Directive influence of a functional group in monosubstituted benzene:
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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}("๐๐ซ๐ญ๐ก๐จ ๐๐ง๐ ๐ฉ๐๐ซ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ:")` The groups which direct the incoming group to ortho and para positions are called `color{green}("๐จ๐ซ๐ญ๐ก๐จ ๐๐ง๐ ๐ฉ๐๐ซ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ")` .Let us discuss the directive influence of phenolic `color{red}((โOH))` 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}(โI)` 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}(โOH)` group activates the benzene ring for the attack by an electrophile. Other examples of activating groups are `color{red}(โNH_2, โNHR, โNHCOCH_3, โOCH_3, โCH_3, โC_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}("๐๐๐ญ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ:")` The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are `color{red}(โNO_2, โCN, โCHO, โCOR, โCOOH, COOR, โSO_3H)`, etc. Nitro group reduces the electron density in the benzene ring due to its strong `color{red}(โI)` 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 โdeactivating 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 `color{green}("๐๐ซ๐ญ๐ก๐จ ๐๐ง๐ ๐ฉ๐๐ซ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ:")` The groups which direct the incoming group to ortho and para positions are called `color{green}("๐จ๐ซ๐ญ๐ก๐จ ๐๐ง๐ ๐ฉ๐๐ซ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ๐ฌ")` .Let us discuss the directive influence of phenolic `color{red}((โOH))` 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}(โI)` 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}(โOH)` group activates the benzene ring for the attack by an electrophile. Other examples of activating groups are `color{red}(โNH_2, โNHR, โNHCOCH_3, โOCH_3, โCH_3, โC_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}("๐๐๐ญ๐ ๐๐ข๐ซ๐๐๐ญ๐ข๐ง๐ ๐ ๐ซ๐จ๐ฎ๐ฉ:")` The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are `color{red}(โNO_2, โCN, โCHO, โCOR, โCOOH, COOR, โSO_3H)`, etc. Nitro group reduces the electron density in the benzene ring due to its strong `color{red}(โI)` 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 โdeactivating 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.
