Directive influence of a functional group in monosubtituted benzene:
When mono substituted benzene undergoes an electrophilic substitution reaction, the rate of the reaction and the site of attack of the incomeing electrophile depends on 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 groups. Those which increases electron density at ‘ortho’ and ‘para’ position are known as ortho-para directors while those which increase electron density at ‘meta’ position is known as meta-directors. Some examples of directive influence of functional groups in mono-substituted benzene are explained below.
All the activating groups are ‘ortho-para’ directors. Example –OH, -NH2, -NHR, -NHCOCH3, -OCH3-CH3 – C2H5 etc. Let us consider the directive influences of phenolic (-OH) group. Phenol is the resonance hybrid of following structures.
In these resonance structures, the (-) charge residue is present on ortho and para position of ring structure. It is quite evident that the lone pair of electron on the atom which is attached to the ring involves in resonance and makes the ring more electron rich than benzene. The electron density at ortho and parapositions increases as compared to the meta position. Therefore phenolic group activates the benzene ring for electrophilic attack at ‘ortho’ and ‘para positions and hence –OH group is an ortho-para director and activator.
In aryl halides, the strong –I effect of the halogens (electron withdrawing tendency) decreases the electron density of benzene ring, thereby deactivating for electrophilic attack. However the presence of lone pair on halogens involved in the resonance with pi electrons of benzene ring, increases electron density at ortho and para position. Hence the halogen group is an ortho-para director and deactivator.
Generally all deactivating groups are meta-directors. For example –NO2, -CN, -CHO, -COR, -COOH, -COOR, -SO3H etc. Let us consider the directive influence of aldehyde (-CHO) group. Benz aldehyde is the resonance hybrid of following structures.
In these resonance structures, the (+) charge residues is present on the ring structure. It is quite evident that resonance delocalizes the positive charge on the atoms of the ring, making the ring less electron rich than benzene. Here overall density of benzene ring decreases due to –I effect of –CHO group there by deactivating the benzene for electrophilic attack. However resonating structure shows that electron density is more in meta position. Compared to o & p-position. Hence –CHO group is a meta-director and deactivator.