Aromatic Hydrocarbons

The term aromatic (Greek; aroma means fragrance) was first used for compounds having pleasant odour although the structure was not known. Now the term aromatic is used for a class of com

pounds having a characteristic stability despite having unsaturation. These may have one or more benzene rings (benzenoid) or may not have benzene ring 

(non-benzenoid). Benzenoid compounds include benzene and its derivatives having aliphatic side chains (arenes) or polynuclear hydrocarbons, eg. naphthalene, anthracene, biphenyl etc.

• Structure of Benzene

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 indicating high unsaturation. However, benzene does undergo addition reactions in contrast to unsaturated hydrocarbons, although it mainly undergoes substitution reactions.

In 1865 Friedrich August Kekule proposed a ring structure for benzene (I). However, many alternative structures have been proposed from time to time by different workers. (II-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.

Refer to the video for structure of benzene

• Stability of Benzene (Resonance)

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. This has been proved by the fact that the enthalpies of hydrogenation and combustion of benzene are lower than expected. Enthalpy of hydrogenation is the change in enthalpy when one mole of an unsaturated compound is hydrogenated. It has been found experimentally that enthalpy of hydrogenation for disubstituted alkenes, R-CH=CH-R varies between 117-125 kJ mol^-1. Accordingly, the values for cyclohexene and cyclohexa-1, 3-diene and hypothetical cyclohexa-1, 3, 5-triene were calculated compared with their experimental values.

While the experimental values of enthalpy of hydrogenation for cyclohexene is similar to the value expected, the variation in the case of cyclohexa-1, 3-diene is small and is due to delocalization. The expected value of enthalpy of hydrogenation of benzene is much higher than the corresponding calculated value for hypothetical cyclohexa-1, 3, 5-triene indicating that benzene does not have this type of structure.

X-ray studies show that it is planar molecule and that all six C-C bonds in benzene are of equal length (139 pm), intermediate between C-C single bond (154 pm) and C=C (134 pm). All six carbons are sp² hybridized and all bond angles are 120^o. Benzene is a hybrid of various resonating structures, the two Kekule structures A and B, being the main contributing forms.

• 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.

The modern theory of aromaticity was advanced by Eric Huckel 1931. Aromaticity is a function of electronic structure. Any polynuclear compound, heterocyclic rings or cyclic ions may be aromatic if these have a specific electronic structure. The important features of the theory are

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 or (4n + 2)p electron rule: 

The rule states that in a conjugated, planar, cyclic system if the number of delocalized p-electrons is (4n + 2) where n is an integer, i.e., 1, 2, 3 etc. Benzene, naphthalene, anthracene and phenanthrene are aromatic as they contain (4n + 2)p electrons i.e. 6, 10, 14, p electrons in a conjugated cyclic system. The cyclopentadiene and cyclooctatetraene are non-aromatic as instead of (4n + 2)p e^-these have 4n p e^-. Moreover, they are non-planar.

Benzene was first isolated by Faraday from cylinders of compressed illuminating gas obtained from the pyrolysis of whale oil.

Benzene was first synthesised by Berthelot by passing acetylene through a red hot tube.

In the laboratory it was first prepared by heating benzoic acid or phthalic acid with calcium oxide.

Refer to the video for Huckel’s rule 

• Properties of Benzene

1. It is a colourless liquid with boiling point of       

2. It is inflammable, burning with a smoky flame due to high carbon content.

3. It is slowly attacked by acid permanganate to form 
   

4. 
5.     

• Reaction of Benzene

Major reactions of benzene ring are electrophilic substitution reactions.

1.  Nitration of Benzene: 

(i) Formation of electrophile 

(base) (acid)     (nitronium ion)

(ii) Electrophilic attack by 

In the intermediate, attacked carbon atom changes its state of hybridization from trigonal to tetrahedral.

(iii) Loss of proton :

2. Friedel – Craft Alkylation:

This reaction involves the introduction of alkyl group into benzene ring in presence of catalyst. The alkylating agent may be R – X, ROH or alkene. The catalyst is a Lewis acid,  

Mechanism:

Step I:

Formation of electrophile (carbonium ion) stability order of carbonium ion is

with RX     

With ROH:

With alkene:

Step II:

Step III:

Examples:

 or carbonium ion.

This is an example of intermolecular Friedel Craft alkylation. But if the side chain contains four or five carbons with halogen at the end then it may undergo intramolecular Friedel – Craft reaction.

3. Friedel – Craft Acylation:

Replacement of  of benzene by  group using carboxylic acid, esters, acid chloride or acid anhydride as acylating agent in the presence of Lewis acid.

Mechanism:

(i)     

        (II) is more stable than (I) because each atom has a complete octet.

Examples:

• Reactions of Side Chains

1. Halogenation:

Within presence of undergoes electrophilic substitution at o- and p- position and in the absence of  and in the presence of light or high temperature undergoes substitution in the side chain (free radical substitution).

• Toluene

1. 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.

2. 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.

• Alkenyl Bemzene

Alkenyl benzenes contain a double bond in the side chain of benzene ring e.g.     

These compounds are prepared as follows:

The reactivity of the double bond is greater than the benzene towards electrophilic reagents. Thus mild conditions are required for the addition reaction across the double bond as compared to those required for the benzene ring.

First step is the formation of carbocation or free radical.

The higher stability of benzyl cation or radical relative to the conjugated alkenyl benzene makes the latter more reactive than simple alkenes.

In the presence of peroxides, the reaction taking place is (peroxide effect) as follows.

Look here for Organic Chemistry Revision Notes, Chemistry Syllabus and Best books of Organic Chemistry.

To read more, Buy study materials of Hydrocarbons comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.