Aromatic (benzene ans its) compounds

These include benzene and compounds that resemble benzene in chemical behavior.

Benzene

Chemical formula: C₆H₆.

Structure

Benzene is a cyclic compound with delocalized conjugated double bonds, i.e. the double bond is free to shift from one place to another through the whole molecule.

Effect of delocalization of double bond into the structure of benzene.

(i) The benzene ring is very stable that it is preserved in most chemical reactions, that is, it’s is generally unreactive towards electrophilic reagents.

(ii) The six carbon-carbon bonds in the benzene are equal in length (i.e. 1.39Å) intermediate between the carbon-carbon double bond (1.34 Å) and carbon-carbon single bond (1.54 Å)

Physical properties

1. It is a colorless with a characteristic aromatic smell.
2. It is insoluble in water but soluble in organic solvents and its self is a good solvent.
3. It burns with a smoky and luminous flame

Industrial preparation

Benzene is obtained by catalytic dehydration of hexane (from petroleum)

Chemical properties

Benzene is not easily attacked by electrophilic reagents. In its reactions, benzene undergoes mostly electrophilic substitution reaction in an electrophile substitutes a hydrogen atom, rather than addition reactions in which the benzene ring system would be destroyed.

Reactions

1.Combustion

Benzene reacts with oxygen to form carbon dioxide and water only.

2C₆H₆ + 15O₂    →     12CO₂ + 6H₂O

2. Catalytic hydrogenation

Benzene undergoes catalytic hydrogenation to cyclohexane

The heats of hydrogenation and combustion of benzene are lower (by 36kcal/mole) than expected.

In other words, benzene is more stable by 30kCal than we would expect cyclohexatriene to be.

The 36kCal is the benzene stabilization energy or resonance energy.

3.Aromatic electrophilic substitution reaction

Here an electrophile substitutes a hydrogen atom on a benzene ring.

General mechanism

(a) Nitration

This is the introduction of the nitro group on the benzene ring using a hot mixture of concentrated sulphuric acid and concentrated nitric acid.

The formation of a yellow liquid of nitrobenzene when benzene is reacted with a mixture of concentrated nitric acid and concentrated sulphuric acid distinguishes benzene from cyclohexane,

Mechanism

Uses

Approximately 95% of nitrobenzene is consumed in the production of aniline,  which is a precursor to rubber chemicals, pesticides, dyes (particularly azo dyes), explosives, and pharmaceuticals.

Specialized applications

Nitrobenzene is also used to mask unpleasant odors in shoe and floor polishes, leather dressings, paint solvents, and other materials. Redistilled, as oil of mirbane, nitrobenzene has been used as an inexpensive perfume for soaps. A significant merchant market for nitrobenzene is its use in the production of the analgesic paracetamol (also known as acetaminophen) (Mannsville 1991). Nitrobenzene is also used in Kerr cells, as it has an unusually large Kerr constant.

(b) Sulphonation

This is the introduction of the sulphonic group (-SO₃H) on to the benzene ring. It’s done by heating benzene with concentrated suphuric acid. A process for the sulphonation of benzene comprises passing benzene vapor into sulphuric acid, the initial temperature of which is between about 95 and about 110⁰C.

Benzene sulphonic acid is a colorless crystalline compound that is soluble in water.

Uses

Benzensulfonic acid is commonly used as the active ingredient in laundry detergent used in clothes washing machines.

A variety of pharmaceutical drugs are prepared as benzenesulfonate salts and are known as besilates (INN) or besylates(USAN).

In a diluted form, it is also used as a polymer remover stripping agent.

Benzenesulfonic acid and related compounds undergo desulfonation when heated in water near 200 °C. The temperature of desulfonation correlates with the ease of the sulfonation:

C₆H₅SO₃H + H₂O → C₆H₆ + H₂SO₄

(c) Halogenation

This requires the use of electron carriers (AlCl₃, FCl₃) as catalysts. The electron carrier is usually a halide of iron or aluminium, the electron carriers’ function is to polarize the halogen molecule by withdrawing the electrons from the bond between the two halogen atoms.

Mechanism

 Chlorobenzene was historically important in the manufacture of chlorinated pesticides, especially DDT, and in the production of phenol and aniline. Monochlorobenzene’s principal current use is as a chemical intermediate in the production of chemicals such as nitrochlorobenzenes and diphenyl oxide. These chemicals are subsequently used in the production of herbicides, dyestuffs, and rubber chemicals. Additionally, monochlorobenzene is used as a solvent in degreasing processes (e.g., in metal cleaning operations), paints, adhesives,waxes and polishes.

(d) Friedel –Craft alkylation

This is a reaction of benzene with an alkyl halide to give an alkyl benzene

The majority of toluene/methylbenzene is used as a component of petrol.

It is also used in paints, lacquers, inks, adhesives, rubber, and cleaning agents. It is used to manufacture benzene, urethane raw materials, and other organic chemicals. It is used in the production of pharmaceuticals, dyes, and cosmetic nail products.

(e) Friedel –Craft acylation

This is the introduction of an acylo group (-OCR) to the benzene ring. The reaction is carried out by reacting benzene with an acid halide in the presence of a halogen carrier.

(f) Reaction with alkene

In the presence of an acid benzene react with alkenes to form alkylbenzene.

MECHANISM

4. Other reactions

(a) Halogenation in the presence of u.v-light

Benzene reacts with chlorine by addition reaction to form saturated product;

1,2,3,4,5,6-hexachlorohexane

Derivatives of benzene

Nomenclature of derivatives of benzene with more than one substituent.

Number the carbon atoms on the ring and include the position(s) of the groups in the name.

Examples

The prefixes; ortho, meta, and para- usually abbreviated as o-, m-, and p- may be used for describing the relative positions of substituents in di-substituted benzene.

Example

Effects of substituent group on the reactivity of the ring

A group attached to the benzene ring may have either of the two effects:

(i)Groups that donate electrons to the benzene ring make the ring more reactive towards electrophilic substitution reaction

(ii) Groups that donate electrons to the benzene ring like alkyl group that those with at least one lone pair of the electron (-NH₂, -OH) direct the incoming groups to position 2 or position 4. For this reason, they are described as 2, 4-directing groups

Example

e.g. sulphonation of methylbenzene.

(iii) Groups that withdraw electron from benzene rings such as nitro or carboxylic groups, make the ring less reactive towards electrophilic reagent.  They direct the incoming group to position 3 and are called meta directing groups.

Example

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