Topics to be covered

`=>` Aromatic hydrocarbon
`=>` Nomenclature and isomerism
`=>` Structure of benzene
`=>` Resonance and stability of benzene
`=>` Aromaticity
`=>` Preparation of benzene
`=>` Physical properties
`=>` Chemical properties


โ˜… These hydrocarbons are also known as `color{green}("โ€˜๐š๐ซ๐ž๐ง๐ž๐ฌโ€™.")`

โ˜… Since most of them possess pleasant odour (๐†๐ซ๐ž๐ž๐ค; ๐š๐ซ๐จ๐ฆ๐š ๐ฆ๐ž๐š๐ง๐ข๐ง๐  ๐ฉ๐ฅ๐ž๐š๐ฌ๐š๐ง๐ญ ๐ฌ๐ฆ๐ž๐ฅ๐ฅ๐ข๐ง๐ ), the class of compounds was named as `color{green}("โ€˜๐š๐ซ๐จ๐ฆ๐š๐ญ๐ข๐œ ๐œ๐จ๐ฆ๐ฉ๐จ๐ฎ๐ง๐๐ฌโ€™.")`

โ˜… Benzene ring is highly unsaturated but in a majority of reactions of aromatic compounds, the unsaturation of benzene ring is retained.

โ˜… Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non benzenoids.

โ˜… Some examples of arenes are given below:

Nomenclature and Isomerism:

โ˜… All six hydrogen atoms in benzene are equivalent; so it forms one and only one type of monosubstituted product.

โ˜… When two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups, three different position isomers are possible.

โ˜… The 1, 2 or 1, 6 is known as the ortho (oโ€“), the 1, 3 or 1, 5 as meta (mโ€“) and the 1, 4 as para (pโ€“) disubstituted compounds.

Structure of Benzene

โ˜… Benzene was isolated by Michael Faraday in 1825.

โ˜… The molecular formula of benzene, `color{red}(C_6H_6)`, indicates a high degree of unsaturation.

โ˜… Benzene was found to be a stable molecule and found to form a triozonide which indicates the presence of three double bonds.

โ˜… Benzene was further found to produce one and only one monosubstituted derivative which indicated that all the six carbon and six hydrogen atoms of benzene are identical.

โ˜… On the basis of this observation August Kekulรฉ in 1865 proposed the following structure for benzene having cyclic arrangement of six carbon atoms with alternate single and double bonds and one hydrogen atom attached to each carbon atom.

โ˜… The Kekulรฉ structure indicates the possibility of two isomeric 1, 2-dibromobenzenes. In one of the isomers, the bromine atoms are attached to the doubly bonded carbon atoms whereas in the other, they are attached to the singly bonded carbons.

โ˜… However, benzene was found to form only one ortho disubstituted product. This problem was overcome by Kekulรฉ by suggesting the concept of oscillating nature of double bonds in benzene as given below.

โ˜… Even with this modification, Kekulรฉ structure of benzene fails to explain unusual stability and preference to substitution reactions than addition reactions, which could later on be explained by resonance.

Resonance and stability of benzene

โ˜… Benzene is a hybrid of various resonating structures The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C) representing the delocalization of the six electrons between the six carbon atoms of the benzene ring.

โ˜… ๐„๐—๐๐‹๐€๐๐€๐“๐ˆ๐Ž๐ ๐Ž๐… ๐’๐“๐‘๐”๐‚๐“๐”๐‘๐„ ๐”๐’๐ˆ๐๐† ๐Ž๐‘๐๐ˆ๐“๐€๐‹ ๐Ž๐•๐„๐‘๐‹๐€๐ ๐‚๐Ž๐๐‚๐„๐๐“:

โ˜… All the six carbon atoms in benzene are `color{red}(sp^2)` hybridized.

โ˜… Two `color{red}(sp^2)` hybrid orbitals of each carbon atom overlap with `color{red}(sp^2)` hybrid orbitals of adjacent carbon atoms to form six `color{red}(Cโ€”C)` sigma bonds which are in the hexagonal plane.

โ˜… The remaining `color{red}(sp^2)` hybrid orbital of each carbon atom overlaps with `color{red}(s)` orbital of a hydrogen atom to form six `color{red}(Cโ€”H)` sigma bonds.

โ˜… Each carbon atom is now left with one unhybridised `p` orbital perpendicular to the plane of the ring as shown below:

โ˜… The unhybridised `color{red}(p)` orbital of carbon atoms are close enough to form a `ฯ€` bond by lateral overlap. There are two equal possibilities of forming three `color{red}(ฯ€)` bonds by overlap of `color{red}(p)` orbitals of `color{red}(C_1 โ€“C_2, C_3 โ€“ C_4, C_5 โ€“ C_6)` or `color{red}(C_2 โ€“ C_3, C_4 โ€“ C_5, C_6 โ€“ C_1)` respectively as shown in the following figures.

โ˜… Structures shown in Fig. 13.7(a) and (b) correspond to two Kekulรฉโ€™s structure with localised `color{red}(ฯ€)` bonds.

โ˜… X-ray diffraction technique is used for the determination of internuclear distance between all the carbon atoms in the ring and it was found to be the same; there is equal probability for the `color{red}(p)` orbital of each carbon atom to overlap with the `color{red}(p)` orbitals of adjacent carbon atoms [Fig. 13.7 (c)]. This can be represented in the form of two doughtnuts (rings) of electron clouds [Fig. 13.7 (d)], one above and one below the plane of the hexagonal ring as shown below:

โ˜… The six `color{red}(ฯ€)` electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 13.6 (a) or (b). The delocalised `color{red}(ฯ€)` electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised `color{red}(ฯ€)` electrons in benzene makes it more stable than the hypothetical cyclohexatriene.

โ˜… X-Ray diffraction data reveals that benzene is a planar molecule. However, X-ray data indicates that all the six `color{red}(Cโ€”C) ` bond lengths are of the same order (139 pm) which is intermediate between `color{red}(Cโ€” C)` single bond (154 pm) and `color{red}(Cโ€”C)` double bond (133 pm).

โ˜… Thus the absence of pure double bond in benzene accounts for the reluctance of benzene to show addition reactions under normal conditions, thus explaining the unusual behaviour of benzene.


For a compound to be aromatic it must possess the following characteristics:

(i) Planarity

(ii) Complete delocalisation of the `color{red}(pi)` electrons in the ring

(iii) Presence of `color{red}((4n + 2) ฯ€)` electrons in the ring where `color{red}(n)` is an integer (`color{red}(n = 0, 1, 2, . . ).`). This is often referred to as Hรผckel Rule.

Preparation of Benzene

`color{green}("๐‹๐š๐›๐จ๐ซ๐š๐ญ๐จ๐ซ๐ฒ ๐ฆ๐ž๐ญ๐ก๐จ๐๐ฌ :")`

(i) `color{green}("๐‚๐ฒ๐œ๐ฅ๐ข๐œ ๐ฉ๐จ๐ฅ๐ฒ๐ฆ๐ž๐ซ๐ข๐ฌ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐ž๐ญ๐ก๐ฒ๐ง๐ž:")` We have already discussed it.

(ii) `color{green}("๐ƒ๐ž๐œ๐š๐ซ๐›๐จ๐ฑ๐ฒ๐ฅ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐š๐ซ๐จ๐ฆ๐š๐ญ๐ข๐œ ๐š๐œ๐ข๐๐ฌ:")`

Sodium salt of benzoic acid on heating with sodalime gives benzene.

(iii) `color{green}("๐‘๐ž๐๐ฎ๐œ๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐ฉ๐ก๐ž๐ง๐จ๐ฅ:")` Phenol is reduced to benzene by passing its vapours over heated zinc dust.

Physical properties:

โ˜… Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. Eg. naphthalene balls are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property.

โ˜… Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.

โ˜… They burn with sooty flame.rbons are immiscible with water but are readily miscible with organic solvents.

โ˜… They burn with sooty flame.

Chemical properties

โ˜… Arenes are characterised by electrophilic substitution reactions. However, under special conditions they
can also undergo addition and oxidation reactions.

Electrophilic substitution reactions:

(i) `color{green}("๐๐ข๐ญ๐ซ๐š๐ญ๐ข๐จ๐ง:")` When benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture) a nitro group is introduced into benzene ring.

(ii) `color{green}("๐‡๐š๐ฅ๐จ๐ ๐ž๐ง๐š๐ญ๐ข๐จ๐ง:")` Arenes react with halogens in the presence of a Lewis acid like anhydrous `color{red}(FeCl_3, FeBr_3)` or `color{red}(AlCl_3)` to yield haloarenes.

(iii) `color{green}("๐’๐ฎ๐ฅ๐ฉ๐ก๐จ๐ง๐š๐ญ๐ข๐จ๐ง:")` The replacement of a hydrogen atom by a sulphonic acid group in a ring is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum).

(iv) `color{green}("๐…๐ซ๐ข๐ž๐๐ž๐ฅ-๐‚๐ซ๐š๐Ÿ๐ญ๐ฌ ๐š๐ฅ๐ค๐ฒ๐ฅ๐š๐ญ๐ข๐จ๐ง ๐ซ๐ž๐š๐œ๐ญ๐ข๐จ๐ง:")` When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.

(v) `color{green}("๐…๐ซ๐ข๐ž๐๐ž๐ฅ-๐‚๐ซ๐š๐Ÿ๐ญ๐ฌ ๐š๐œ๐ฒ๐ฅ๐š๐ญ๐ข๐จ๐ง ๐ซ๐ž๐š๐œ๐ญ๐ข๐จ๐ง:")` The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (`color{red}(AlCl_3)`) yields acyl benzene.

If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous `color{red}(AlCl_3)` in dark yields hexachlorobenzene (`color{red}(C_6Cl_6)`)

Mechanism of electrophilic substitution reactions

According to experimental evidences, SE (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps:

(๐š) `color{green}("๐†๐ž๐ง๐ž๐ซ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐ญ๐ก๐ž ๐ž๐ฅ๐ž๐ญ๐ซ๐จ๐ฉ๐ก๐ข๐ฅ๐ž")`

(๐›) `color{green}("๐…๐จ๐ซ๐ฆ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐œ๐š๐ซ๐›๐จ๐œ๐š๐ญ๐ข๐จ๐ง ๐ข๐ง๐ญ๐ž๐ซ๐ฆ๐ž๐๐ข๐š๐ญ๐ž")`

(๐œ) `color{green}("๐‘๐ž๐ฆ๐จ๐ฏ๐š๐ฅ ๐จ๐Ÿ ๐ฉ๐ซ๐จ๐ญ๐จ๐ง ๐Ÿ๐ซ๐จ๐ฆ ๐ญ๐ก๐ž ๐œ๐š๐ซ๐›๐จ๐œ๐š๐ญ๐ข๐จ๐ง ๐ˆ๐ง๐ญ๐ž๐ซ๐ฆ๐ž๐๐ข๐š๐ญ๐ž")`

(a) `color{green}("๐†๐ž๐ง๐ž๐ซ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐ž๐ฅ๐ž๐œ๐ญ๐ซ๐จ๐ฉ๐ก๐ข๐ฅ๐ž" E^(โŠ•))` : During chlorination, alkylation and acylation of benzene, anhydrous `color{red}(AlCl_3)`, being a Lewis acid helps in generation of the elctrophile `color{red}(Cl^(โŠ•), R^(โŠ•), RC^(โŠ•)O)` (acylium ion) respectively by combining with the attacking reagent.

In the case of nitration, the electrophile, nitronium ion, `color{red}(overset(+)(N)O_2)` is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner:
Step I

It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base.
Thus, it is a simple acid-base equilibrium.

(b) `color{green}("๐…๐จ๐ซ๐ฆ๐š๐ญ๐ข๐จ๐ง ๐จ๐Ÿ ๐‚๐š๐ซ๐›๐จ๐œ๐š๐ญ๐ข๐จ๐ง (๐š๐ซ๐ž๐ง๐ข๐ฎ๐ฆ ๐ข๐จ๐ง):")` Attack of electrophile results in the formation of ฯƒ-complex or arenium ion in which one of the carbon is `color{red}(sp^3)` hybridised.

The arenium ion gets stabilised by resonance:

Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at `sp^3` hybridised carbon.

(c) `color{green}("๐‘๐ž๐ฆ๐จ๐ฏ๐š๐ฅ ๐จ๐Ÿ ๐ฉ๐ซ๐จ๐ญ๐จ๐ง:")` To restore the aromatic character, `color{red}(ฯƒ )`-complex releases proton from `color{red}(sp^3)` hybridised carbon on attack by `color{red}([AlCl_4]^-)` (in case of halogenation, alkylation and acylation) and `color{red}([HSO_4]^โ€“)` (in case of nitration).

Addition reactions:

Under vigorous conditions, i.e., at high temperature and/ or pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane.

Under utra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, `color{red}(C_6H_6Cl_6)` which is also called gammaxane.


When heated in air, benzene burns with sooty flame producing `color{red}(CO_2)` and `color{red}(H_2O)`

`color{red}(C_6H_6 + 15/2 O_2 โ†’ 6CO_2 + 3H_2O) ` .............(13.82)

General combustion reaction for any hydrocarbon may be given by the following chemical equation:

`color{red}(C_x+H_y + ( x+ y/4) O_2 โ†’ xCO_2 + y/2 H_2O )` ......................(13.83)