Chemistry Directive influence of a functional group group in monosubstituted benzene, Carcinogenity and toxicity

### Topics to be covered

=> Directive influence of a functional group group in monosubstituted benzene
=> Carcinogenity and toxicity

### Directive influence of a functional group in monosubstituted benzene:

★ Depending on the nature of substituent present in benzene ring either ortho and para products or meta product are predominantly formed. This is known as directive influence of substituents.

★ color{green}("𝐎𝐫𝐭𝐡𝐨 𝐚𝐧𝐝 𝐩𝐚𝐫𝐚 𝐝𝐢𝐫𝐞𝐜𝐭𝐢𝐧𝐠 𝐠𝐫𝐨𝐮𝐩𝐬:") The groups which direct the incoming group to ortho and para positions are called color{green}("𝐨𝐫𝐭𝐡𝐨 𝐚𝐧𝐝 𝐩𝐚𝐫𝐚 𝐝𝐢𝐫𝐞𝐜𝐭𝐢𝐧𝐠 𝐠𝐫𝐨𝐮𝐩𝐬") .Let us discuss the directive influence of phenolic color{red}((–OH)) group. Phenol is resonance hybrid of following structures:

★ As can be seen from the above resonating structures that the electron density is more on color{red}(o –) and color{red}(p –) positions. Hence, the substitution takes place mainly at these positions. But due to color{red}(–I) effect of color{red}(– OH) group 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, color{red}(–OH) group activates the benzene ring for the attack by an electrophile. Other examples of activating groups are color{red}(–NH_2, –NHR, –NHCOCH_3, –OCH_3, –CH_3, –C_2H_5), etc.

★ In the case of aryl halides, halogens are moderately deactivating. Because of their strong color{red}(– I) effect, overall electron density on benzene ring decreases. It makes further substitution difficult. However, due to resonance the electron density on color{red}(o–) and color{red}(p–) positions is greater than that at the color{red}(m)-position. Hence, they are also color{red}(o–) and color{red}(p–) directing groups.

★ color{green}("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 color{red}(–NO_2, –CN, –CHO, –COR, –COOH, COOR, –SO_3H), etc. Nitro group reduces the electron density in the benzene ring due to its strong color{red}(–I) effect. Nitrobenzene is a resonance hybrid of the following structures.

★ 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 color{red}(o–) and color{red}(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.