We are familiar with the aliphatic hydrocarbons-alkanes, alkenes and alkynes as different homologous series. There are a number of homologous series of the aromatic hydrocarbons or arenes. The simplest, based on benzene, C6H6, and having one ring of six carbon atoms encompass a general formula CnH2n-6 (n > 6). The aromatic compounds were originally so named as most of them were fragrant and the term aromatic is derived from the Greek term aroma, meaning pleasant smell. The structure of benzene is symbolized simply as a conjugated (that is, alternating single and double bonds) system of three double bonds making a hexagonal ring with the carbon atoms.
Fig: Benzene ring
The benzene ring as illustrated above is not strictly on alternating single and double bonds in cyclic form however an acceptable and suitable manner of drawing the structure of benzene.
The other members of the benzene homologous series are made by successive substitution of hydrogen atoms by-CH3 groups.
Structure and Bonding in Benzene:
Benzene is generally drawn as a six-membered carbocyclic ring having alternating single and double carbon-to-carbon bonds. Each and every carbon is linked to single hydrogen. Now a question arises 'Is benzene simply a cyclic alkene?' On the account of structure, the C = C must behave like those found in the alkenes, however benzene doesn't behave like alkenes that is, it doesn't give addition products across the C = C like alkenes.
The studies have illustrated that benzene is a regular planar hexagon, having the carbon-carbon bond length of 1.39 nm. This bond length is shorter than the C-C bond length of alkanes (1.54 nm) and longer than C = C of alkanes (1.34 nm). For benzene, though, the carbon-to-carbon bond for all the six carbon atoms in the ring is 1.39 nm; an intermediate value among the single and double carbon-to-carbon bond. How can this observation be described? The solution lies in the theory of a phenomenon termed as resonance.
Resonance in Benzene:
To describe the unusual observation in respect of the benzene structure, we have to remember that we have illustrated bonding only in terms of electron pairs related with two nuclei. These we might term as localized electron that is, shared bonding electrons situated between the two bonded atoms. The fact, though, is that bonding electrons can be related with more than two nuclei, and there is a measure of stability to be gained by this as the degree of bonding increases if the electrons can distribute themselves over a greater volume. This effect is termed as an electron delocalization or resonance.
In this topic, we are going to see how this concept can be employed to describe the benzene ring structure that is, all the carbon-to-carbon bonds in the ring are usual. We are familiar that carbon consists of four electrons available for bonding. Let us look at a typical six carbon ring system similar to
Fig: Resonance in Benzene
that proposed for benzene however with only the single bonds illustrated. Concentrate your attention on the asterisk carbon atom. It consists of four valence electrons, one is employed for bonding with hydrogen atom and one used each to bond with one carbon to the right and one to the left, making a sum of three electrons utilized for bonding and one electron unused. This statement is true for all the six carbon atoms in the ring, providing a sum of six unused electrons. Instead of localizing any of these six electrons, as pairs of electrons among the two adjacent carbon atoms, to form three bonds generally illustrated as the three double bonds (π bonds) in the benzene; the six electrons are delocalized (that is, resonance) between all the six carbon atoms. In another words, the six unused electrons are pooled altogether as electron cluster from which any of the six carbon atoms can pool from.
Fig: Benzene molecule
The structure above provides a better picture of the benzene molecule however the earlier shown is still acceptable once it is clear that the bonds are not in reality alternating single and double bonds. The true structure of benzene is a resonating hybrid of different resonance structure in which there is movement of electrons, most of the time delocalized however at times localized in a dynamic equilibrium.
This suggestion is in conformity with the fact that all the carbon-to-carbon bonds in benzene are of similar length, and intermediate between the C-C single bond of alkanes and the C double bond of alkenes. This concept of resonance confers stability on the benzene, therefore any attempt to change (example: addition reaction), this arrangement can lead to instability and it is in general resisted.
It is the lack of simple C - C and C = C bonds that make the properties of benzene different from those of the alkanes and alkenes, and the delocalization in the benzene molecule are the cause of aromatic character.
Thus, benzene although unsaturated like alkenes and alkynes doesn't decolorize bromine water and aqueous potassium tetraoxomanganate (VII) as it doesn't experience substitution reaction.
Chemical Properties of Benzene:
The stability of the aromatic nucleus in benzene due to resonance signifies that it doesn't undergo similar reactions such as alkenes; even though it is as well unsaturated. Benzene reacts mainly by, substitution in which the hydrogen atoms are substituted with other groups while still retaining its aromatic stabilization. In certain reactions, though, this aromatic stabilization is lost and benzene experiences addition reactions like catalytic hydrogenation.
Substitution reactions of benzene:
The mixture of concentrated trioxonitrate (V) HNO3 and tetraoxosulphate (VI), H2SO4; acids reacts by benzene, at 60°C, to form nitrobenzene.
Benzene reacts with bromine or chlorine in the presence of a reagent known as Lewis acid (FeBr3 or FeCl3) to form bromo-or chloro-benzene. Light should be excluded from the reaction mixture.
Benzene reacts on heating by concentrated tetraoxosulphate (VI) acid, H2SO4, to form, benzenesulphonic acid.
Benzene and halogenoalkanes example: chloromethane react, in the presence of the Lewis acids, AlCl3, as catalyst, to form alkyl benzenes example - methylbenzene. The reaction is termed as the Friedel-Crafts reaction.
Addition reactions of benzene:
Whenever a mixture of hydrogen gas and benzene vapor is passed over finely-divided nickel at 150°C, the benzene is reduced to cyclohexane.
Chlorine and bromine add on to benzene in the presence of sunlight or ultra-violet light producing, for illustration 1,2,3,4,5,6,-hexachlorocyclohexane.
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