-complex releases proton from sp hybridised carbon on attack by [AlCl ] – (in case of halogenation, alkylation and acylation) and [HSO ] – (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. Cyclohexane ( . ) Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C H Cl which is also called gammaxane.
Benzene hexachloride, (BHC) ( . ) Combustion: When heated in air, benzene burns with sooty flame producing CO and H O C H O 6CO +3H O → ( . ) General combustion reaction for any hydrocarbon may be given by the following chemical equation: C x H y + ( x + ) O → x CO + H O n ( . ) .
. Directive influence of a functional group in monosubstituted benzene When monosubstituted benzene is subjected to further substitution, three possible disubstituted products are not formed in equal amounts. Two types of behaviour are observed. Either ortho and para products or meta product is predominantly formed.
It has also been observed that this behaviour depends on the nature of the substituent already present in the benzene ring and not on the nature of the entering group. This is known as directive influence of substituents. Reasons for ortho / para or meta directive nature of groups are discussed below: Ortho and para directing groups: 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