Question

Indicate whether or not each of the following structures is considered to be aromatic.

Answer 1

Huckel's rule for Aromaticity:

1.   the molecule must follow Huckel's rule, having 4n+2 electrons in the delocalized, and conjugated p-orbital cloud;

2.   the molecule must be able to be planar

3.   the molecule must be cyclic

4.   Every atom in the ring must be able to participate in delocalizing the electrons by having a p-orbital or an unshared pair of electrons.

Structure A is aromatic. All carbons SP² hybridized and all are conjugated double bonds. Follow (4n+2) n= no. of rings 2 => 10? e^- formula.

Structure B is non aromatic. There is a SP³ hybridized carbon adjacent to the Positive charge. Follow 4n+2 formula. So delocalization not possible. Rule 4 not obeyed.