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:

โ˜… 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.

โ˜… `color{green}("๐Ž๐ซ๐ญ๐ก๐จ ๐š๐ง๐ ๐ฉ๐š๐ซ๐š ๐๐ข๐ซ๐ž๐œ๐ญ๐ข๐ง๐  ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ:")` The groups which direct the incoming group to ortho and para positions are called `color{green}("๐จ๐ซ๐ญ๐ก๐จ ๐š๐ง๐ ๐ฉ๐š๐ซ๐š ๐๐ข๐ซ๐ž๐œ๐ญ๐ข๐ง๐  ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ")` .Let us discuss the directive influence of phenolic `color{red}((โ€“OH))` group. Phenol is resonance hybrid of following structures:

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

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

โ˜… `color{green}("๐Œ๐ž๐ญ๐š ๐๐ข๐ซ๐ž๐œ๐ญ๐ข๐ง๐  ๐ ๐ซ๐จ๐ฎ๐ฉ:")` The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are `color{red}(โ€“NO_2, โ€“CN, โ€“CHO, โ€“COR, โ€“COOH, COOR, โ€“SO_3H)`, etc. Nitro group reduces the electron density in the benzene ring due to its strong `color{red}(โ€“I)` effect. Nitrobenzene is a resonance hybrid of the following structures.

โ˜… In this case, the overall electron density on benzene ring decreases making further substitution difficult, therefore these groups are also called โ€˜deactivating groupsโ€™. The electron density on `color{red}(oโ€“)` and `color{red}(pโ€“)` position is comparatively less than that at meta position.
Hence, the electrophile attacks on comparatively electron rich meta position resulting in meta substitution.


โ˜… Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property.

โ˜… Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco, coal and petroleum.

โ˜… They enter into human body and undergo various biochemical reactions and finally damage DNA
and cause cancer.