JEE Advanced Mi...

On the basis of aromaticity, there are three types of compounds i.e. aromatic, non-aromatic and antiaromatic. The increasing order of stability of these compounds are is under: Anti-aromatic compound < non-aromatic compound < aromatic compounds. Compounds to be aromatic follow the following conditions (according to valence bond theory)

(i) The compounds must be be cyclic in structure having (4n + 2)π e^–, where n = Hückel’s number = 0, 1, 2, 3 et.c

(ii) The each atoms of the cyclic structure must have unhybridised p-orbital i.e. the atoms of the compounds have unhybridised p-orbital i.e. usually have sp² hybrid or planar.

(iii) There must be a ring current of π electrons in the ring or cyclic structure i.e. cyclic structure must undergo resonance .

Compounds to be anti-aromatic, it must have 4nπe^– where n = 1, 2… and it must be planar and undergo resonance. Non-aromatic compounds the name itself spells that compounds must be non-planar irrespective of number of π electrons. Either it has 4nπe^– or (4n + 2) π electrons it does not matter. The rate of reaction of any aromatic compounds depends upon the following factors:

(i) Electron density

(ii) stability of carbocation produced

Higher the amount of electron density of the ring, higher will be its rate towards aromatic electrophilic substitution and vice-versa. Similarly, higher will be the stability of the produced carbocation after the attack of electrophile, higher will be its rate towards aromatic electrophilic substitution. There is a great effect of kinetic labelling on the rate of aromatic electrophilic substitution. As we known that higher the atomic weight or, molecualr weight, higher will be the van der Waal’s force of attraction or, bond energy. Since there will be effect of kienetic labelling if the 2^nd step of the reaction will be the slow step, (r.d.s.) otherwise there will be no effect of kinetic labelling.