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 –NH , –NHR, –NHCOCH , –OCH , –CH , –C H , 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: In this case, the overall electron density on benzene ring decreases making further substitution difficult, therefore these groups are also called ‘ deactivating groups ’. The electron density on o – and p – position is comparatively less than that at meta position. Hence, the electrophile attacks on comparatively electron rich meta position resulting in meta substitution.
. Carcinogenicity and Toxicity Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property. Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco, coal and petroleum. They enter into human body and undergo various biochemical reactions and finally damage DNA and cause cancer.
Some of the carcinogenic hydrocarbons are given below (see box). 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 –NO , –CN, –CHO, –COR, –COOH, –COOR, –SO H, 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. SUMMARY Hydrocarbons are the compounds of carbon and hydrogen only. Hydrocarbons are mainly obtained from coal and petroleum, which are the major sources of energy .
Petrochemicals are the prominent