Benzenoids

BENZENOIDS

Nomenclature and Isomerism of benzenoids

The nomenclature and isomerism of aromatic hydrocarbons has already been discussed in Unit 12. 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. A few examples of derivatives of benzene are given below:

Methylbenzene(Toluene) 1,2-Dimethylbenzene (o-Xylene)

 

1,3 Dimethylbenzene (m-Xylene)  1,4-Dimethylbenzene ( p-Xylene)

 

Structure of Benzene

  • Benzene is a highly unsaturated hydrocarbon with the molecular formula C6H6.
  • Its structure was initially unknown, but in 1865, August Kekulé proposed the cyclic structure of benzene, consisting of a ring of six carbon atoms with alternating single and double bonds.
  • This structure explained benzene's unique properties, such as its stability and the fact that it produces only one monosubstituted derivative.
  • Kekulé's structure also indicated the possibility of two isomeric 1,2-dibromobenzenes, but the observation of only one ortho disubstituted product presented a problem.
  • Kekulé suggested that the double bonds in benzene were not fixed but rather oscillated between the carbon atoms.
  • However, Kekulé's structure still failed to explain why benzene prefers substitution reactions over addition reactions, which could later be explained by resonance.

 

 

Resonance and stability of benzene

  • According to Valence Bond Theory, the oscillating double bonds in benzene are explained by resonance, where benzene is a hybrid of various resonating structures.
  • The two main contributing structures are the Kekulé structures A and B.

 

         A                    B                                  C                                                                                 

  • The hybrid structure is represented by inserting a circle or dotted circle in the hexagon.
  • All six carbon atoms in benzene are sp2 hybridized, forming six C—C sigma bonds and six C—H sigma bonds.

  • Each carbon atom has one unhybridised p orbital perpendicular to the plane of the ring.
  • The unhybridised p orbitals overlap to form a delocalised π bond by lateral overlap.
  • There are two equal possibilities of forming three π bonds by overlap of p orbitals.
  • X-ray diffraction data reveals that benzene is a planar molecule with equal C—C bond lengths of 139 pm.
  • The delocalised π electron cloud in benzene is more stable than the hypothetical cyclohexatriene, and this explains the unusual behaviour of benzene in not showing addition reactions under normal conditions.

 

Aromaticity

 Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene ring, possessing following characteristics.

(i) Planarity

(ii) Complete delocalisation of the π electrons in the ring

(iii) Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .).

This is often referred to as Hückel Rule.

Some examples of aromatic compounds are given below:

 

 

 

 

Preparation of Benzene

Cyclic polymerization of ethyne

Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene, which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds as discussed below:

Decarboxylation of aromatic acids

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

 

 

Reduction of phenol

Phenol is reduced to benzene by passing its vapours over heated zinc dust

Physical properties of benzenoids

Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. You are also familiar with naphthalene balls which 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.

 

Chemical properties of benzenoids

Electrophilic aromatic substitution reaction

The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E+)

Nitration of benzene

A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture).

 

 

Aromatic halogenation

Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes.

 

Aromatic 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).

 

Friedel-Crafts alkylation reaction

When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.

 

Friedel-Crafts acylation reaction

The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids (AlCl3) yields acyl benzene.

 

Mechanism of electrophilic substitution reaction

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

(a) Generation of the eletrophile

(b) Formation of carbocation intermediate

(c) Removal of proton from the carbocation intermediate

(a) Generation of electrophile E: During chlorination, alkylation and acylation of benzene, anhydrous AlCl3, being a Lewis acid helps in generation of the elctrophile Cl, R, RCO (acylium ion) respectively by combining with the attacking reagent.

In the case of nitration, the electrophile, nitronium ion,  is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner:

Step I

 

Step II

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) 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 sp3 hybridised.

sigma complex (arenium ion)

 

The arenium ion gets stabilised by resonance:

Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at sp3 hybridised carbon.

(c) Removal of proton: To restore the aromatic character, σ -complex releases proton from sp3 hybridised carbon on attack by [AlCl4] (in case of halogenation, alkylation and acylation) and [HSO4] (in case of nitration).

 

Addition reactions of benzene

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

                                                                                                 Cyclohexane 

Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C6H6Cl6 which is also called gammaxane.

 

Combustion of benzene

When heated in air, benzene burns with sooty flame producing CO2 and H2O

2O

 

Activating and deactivating group

ortho and para directing group

The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups. As an example, let us discuss the directive influence of phenolic
(–OH) group. Phenol is resonance hybrid of following structures:

It is clear from the above resonating structures that the electron density is more on o – and p – positions. Hence, the substitution takes place mainly at these positions. However, it may be noted that –I effect of – OH group also operates due to which the electron density on ortho and para positions of the benzene ring is slightly reduced. But the overall electron density increases at these positions of the ring due to resonance. Therefore, –OH group activates the benzene ring for the attack by an electrophile. Other examples of activating groups are –NH2, –NHR, –NHCOCH3, –OCH3, –CH3, –C2H5, etc.

In the case of aryl halides, halogens are moderately deactivating. Because of their strong – I effect, overall electron density on benzene ring decreases. It makes further substitution difficult. However, due to resonance the electron density on o– and p– positions is greater than that at the m-position. Hence, they are also o– and p– directing groups. Resonance structures of chlorobenzene are given below:

 

meta directing group

The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc.

Let us take the example of nitro group. Nitro group reduces the electron density in the benzene ring due to its strong–I effect. Nitrobenzene is a resonance hybrid of the following structures.