In 1931, who recognised that an aromatic compound must have an odd number of pairs of electrons, which can mathematically be written as 4n + 2 (n = 0, 1, 2, 3 etc.) ?

Erich Huckel

The correct answer is Option (2) - Erich Huckel

Erich Huckel, a German physicist and chemist, developed a theory in 1931 that provided a mathematical explanation for the stability of certain organic molecules known as aromatic compounds. This theory, known as Huckel's rule, states that an aromatic compound must have a specific number of π-electrons, which are delocalized within a ring structure.

Huckel proposed that for a molecule to exhibit aromaticity, it must fulfill the following criteria:

1. The molecule must be planar.
2. It must be cyclic.
3. It must have a continuous ring of overlapping p-orbitals.
4. The number of π-electrons in the cyclic system must follow the formula 4n + 2, where 'n' is an integer (0, 1, 2, 3, etc.).

This mathematical expression (4π + 2) indicates that the number of π-electrons in an aromatic compound can only be certain values that satisfy this equation, such as 2, 6, 10, 14, etc. This concept explains why many aromatic compounds contain 6, 10, or 14π-electrons, which correspond to stability due to electron delocalization.

Huckel's rule is fundamental in understanding the stability and reactivity of aromatic compounds and has widespread applications in organic chemistry. It provides a theoretical framework for predicting the aromatic character of molecules and has contributed significantly to the development of aromatic chemistry.