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