Huckel 4n + 2 rule

Cycloheptatrienyl radical has seven tt electrons Therefore it does not satisfy the Huckel 4n + 2 rule and is not aromatic... 

Let s look at some examples to see how the Huckel 4n + 2 rule work... 

The terms aromatic and antiaromatic have been extended to describe the stabilization or destabilization of TRANSITION STATES of PERICYCLIC REACTIONS. The hypothetical reference structure is here less clearly defined, and use of the term is based on application of the Huckel (4n+2) rule and on consideration of the topology of orbital overlap in the transition state. Reactions of molecules in the ground state involving antiaromatic transition states proceed, if at all, much less easily than those involving aromatic transition states. 

Hofmann rearrangement, 118 Homotopic groups, 9 Homotropenylium, 152 Huckel array, 85 Huckel MO theory, 35, 86 Huckel 4n + 2 rule, 150, 151 Hund s rule, 114 Hunsdiecker reaction, 305-307 Hybridization, 15, 16... 

Figure 5.21 Equivalence of Huckel 4n + 2 pi electron rule and the mno rule Huckel 4n + 2 rule can be viewed as a special case of the mno rule.

Mayer I (2010) Analytical derivation of the Huckel 4n + 2 rule . Theoiet Chem Acc 125 203-206... 

Tropylium cation and the substituted cyclopropenyl cations cited in Table 5.2 are of interest in that their ease of formation provides experimental support for the Huckel 4n + 2 rule of aromatic stability. Both types contain planar monocyclic systems of 5p -hybridized atoms with two tt electrons (n = 0) in the cyclopropenyl cations and six tt electrons (n = 1) in tropylium cation. 

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