Aromatic Hydrocarbons Notes

Aromatic Hydrocarbons Notes Introduction Greek word aroma means fragrance and so the term "aromatic compounds" was originall...

Aromatic Hydrocarbons Notes

Introduction

Greek word aroma means fragrance and so the term "aromatic compounds" was originally applied to various fragrant organic compounds.Chemical analysis of most of the natural fragrant organic compounds has shown that they are made up of one or more benzene rings containing six carbon atoms and carbon content in them is greater than the corresponding aliphatic compounds. Moreover, these aromatic compounds can be converted into benzene and benzene derivatives by chemical reactions.   

Similarly, different aromatic compounds can be synthesised from benzene. Thus, benzene is called the parent hydrocarbon of aromatic compounds and all aromatic compounds are considered as benzene derivatives. Hence, it may simply be said that the aromatic compounds are actually benzene and benzene derivatives. Aromatic compounds containing benzene ring are also called benzenoids. 

Later on, some polycyclic compounds like naphthalene, anthracene, etc. and some heterocyclic compounds like pyrrole, furan, pyridine etc. are also considered as aromatic compounds.These aromatic compounds are called non-benzencids. Therefore, the presence of benzene ring in an aromatic compound is not essential. 

All aomatic compounds do not necessarily possess sweet smell. Many aromatic compounds are odourless and many of them have bad odour. There is also a large number of non-aromatic compounds having characteristic sweet smell. Hence, all sweet smelling organic compounds are aromatic compounds and all aromatic compounds must possess sweet smell-this concept is baseless. some 3, 4, 5 or 7 membered cyclic polyenes (cations, anions or neutral compounds) also included in aromatic group.

ARENES
Aromatic hydrocarbons containing one or more benzene rings are called arenes
Example of arenes containing only one benzene ring is- Benzene, Toluene, Ethylbenzene, xylene etc


If arenes contain more than one benzene then the rings may remain fused and those are called as fused Benzene Aromatic compound.

  • naphthalene is fused bicyclic arenes.
  • Anthracene and Phenanthrene is fused tricyclic arenes.

Aromatic hydrocarbons containing fused rings are also called polynuclear aromatic hydrocarbons. Their general formula is CnH2n-6m  ; 
where n= number of carbon atom and m= number of rings. 
For the bicyclic arene naphthalene n =10 and m = 2. 
For tricyclic arenes such as anthracen and phenanthrene, n = 14 and m = 3.

Structure of Benzene Molecule

Benzene has been known since 1825 when it was first isolated by Michel Faraday. Form elemental analysis and molecular mass determination, it was found that the molecular formula of benzene is C6H6 and had eight hydrogen atoms less than the corresponding saturated hydrocarbon (n-Hexane C6H14). Therefore it is expected to be highly unsaturated compound.

[1] Benzene undergoes addition reaction only under drastic condition 
  • Hydrogenation with Raney Ni/ H2
  • Halogenation in presence of Sunlight.
  • ozonolysis reaction
these three reaction suggest that benzene is unsaturated compound containing 3 double bonds.
[2] Benzene does not react with cold alkaline KMnO4 Solution and halogen acids.Therefote, the type of unsaturation present in benzene is quite different from that of aliphatic unsaturation.

In 1865 Friedrich August Kekule proposed a ring structure for benzene(1) . However, many alternative structures have been proposed from time to time by different workers(ll - IV).
Then main objection against Kekule structure was that it should yield two ortho disubstituted products when it reacts with bromine. However, experimentally benzene was found to yield only one product.

Kekule removed this objection by proposing that the double bonds in benzene are continuously oscillating back and forth between two adjacent positions. Since positions of double bonds are not fixed, only one product is formed. This structure came to be known as Kekule’s dynamic formula, which formed the basis for the present electronic structure of benzene.

Stability of Benzene (Resonance)

Stability of Benzene (Resonance)


The High Stability of Benzene
Benzene resists addition whereas it readily undergoes substitution reactions, like nitration, halogenation etc. This indicates that benzene is more stable than the hypothetical cyclohexatriene molecule.

Explanation of the abnormal behavlour of benzene by resonance
[1] The three double bonds of benzene are not active in forming addition compounds like the olefinic double bonds because their participation in addition reaction causes loss of resonance stability of benzene. But, in substitution reaction there is no net effect on the formation of resonance hybrid. So, benzene tends to participate in substitution reaction in preference to addition reaction.

[2] Because of hybrid structure, all the carbon-carbon bonds in benzene are equivalent. So, there is no difference between two apparently different disubstituted ortho -isomers. Two H- atoms of benzene, on being replaced by same or different atoms or groups, can form three isomers (ortho, meta & para).

[3] Since benzene exists as a resonance hybrid, there is no real existence of carbon-carbon double bonds (C=C) or carbon-carbon single bonds (C-C) in the molecule. All the carbon- carbon bonds are equivalent. It has been observed experimentally that in benzene molecule all the carbon-carbon bonds are equal in length and its value (1.39A) is intermediate between carbon-carbon single bond length (1.54 A) and carbon-carbon double bond length (1.34 A).

Orbital Structure of benzene:
sigma bond bond orbitals bond length stability benzene skeleton of benzene
rings of pi electron cloud above and below the carbon skeleton of benzene molecules



Representation of benzene molecule:


The hexagonal structure of benzene is known as benzene ring. Benzene ring is generally represented by any one of the two Kekule' structures. Benzene ring is also represented by drawing a circle inside a regular hexagon. In representing benzene molecule, C and H-atoms are not generally written.
Representation of benzene molecule


Aromaticity in Benzene and Related Systems

After the structure of benzene was established, the term aromatic was adapted for such compounds which despite having p bonds (unsaturation) resist addition and instead undergo substitution. The aromaticity in benzene is attributed to the six delocalized pi electrons in the coplanar carbon hexagon. When a bonding orbital is not restricted to two atoms but is spread over more than two atoms, e.g. six in benzene, such bonding orbitals are said to be delocalized. Delocalisation results in greater stability.

  1. Delocalization: Complete delocalization of p electron cloud of the ring system is a necessary requirement for aromatic character.
  2. Planarity: Complete delocalization of p-electron cloud is possible only if the ring is planar. This is the reason that benzene is aromatic but cyclooctatetraene is not, since the latter is not a planar molecule.

Huckel's rule for aromaticity or (4n-+2) rule:

According to German scientist Huckel, planar conjugated polyene systems (cation, anion or neutral species) containing (4n + 2) delocalised pi-electrons (n = 0, 1, 2, 3,..) exhibit aromatic property. Therefore, monocyclic conjugated polyene systems containing 2(n = 0),6(n= 1), 10 (n = 2), 14 (n = 3), . etc. delocalised pi-electrons possess aromatic character and they are unusually stable.
Huckel's rule for aromaticity


Classification of Aromatic Compounds:

  1. Benzenoid aromatic compounds
  2. Non Benzenoid aromatic compounds
Benzenoid  and non Benzenoid aromatic compounds

Antiaromatic Compounds:

Definition of antiaromatic compounds. Compounds that contain 4n (n not equal to 0) pi-electrons in a cyclic planar, or nearly planar, system of alternating single and double bonds.

Non Aromatic Compounds:

A cyclic compound which doesn't necessitate a continuous form of overlapping ring of p orbitals need not be considered as aromatic or even antiaromatic and hence these are termed as nonaromatic or aliphatic.

Nomenclature of Benzene Derivatives

Physical Properties of Benzene

Chemical Reaction of Benzene

Major reactions of benzene ring are electrophilic substitution reactions.

Substitution reaction of Aromatic Compounds:

Like aliphatic compounds, aromatic compounds undergo three types of substitution reactions:

[1] Electrophilic substitution reactions :Inreactions, the benzene ring is attacked by electrophilic reagents or electrophiles in the first step.
  • Nitration of Benzene
  • Halogenation of Benzene
  • Friedel Craft alkylation
  • Friedel Craft acylation
  • Sulphonation of Benzene
[2] Nucleophilic substitution reactions : In reactions, the benzene ring is attacked by nucleophilicreagents or nucleophiles in the first step.
[3] Free radical substitution reactions: This type of reactions involve the attack of free radicals on the benzene ring.

Reactions of Side Chains and Nucleus of Aromatic compounds

Halogenation of Toluene:
For side chain halogenation, case of abstraction of hydrogen atom is as follows:
the stability order of free radicals is
Bromine is more selective than chlorine.

Oxidation of Toluene:
Benzene ring is usually very resistant to oxidation, hence the side chain is always attacked. Whatever the length of side chain the ultimate oxidation product is benzoic acid.

When two side chains are present, it is possible to oxidize them at same time.

But, if the C attached to benzene ring does not have any hydrogen then it will not give benzoic acid.

When an electron withdrawing group (-I and / or –R) is present, the ring is stable and the result of oxidation is a substituted benzoic acid.

If –OH or  is present, the ring is very sensitive to oxidation and is largely broken down, whatever be the nature of oxidizing agent. Ring rupture can be prevented by protection of the group.

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Advanced Chemistry: Aromatic Hydrocarbons Notes
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