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Write the requirements for aromatic character in compound. |
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Answer» Solution :Benzene was considered as parent .aromatic. compound. Now the name is applied to all the ring system whether or not having benzene ring, possessing following characteristics. (a) Requirements for aromatic character in compound : (i) Compound must havr cyclic (ring) STRUCTURE. (ii) Cyclic compound must have all atoms in one planer structure. (iii) Complete delocalisation of the `pi` electronsin the ring. (iv) Presence of `(4n+2)pi` electrons in the rinf where .n. is an inetger (n=0, 1, 2,...). This is often referred to as Huckel Rule. (b) Huckel.s rule for aromaticity : If the number of electrons in planar cyclic compound equals to (4n+2), then these compounds possess aromatic properties, (aromaticity) where n = number of cyclic structures, (4n+2) = total delocalized electron of atoms present in aromatic ring). (c) Aromatic hydrocarbon compounds : (i) Benzene is aromatic in nature. Number of rings in benzene = n =1 NUmber of double bonds in benzene = 3 so, number of `PI2(p)` electrons = 6 `therefore` According to Huckel rule, if (4n+2) `pi` electrons are present in structure then it is considered as aromatic. So for benzene , n=1 and (4n+2) = 4(1) + 2 = 6, so benzen obeys Huckel rules and benzene is aromatic in nature and it possess aromatic characteristics.  (ii) Cyclopentadienyl anion aromatic in nature. According to Huckel rule, n = 1 and hence (4n+2) = 6 In cyclopentadienail anion TWO double bond have `2 xx 2 = 4pi` + two lone `pi` electrons, so it is aromatic in nature.  (iii) Cycloheptatrienyl cation is aromatic in nature. Number of ring = n = 1 `therefore (4n+2) = 6`  In this compound three double bonds are present, so no. of `pi` electrons `= (3 xx 2) = 6`. So it follows Huckel rule, so cycloheptatriene, n = 1(4n+2)=6, cation is aromatic in nature. It possess positive charge `sp^(2)` carbon so it also possess PLANARITY. (iv) Nepthalene is aromatic in nature.  Number of ring = n= 2 It having five double bonds so no. of `pi` electrons `= (5 xx 2) = 10` electrons Napthalene follows Huckel rules and so it is aromatic innature. (v) Anthrecene and Phenanathrene are aromatic compounds. 
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