BENZENE
Arenes are hydrocarbons with alternating single and double bonds between carbon atoms forming rings. They are classified based on the number of fused rings that is present in the molecule. They have the general molecular formula CnH2n-6m, where n is the number of carbon atoms and m is the number of rings. Arene may be monocyclic (contain only one ring) examples include benzene and toluene, or they may be polycyclic (contain more than one ring) examples include anthracene and naphthalene.
Benzene is the simplest arenes.
BENZENE
Benzene is a colorless, sweet-smelling chemical compound that is characterized by its aromatic properties. It is represented by the chemical formula C6H6, meaning it consists of six carbon atoms and six hydrogen atoms arranged in a ring structure.
Aromatic hydrocarbons are planer compounds which usually have one or more rings of six carbon atoms, which usually have alternating single and double bonds. The value and the strength of these single and double bonds are identical. To explain these alternating bonds benzene is drawn as a resonance structure showing the alternating bonds.
Structures of benzene
These resonance structures of benzene also known as Kekule’ structures were proposed by a German chemist in1865 know as August Kekule. He proposed that 1. The six carbon atoms in benzene are arranged in a planar hexagonal ring with alternating double and single bonds around the ring.
2. That the pi electrons from the double bonds are delocalized across the entire ring and not localized between individual carbon-carbon bonds.
These resonance structures can be drawn as two hexagons with alternating single and double bonds or as a single hexagon with a circle drawn in the center representing the delocalized pi- electrons.
Benzene Derivatives These are compounds that look like benzene which have benzene as the parent or main compound with one or more of the hydrogens substituted by other groups. Some examples of these compounds include.
I. Methylbenzene (toluene) CH3
ii. Phenol
iii. Xylene
iv. Nitrobenzene Phenylamine (Aniline)
v. Benzoic acid
Physical Properties of benzene
1. Benzene is a colourless liquid
2. has a sweet smell.
.3. It is insoluble in water.
4. It has a boiling point of 800C
Chemical properties
1. Addition reactions:
i. Addition reactions:
(i) Benzene combines with hydrogen to yield cyclohexane at 1800C and presence of Ni catalyst
C6H6 + 3H2 C6H12 + 3H2
Benzene Cyclohexane
(ii). Halogenation: Benzene reacts with the halogens to produce cyclic compounds in presence of UV light
C6H6 + 3Cl2 UV light C6H6Cl6
2. Substitution reaction: - benzene undergoes substitution reaction with the halogens to e.g Cl2, Br2, I2 to yield halogenated products. The presence of single bonds in benzene is responsible for its substitution reactions
i. Halogenation
C6H6 + Br2 C6H5Br + HBr
ii. In the absence of sunlight using aluminum chloride as catalyst benzene react with chlorine to form chlorobenzene
2. Benzene reacts to form methylbenzene when combined with monochloromethane, the reaction is catalyzed by aluminium chloride
ii. Nitration: This occurs in the mixture of HNO3 and H2SO4 together with benzene.
C6H6 + HNO3 C6H5NO2
iii. Sulphonation:- Benzene reacts with conc. H2SO4 to form benzene sulphonic acid.
iv. Alkylation: - These are reactions in which benzene reacts with halo-alkanes in the presence of AlCl3.
Uses of benzene
1. Benzene and its derivatives like methylbenzene are used as additives to improve the quality of petrol.
2. It is used for the manufacture of some drugs like aspirin.
3. It is used for manufacture of explosives like 2,4,6-trinitromethyl-benzene (TNT)
4. It is used for the manufacture of some dyes.
5. It is used to produce phenylethene (styrene) a monomer used to produce polystyrene.
6. It is used in the preparation of detergents.
7. Preparation of insecticides
NOTE: Benzene is a known carcinogen and should be handled with great care. Avoid prolonged exposure to benzene