Chemistry · General Principles Of Organic Chemistry · NEET
Count 2 pi electrons for every double bond that is part of the ring. For a lone pair on a ring atom (like the negative charge in cyclopentadienyl anion or the N in pyrrole), add 2 more only if that lone pair sits in a p-orbital parallel to the ring. Then check: does the total fit 4n+2 (2, 6, 10, 14...)? Benzene = 3 double bonds = 6 pi electrons = 4n+2 with n=1, so aromatic.
Cyclohexane has only single bonds, so it has zero pi electrons and no delocalisation — it cannot be aromatic. Benzene is cyclic, flat (planar), and has 6 fully delocalised pi electrons in the ring, meeting all of Huckel's conditions. This delocalisation is why benzene resists addition reactions.
No. Cyclooctatetraene has 8 pi electrons (4 double bonds). 8 fits 4n (n=2), not 4n+2, so it is NOT aromatic. To avoid this unstable arrangement it puckers into a non-planar tub shape, which also breaks the planarity condition. This is a classic NEET trap.
Aromatic = cyclic + planar + delocalised + (4n+2) pi electrons (extra stable, e.g. benzene). Antiaromatic = cyclic + planar + delocalised but (4n) pi electrons (extra unstable, e.g. planar cyclobutadiene, 4 pi). Non-aromatic = missing a condition, usually not planar or not fully conjugated (e.g. cyclooctatetraene, cyclohexane).
Yes. Its ring has 2 double bonds (4 pi electrons) plus a lone pair from the negative carbon sitting in a p-orbital = 6 pi electrons total = 4n+2 (n=1). It is planar and cyclic, so it is aromatic. The tropylium cation (7-membered, 6 pi electrons) is aromatic for the same reason.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT lists three: (i) planarity of the ring, (ii) complete delocalisation of the pi electrons in the ring, and (iii) presence of (4n+2) pi electrons where n is an integer (0, 1, 2...). This third condition is Huckel's Rule.
For benzene, n = 1, giving 4(1)+2 = 6 pi electrons. This matches benzene's three delocalised double bonds.
Yes. With n = 0, the rule gives 4(0)+2 = 2 pi electrons. The cyclopropenyl cation has 2 pi electrons in a planar 3-membered ring and is aromatic.
Because its 6 pi electrons are completely delocalised over the ring, giving extra stability. This aromatic stability makes benzene prefer substitution over addition, which would destroy the delocalisation.
No. 'n' is just any non-negative integer (0, 1, 2...) used to generate the allowed pi electron counts 2, 6, 10, 14. It is not the count of double bonds or ring atoms.
