A New Method for Determining Aromaticity through Comparison of Inclusive vs. Non-Inclusive Steric Number Calculation in Conjunction with an Alternative Approach to Reveal Conjugated Pi Systems in Alkyne-Containing Compounds

A required aspect for the classification of a compound as aromatic is it must have a conjugated pi system, which is constituted by a region of overlapping p-orbitals or, in other words, a continuous system of sp2-hybridized atoms. This is usually determined by drawing a valid resonance structure (one with no more than 2 formal charges) where all atoms are sp2-hybridized for the compound of interest, but we propose a new method intuitively suggesting resonance (if the atom of interest is determined to be sp2-hybridized) involving the comparison of inclusive vs. non-inclusive steric number calculation. We define inclusive steric number calculation as the number of sigma bonds plus the number of lone pairs bonded to the atom (both localized and delocalized), and define non-inclusive as the number of sigma bonds plus the number of localized lone pairs only. In the non-inclusive steric number calculation, the delocalized lone pairs count towards the Huckel number of pi electrons in the compound (4n+2 where n is an integer). Once both are calculated for the atom of interest and the rest of the following requirements for aromaticity are evaluated (i.e. compound is planar, cyclic, and has Huckel number of electrons) the version of the compound with the most stable aromatic configuration is selected as the correct configuration. For alkyne-containing compounds, we devised an alternative method for determining sp2-hybridization of atoms involved in an alkyne to ultimately reveal a conjugated pi system in the compound that agrees with our inclusive vs. non-inclusive steric number calculation. This approach involves the idea that the pi electrons of a pi bond within an alkyne that are not involved in the conjugated pi system (due to these p-orbitals being perpendicular relative to the parallel p-orbitals contributing to conjugation) can occupy a single atom in the alkyne, but the specific atom they occupy is unclear. Therefore, we present a “lone pair hybrid” that accounts for both atoms being equally able to receive the pi electrons as a lone pair. These hypothetical lone pairs are localized due to an alkene still bonding the atoms and makes the steric number calculation for both the inclusive and non-inclusive approach 2 sigma bonds + 1 lone pair = 3, sp2, making the atoms sp2-hybridized and thus revealing the conjugated pi system.