Addition Reactions of Benzene:

We have already studied in chapter 5 that the chlorination of an alkene provides 1, 2-dichloroalkane.

In contrast to this, the addition of chlorine to benzene occurs through numerous difficulties and produces numerous isomers of 1, 2, 3, 4, 5, 6-hexachlorocyclohexanes. When treated through chlorine or bromine in the existence of sunlight, benzene forms the hex halides, C₆H₆Cl₆ and C₆H₆Br₆, correspondingly. The addition reaction continues via the free radical mechanism.

The 1,2,3,4,5,6- hexachlorocyclohexane, theoretically can exist in eight stereoisomeric forms but only seven of these are known. One of the isomers, identified as Gammexane, is an influential insecticide. It is extremely constant and acts more rapidly than D.D.T. All of the isomers have given to exist in the chair form.

Reduction:

Hydrogenation of benzene at higher temperature and under pressure yields cyclohexane.

Even though benzene isn't diminished through metal and acid, or via sodium in ethanol, it is decreased via sodium in liquid ammonia in the occurrence of ethanol (Birch reduction) to give 1, 4-dihydrobenzene (cyclohexa-l, 4-diene). This reaction has as well been given to have a free radical mechanism.

Lithium in anhydrous ethylamine, though, decreases benzene to cyclohexene and cyclohexane.

Reactions of Side-chain:

Substitution in the Side-chain

Alkylbenzene visibly proposes for attack via halogen: the ring and the side chain. We can

Manage the position of attack through choosing the proper reaction situations. Halogenation of alkanes entails condition under that halogen atoms are shaped through homolyses of halogen molecules, which is, elevated temperature or light. Halogenation of benzene, on the other hand, includes shift of positive halogen that is promoted via Lewis acid catalyst like ferric chloride.

The position of attack in methylbenzene (toluene) would be decided by the nature of the attacking particle and by the condition employed. If the reaction is carried out in the presence of light, substitution occurs almost exclusively in the side chain. In the absence of light and in the presence of ferric chloride, substitution occurs mostly in the ring. For instance;

Chlorination of methylbenzene (toluene), in the presence of light, occurs via free radical chain mechanism as shown below:

In alkyl benzenes through side chains larger than methyl, it is supposed that the free radical substitution may take place on any of the side chain carbon atoms; so we must think the likelihood of gaining a mixture of isomers. For instance, chlorination of ethyl benzene should provide 2 isomeric products; 1-chloro-1-phenylethane is the major (91%) product,

We can ask why it is so. This is since the bond dissociation energy of benzylic C- H.

                 CH₃

                  |

Bond, C₆H₅CH - H, (355 kJ mol) is less than β -phenyl ethyl C - H bond, C₆H₅ - CH₂CH₂ - H (435 K j mol^-1). That means, less energy is required for the homolylic fission of benzylic C-H bond. In other terms, benzyl radical is more constant. The superior stability of benxyl radical is, delocalization of the odd electron over the ring as given below:

Because the benzylic radical formed is more stable, 1-chloro-1-phenylethane is the major product.

The Oxidation of Side-Chain

Even though benzene and alkanes are fairly unreactive towards the common oxidizing agents (KMnO₄, K₂Cr₂O₇ etc), the benzene ring renders an aliphatic side chain quite susceptible to oxidation. The side chain, irrespective of its length, is oxidized to a carboxyl group (-COOH). Tertiary alkyl reserved aromatic compounds don't follow this reaction. For example, toluene, propylbenzene, (1-methylethyl) benzene are oxidized to benzoic acid in higher yields. P-Methyltoluene on oxidation provides terephthalic (benzene-1, 4-dicarboxylic) acid but tertiary butyl benzene is not affected.

The numeral and the location of the carboxylic groups generated signify the number and position of alkyl chain connected to the aromatic ring.

This reaction is helpful for 2 reasons:

• synthesis of carboxylic acids
• identification of alkyl benzenes

Polynuclear hydrocarbons

Polynuclear hydrocarbon is an assembly of more than one benzene ring in the molecule. Depending upon the mode of attachment of various rings, the polynuclear aromatic hydrocarbons may be classified into two broad classes: (i) isolated benzenoid hydrocarbon, and (ii) condensed or fused benzenoid hydrocarbons.

I. Isolated benzenoid hydrocarbon: In isolated systems, two or more rings are connected to each other either directly or through carbon chain. Several general instances are biphenyl, diphenylmethane and triphenylmethane.

II. Condensed or Fused Benzenoid Hydrocarbon: The condensed or fused benzenoid hydrocarbons are those in which two or more benzene rings are fused together at the ortho position in such a way that each pair of rings shares two carbons. They contain compounds as naphthalene, anthracene; phenanthrene etc.

The condensed polynuclear is via far the larger and the more significant group. A large number of them have been found to possess carcinogenic (cancer producing) activity. In this unit we will discuss the chemistry of naphthalene only.

Naphthalene

Naphthalene is the parent compound of polynuclear hydrocarbons. Naphthalene, m.p. 355 K, is a colourless volatile crystalline solid.

Naphthalene has three resonance hybrid structures. The bonds aren't all of similar length, but are close to the benzene value of 139.7 p.m. How can we clarify the aromaticity of naphthalene?

The above structure of naphthalene shows that it has 10 π electrons and 10 is the Huckel number which indicates that naphthalene should be an aromatic molecule.

Like benzene, naphthalene undergoes usual electrophilic substitution reactions. It as well undergoes oxidation or reduction more readily than benzene. Several significant reactions of naphthalene are following in Table

Table:  Electrophilic substitution reactions of naphthalene

Electrophilic Substitution of Naphthalene: Polynuclear hydrocarbons are more reactive towards electrophilic attack than benzene. Naphthalene undergoes a number of usual electrophilic substitution reactions, such as nitration, halogenation, sulphonation, Friedel-Crafts alkylation, Friedel-Crafts acylation, etc.

The method for naphthalene substitution reaction is similar to that of benzene substitution.

The 1^st substituent goes to 1-position (α -position); that denotes, the 1-position is more reactive than the 2-position (β -position). We can ask why it is so. To comprehend this, let us examine the resonance structures of the 2 intermediate carbocations effecting from the particular attacks.  

In both cases, the positive charge can be allocated to 5 different positions, but such carbocations aren't the same in energy. In the 1^st case, the first two structures have their benzene ring intact and are consequently more stable than the continuing three structures. In the 2^nd case, only one resonance structure has a benzenoid ring intact.

The resulting resonance hybrid has higher energy in the 2^nd case than in the first case. The middle carbocation in the 1^st case is more stabilized via resonance, and its conversion state is of lower energy. For this cause, the intermediate in the 1^st case is shaped faster and 1-position is more reactive.

Oxidation and Reduction of Naphthalene: Under managed conditions, naphthalene is oxidized to 1, 4-naphthoquinone, but the yield is generally low.

More vigorous oxidation results in the loss of one ring and yields phthatic anhydride. This reaction probably continues through the formation of ortho-phthatic acid.

Unlike benzene, naphthalene can be partially hydrogenated with no heat and pressure, or that can be decreased through sodium and ethanol.

For entire hydrogenation of naphthalene, it needs heat and pressure just as in the case of benzene.

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