Chemistry AROMATIC HYDROCARBONS

### 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

### AROMATIC HYDROCARBON

★ These hydrocarbons are also known as color{green}("‘𝐚𝐫𝐞𝐧𝐞𝐬’.")

★ Since most of them possess pleasant odour (𝐆𝐫𝐞𝐞𝐤; 𝐚𝐫𝐨𝐦𝐚 𝐦𝐞𝐚𝐧𝐢𝐧𝐠 𝐩𝐥𝐞𝐚𝐬𝐚𝐧𝐭 𝐬𝐦𝐞𝐥𝐥𝐢𝐧𝐠), the class of compounds was named as color{green}("‘𝐚𝐫𝐨𝐦𝐚𝐭𝐢𝐜 𝐜𝐨𝐦𝐩𝐨𝐮𝐧𝐝𝐬’.")

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

★ Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non benzenoids.

★ Some examples of arenes are given below:

### Nomenclature and Isomerism:

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

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

★ The 1, 2 or 1, 6 is known as the ortho (o–), the 1, 3 or 1, 5 as meta (m–) and the 1, 4 as para (p–) disubstituted compounds.

### Structure of Benzene

★ Benzene was isolated by Michael Faraday in 1825.

★ The molecular formula of benzene, color{red}(C_6H_6), indicates a high degree of unsaturation.

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

★ 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.

★ On the basis of this observation August Kekulé 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.

★ The Kekulé 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.

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

★ Even with this modification, Kekulé 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

★ 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.

★ 𝐄𝐗𝐏𝐋𝐀𝐍𝐀𝐓𝐈𝐎𝐍 𝐎𝐅 𝐒𝐓𝐑𝐔𝐂𝐓𝐔𝐑𝐄 𝐔𝐒𝐈𝐍𝐆 𝐎𝐑𝐁𝐈𝐓𝐀𝐋 𝐎𝐕𝐄𝐑𝐋𝐀𝐏 𝐂𝐎𝐍𝐂𝐄𝐏𝐓:

★ All the six carbon atoms in benzene are color{red}(sp^2) hybridized.

★ 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}(C—C) sigma bonds which are in the hexagonal plane.

★ 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}(C—H) sigma bonds.

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

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

★ Structures shown in Fig. 13.7(a) and (b) correspond to two Kekulé’s structure with localised color{red}(π) bonds.

★ 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:

★ The six color{red}(π) 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}(π) 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}(π) electrons in benzene makes it more stable than the hypothetical cyclohexatriene.

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

★ 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.

### Aromaticity:

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) π) electrons in the ring where color{red}(n) is an integer (color{red}(n = 0, 1, 2, . . ).). This is often referred to as Hückel Rule.

### Preparation of Benzene

color{green}("𝐋𝐚𝐛𝐨𝐫𝐚𝐭𝐨𝐫𝐲 𝐦𝐞𝐭𝐡𝐨𝐝𝐬 :")

(i) color{green}("𝐂𝐲𝐜𝐥𝐢𝐜 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐢𝐬𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐞𝐭𝐡𝐲𝐧𝐞:") We have already discussed it.

(ii) color{green}("𝐃𝐞𝐜𝐚𝐫𝐛𝐨𝐱𝐲𝐥𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐚𝐫𝐨𝐦𝐚𝐭𝐢𝐜 𝐚𝐜𝐢𝐝𝐬:")

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

(iii) color{green}("𝐑𝐞𝐝𝐮𝐜𝐭𝐢𝐨𝐧 𝐨𝐟 𝐩𝐡𝐞𝐧𝐨𝐥:") Phenol is reduced to benzene by passing its vapours over heated zinc dust.

### Physical properties:

★ 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.

★ Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.

★ They burn with sooty flame.rbons are immiscible with water but are readily miscible with organic solvents.

★ They burn with sooty flame.

### Chemical properties

★ 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}("𝐍𝐢𝐭𝐫𝐚𝐭𝐢𝐨𝐧:") 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}("𝐇𝐚𝐥𝐨𝐠𝐞𝐧𝐚𝐭𝐢𝐨𝐧:") 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}("𝐒𝐮𝐥𝐩𝐡𝐨𝐧𝐚𝐭𝐢𝐨𝐧:") 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}("𝐅𝐫𝐢𝐞𝐝𝐞𝐥-𝐂𝐫𝐚𝐟𝐭𝐬 𝐚𝐥𝐤𝐲𝐥𝐚𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:") When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.

(v) color{green}("𝐅𝐫𝐢𝐞𝐝𝐞𝐥-𝐂𝐫𝐚𝐟𝐭𝐬 𝐚𝐜𝐲𝐥𝐚𝐭𝐢𝐨𝐧 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧:") 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:

(𝐚) color{green}("𝐆𝐞𝐧𝐞𝐫𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐭𝐡𝐞 𝐞𝐥𝐞𝐭𝐫𝐨𝐩𝐡𝐢𝐥𝐞")

(𝐛) color{green}("𝐅𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐜𝐚𝐫𝐛𝐨𝐜𝐚𝐭𝐢𝐨𝐧 𝐢𝐧𝐭𝐞𝐫𝐦𝐞𝐝𝐢𝐚𝐭𝐞")

(𝐜) color{green}("𝐑𝐞𝐦𝐨𝐯𝐚𝐥 𝐨𝐟 𝐩𝐫𝐨𝐭𝐨𝐧 𝐟𝐫𝐨𝐦 𝐭𝐡𝐞 𝐜𝐚𝐫𝐛𝐨𝐜𝐚𝐭𝐢𝐨𝐧 𝐈𝐧𝐭𝐞𝐫𝐦𝐞𝐝𝐢𝐚𝐭𝐞")

(a) color{green}("𝐆𝐞𝐧𝐞𝐫𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐩𝐡𝐢𝐥𝐞" E^(⊕)) : 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^(⊕), R^(⊕), RC^(⊕)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}("𝐅𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧 𝐨𝐟 𝐂𝐚𝐫𝐛𝐨𝐜𝐚𝐭𝐢𝐨𝐧 (𝐚𝐫𝐞𝐧𝐢𝐮𝐦 𝐢𝐨𝐧):") Attack of electrophile results in the formation of σ-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}("𝐑𝐞𝐦𝐨𝐯𝐚𝐥 𝐨𝐟 𝐩𝐫𝐨𝐭𝐨𝐧:") To restore the aromatic character, color{red}(σ )-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]^–) (in case of nitration).

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.

### Combustion

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 → 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 → xCO_2 + y/2 H_2O ) ......................(13.83)