Aromaticity

Aromaticity requires a cyclic, planar, fully conjugated system whose delocalised pi electrons number 4n + 2, where n is a non-negative integer; this is the Huckel rule. Testing the choices, six electrons correspond to n = 1, since 4(1) + 2 = 6, exactly satisfying the rule, which is why benzene with its six pi electrons is the archetypal aromatic molecule. Four pi electrons fit 4n with n = 1 and represent an antiaromatic count, the opposite of aromatic stabilisation, so that option is wrong. Eight pi electrons also equal 4n with n = 2, again antiaromatic rather than aromatic. Five pi electrons is an odd number that fits neither the aromatic nor a closed-shell pattern and cannot satisfy 4n + 2. Hence six delocalised pi electrons is the correct aromatic count, matching the NCERT description of benzene-like stability. Aromatic systems show enhanced thermodynamic stability, reduced reactivity toward addition, and a preference for substitution reactions that preserve the closed pi shell. The same 4n + 2 test extends to charged rings and heterocycles once their pi electrons, including relevant lone pairs, are counted correctly. Plausibility check: substituting n = 1 into 4n + 2 directly yields six, the smallest aromatic number beyond the trivial case, confirming the answer.