Aromatic Hydrocarbons Introduction Kekule proposed the structure of

Aromatic Hydrocarbons Introduction Kekule proposed the structure of benzene Resonance Theory The Stability of Benzene The Criteria for Aromaticity—Hückel’s Rule Dr. Manal Fawzy Abou Taleb

Introduction Benzene (C 6 H 6) 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) Kekulé 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 • All C-C bonds are identical Kekulé structure cannot explain the behaviour of benzene • Experiments show that the Kekulé structure is not correct. • A correct description is given by resonance theory or by orbital models – valence bond or molecular orbital.

Resonance Theory 1. 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)

Stability of Benzene • 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. 9

Stability of Benzene • 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 Br 2 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. 10

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 The Resonance Explanation of the Structure 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 Stability of Benzene All carbon atoms in benzene are sp 2 -hybridized The side-way overlap of unhybridized 2 p 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 Why due to resonance delocalization of -electrons.

31. 3 The Stability of Benzene (SB p. 155) Structure of Methylbenzene • All C atoms in the ring is sp 2 -hybridized • The C atom in the methyl group is sp 3 -hybridized • The delocalized electron clouds give rise to extra stability

The Criteria for Aromaticity—Hückel’s Rule Four structural criteria must be satisfied for a compound to be aromatic. [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. 17

[2] A molecule must be planar. 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. 18

[3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. [4] A molecule must satisfy Hückel’s rule, and contain a particular number of electrons. 19

[4] A molecule must satisfy Hückel’s rule, and contain a particular number of electrons. Hückel's rule: Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons. 20

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 may be aromatic Ring with 8, 12 or 16 pi electrons will not be aromatic 21

For aromaticity, all pi (π)� electrons must be ( paired and all bonding orbitals filled Maximum and complete overlap is required for stabilization With unpaired pi (π )� electrons, overlap is not maximized The pi (π ( )�electrons in an aromatic compound are delocalized over the entire ring leading to stabilization

Summary • Aromatic: cyclic, planar, completely conjugated compound with 4 n + 2 π electrons • Anti-aromatic: cyclic, planar, completely conjugated compound with 4 n π electrons • Non-aromatic: a compound that lacks one or more of the following requirements: being cyclic, planar, or completely conjugated. 23

So what makes a molecule aromatic? It must be cyclic It must be conjugated It must be flat so that the p orbital overlap can occur It must also have 4 n + 2 pi electrons… 24

Examples of Aromatic Rings • Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4 n + 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. 25

• [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. 26

• 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. 27

ﺛﻴﻮﻓﻴﻦ Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 10 Øelectrons—four from the bonds and two from the lone pair. Ø 10= 4 n+2 Øn= 2 Øso it is aromatic ﺑﻴﺮﻭﻝ ﻓﻴﻮﺭﺍﻥ Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 6 Øelectrons—four from the bonds and two from the lone pair. Ø 6= 4 n+2 Øn= 1 Øso it is aromatic

Both negatively and positively charged ions can be aromatic if they possess all the necessary elements. Tropylium anion Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 8 Øelectrons—six from the bonds and two from the negative charge. Ø 8= 4 n+2 Øn= 1. 5 Øso it is not aromatic cyclopentadienyl anion Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 6 Øelectrons—four from the bonds and two from the negative charge. Ø 6= 4 n+2 Øn= 1 Øso it is aromatic

Tropylium cation Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 6 Øelectrons—six from the bonds and zero from the positive charge. Ø 6= 4 n+2 Øn= 1 Øso it is aromatic Øcyclic ØPlanar Øcompletely conjugated ØΠ electron = 2 Øelectrons— two from the bonds and zero from the positive charge. Ø 2= 4 n+2 Øn= 0 Øso it is aromatic

Structure Resonance theory of benzene All bonds are equivalent! electrons are delocalised around the ring

Aromaticity Example 1: Benzene cyclic planar conjugated 6 electrons

Aromaticity cyclooctatetraene is nonaromatic It is not planar © Prentice Hall 2001 Chapter 14 34

Classify the following molecules as aromatic, antiaromatic, or non-aromatic

The END