F number (chemistry)


F number is a correlation number used in the analysis of polycyclic aromatic hydrocarbons as a descriptor of their hydrophobicity and molecular size. It was proposed by Robert Hurtubise and co-workers in 1977.

Calculation

The F number is calculated using the formula:
where:

Example

For fluorene, there are 6 apparent double bonds ; the central ring has one secondary carbon and is non-aromatic. Therefore:

Correlation

It has been found that the F number linearly correlates with the log k' value in aqueous reversed-phase liquid chromatography. This relationship can be used to understand the significance of different aspects of molecular architecture on their separation using different stationary phases. This size analysis is complementary to the length-to-breadth ratio, which classifies molecules according to their "rodlike" or "squarelike" shape.