that Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring.
Ring with 2, 6, 10 or 14 pi electrons Ring with 2, 6, 10 or 14 pi electrons maymay be be aromaticaromatic Ring with 8, 12 or 16 pi electrons Ring with 8, 12 or 16 pi electrons will notwill not be aromatic be aromatic
For aromaticityFor aromaticity, all pi (, all pi (π) electrons must be electrons must be paired and all bonding orbitals filledpaired and all bonding orbitals filled
Maximum and complete overlap is required for Maximum and complete overlap is required for stabilizationstabilization
With unpaired pi (With unpaired pi (π ) electrons, overlap is not electrons, overlap is not maximizedmaximized
The pi (The pi (π ) electrons in an aromatic compound electrons in an aromatic compound are delocalized over the entire ring leading to are delocalized over the entire ring leading to
stabilizationstabilization
• Aromatic: cyclic, planar, completely conjugated compound with 4n + 2 π electrons
• Anti-aromatic: cyclic, planar, completely conjugated compound with 4n π electrons
• Non-aromatic: a compound that lacks one or more of the following requirements: being cyclic, planar, or completely conjugated.
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Summary
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It must be cyclicIt must be cyclic It must be conjugatedIt must be conjugated It must be flat so that the It must be flat so that the pp orbital overlap can occur orbital overlap can occur It must also have It must also have
4n + 2 pi electrons…4n + 2 pi electrons…
So what makes a molecule aromatic?
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Examples of Aromatic Rings
• Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 electrons.
• Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes.
• To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene.
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• [10]-Annulene has 10 electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain.
• Since [10]-annulene is not planar, the 10 electrons can’t delocalize over the entire ring and it is not aromatic.
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• Two or more six-membered rings with alternating double and single bonds can be fused together to form polycyclic aromatic hydrocarbons (PAHs).
• There are two different ways to join three rings together, forming anthracene and phenanthrene.
• Heterocycles containing oxygen, nitrogen or sulfur, can also be aromatic.
• With heteroatoms, we must determine whether the lone pair is localized on the heteroatom or part of the delocalized system.
• An example : pyridine.
ON
H
S
بيروفيورانل
ثيوفين
cyclicPlanarcompletely conjugatedΠ electron = 6electrons—four from the bonds and two from the lone pair. 6= 4n+2n= 1so it is aromatic
N
H
cyclicPlanarcompletely conjugatedΠ electron = 10electrons—four from the bonds and two from the lone pair. 10= 4n+2n= 2so it is aromatic
Both negatively and positively charged ions can be aromatic if they possess all the necessary elements.
cyclopentadienyl anion
cyclicPlanarcompletely conjugatedΠ electron = 6electrons—four from the bonds and two from the negative charge. 6= 4n+2n= 1so it is aromatic
Tropylium anioncyclicPlanarcompletely conjugatedΠ electron = 8electrons—six from the bonds and two from the negative charge. 8= 4n+2n= 1.5so it is not aromatic
cyclicPlanarcompletely conjugatedΠ electron = 2electrons— two from the bonds and zero from the positive charge. 2= 4n+2n= 0so it is aromatic
Tropylium cation
cyclicPlanarcompletely conjugatedΠ electron = 6electrons—six from the bonds and zero from the positive charge. 6= 4n+2n= 1so it is aromatic
Structure
Resonance theory of benzeneResonance theory of benzene All bonds are equivalent!
electrons are delocalised around the ring
AromaticityExample 1: BenzeneExample 1: Benzene
cyclic
planar
conjugated
6 electrons
© Prentice Hall 2001© Prentice Hall 2001 Chapter 14Chapter 14 3434
AromaticityAromaticity
cyclooctatetraene cyclooctatetraene is is nonnonaromaticaromatic
It is It is notnot planar planar
Classify the following molecules as aromatic, anti-aromatic, or non-aromatic
O
The END