Directive Influence of Functional Group in Mono Substituted Benzene
When mono substituted benzene undergoes an electrophilic attack, the rate of reaction and the site of attack vary with the functional group already attached to it. Some groups increase the reactivity of benzene ring and are known as activating groups while others which decrease the reactivity are known as deactivating groups.
We further divide these groups into two categories depending on the way they influence the orientation of attack by the incoming electrophile. Those which increase the electron density at ”ortho” and “para” positions are known as ortho-para directors while those which increase the electron density at “meta” position are known as meta directors. Some examples of directive influence of functional group in mono substituted benzene are explained below:
Ortho-para directing group: As discussed earlier, these groups direct the electrophilic attack on “ortho” and “para” positions. All activating groups are “ortho-para” directors, for example –NH2, –NHR, –NHCOCH3, –OCH3,–CH3, –C2H5, etc.
From the figure, it is quite evident that the electron density at ortho and para position increases due to the resonance of benzene ring. Therefore, phenols have high affinity for electrophilic attack at “ortho” and “para” positions. As a result, we can say that “–OH” group is an ortho-para director. Due to the resonance in the benzene ring, electron density at “ortho” and “para” position increases as compared to the “meta” position. As a result, halogens too are ortho-para directors, irrespective of the fact that they are deactivators due to “-I” effect.
Meta directing groups: These groups direct the electrophilic attack on “meta” positions of the associated benzene ring. Generally deactivating groups are meta directors, for example –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc.
Nitro group is a ring deactivating group; they decrease the electron density of the benzene ring attached. As we can see from the figure the electron density at “meta” position is quite high in comparison to “ortho” and “para” position. Hence, these groups facilitate electrophilic substitution of the ring at “meta” positions and are called meta directors.