# Why is logP quite a bit lower in cyclohexane than in hexane?

http://www.rsc.org/suppdata/nj/b3/b303016d/b303016d.pdf

If anyone doesn't know the definition, logP is the the log of the ratio of a molecule in dioxane to water. So a high logP would mean the molecule is mostly in the organic dioxane layer rather than water. E.g a molecule with a logP of 2 would prefer the organic layer 100 times more than water, and would be considered hydrophobic/lipophilic.

Hexane logP=4.15, very hydrophobic and almost all of it will stay in the organic layer.

Cyclohexane logP=3.66, still very hydrophobic, but hexane is almost 4 times more hydrophobic than cyclohexane. Can anyone please try to explain why the cyclic molecule appears to like water slightly more than its straight chain counterpart?

Think of the hole in the water that the molecules occupies as a bubble. The bigger the bubble the greater the area over which surface tension is acting.

The highest volume to surface ratio is for a sphere. So hexane is like a rod and cyclohexane is like hexagonal pyramid. Guess which has a higher volume to surface ratio?

Not that it is exactly the same, but look at the densities of the liquids. The cyclohexane molecule occupies a small volume in the liquid as compared to hexane, hence it is denser, even though it has 2 hydrogen atoms less per molecule.

Now water has 18 ml/mole so we can figure out roughly how many water molecules are displaced by a molecule of each hydrocarbon.

                                                     # water
molecules
density         molecular mass  ml/mole displaced
Cyclohexane  0.7781 g/mL     84.16 g/mol     108.2    6.0
Hexane       0.6548 g/mL     86.18 g/mol     131.6    7.3

• I'm not fully convinced/haven't grasped this fully so if a bit more depth could be possible that would be brilliant! – Laksh Nov 3 '15 at 22:32
• @Aced125 - I added some additional information which might be helpful. – MaxW Nov 4 '15 at 1:24
• Ok, I don't seem to understand how a greater amount of water molecules displaced correlates to a higher logP though. – Laksh Nov 5 '15 at 18:02
• Neither hexane nor cyclohexane will participate in much hydrogen bonding. So essentially the molecule has to make a "hole" in the hydrogen bonds between the water molecules that would be there otherwise. So it is like the molecules need a "bubble" devoid of anything into which that can slip. Hexane requires a bigger bubble, thus takes more energy and is hence less favorable. – MaxW Nov 5 '15 at 18:07