In Normal Phase HPLC which elutes first n-hexane or benzene Both are non-polar so the retention time on the column should be low but I don't know which one is most nonpolar.?

Answer 1

You can look up solvent polarity by looking up their dielectric constant (the higher the constant the more polar it is).

n-hexane has a dielectric constant of 2.0, where as benzene has a dielectric constant of 2.3. This means that benzene is more polar than n-hexane. (Source: http://en.wikipedia.org/wiki/Solvent).

On a Normal Phase HPLC column, the column is coated in silica gel which is very polar. The rule "like dissolves like" is important, as you pass your sample through the column the polar molecules will grip on tighter to the silica gel, where as the non-polar molecules will flow through easier.

This means that in a Normal Phase HPLC "race" down the column, it is the n-hexane that will come out first.

Answer 2

In Normal Phase HPLC, n-hexane would typically elute first compared to benzene. This is because n-hexane is more nonpolar than benzene due to its longer hydrocarbon chain, leading to weaker interactions with the polar stationary phase, resulting in faster elution.