F number is a correlation number used in the analysis of
polycyclic aromatic hydrocarbon
A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
s (PAHs) 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:
: ''B''
2 is the number of
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...
s
: ''C''
12 is the number of
primary carbon
A primary carbon is a carbon atom which is bound to only one other carbon atom. It is thus at the end of a carbon chain. In case of an alkane, three hydrogen atoms are bound to a primary carbon (see propane in the figure on the right). A hydrogen ...
and
secondary carbon atoms
: ''R'' is the number of
non-aromatic rings.
Example
For
fluorene, there are 6 apparent double bonds (three
pi bond
In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
s in each side
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
-like ring); 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 (
logarithm
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...
of the
retention factor In chromatography, the retardation factor (''R'') is the fraction of an analyte
An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest ...
) in aqueous
reversed-phase liquid chromatography
In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...
. 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 (L/B) ratio, which classifies molecules according to their "rodlike" or "squarelike" shape.
References
External links
List of F numbers for PAHs
Polycyclic aromatic hydrocarbons
Chromatography
{{chromatography-stub