CCL: Why Chlorine atom is more hydrophobic than Hydrogen

[Andreas Klamt <klamt{:}cosmologic.de>]

 Sent to CCL by: Andreas Klamt [klamt%%cosmologic.de]
 Hi Mannan,
this can be quite nicely explained  within COSMO-RS theory. Basically
 the interactions in liquids are surface contacts. Hydrophobicity is
 sloppy expression for the fact that water does not like to make
 contayts
 to non- or slightly polar surface area. By the way, this is an entropic
 effect, because the really polar parts of water essentially will make
 no
 contacts to slightly polar surfaces, and the free high free energy of
 contact results from the entropy loss which goes along with placing the
 few "green" (in COMSO-RS the non-polar surface is colored in
 green)
 surface pieces of water on the non-polar solute surface area.
Anyway, the larger the non-polar surface area, the more of the green
 water patches are required as contact partners and the higher the
 "hydrophobic" costs. And since the vdW-radius of hydrogen is
 ~ 1.1
 Angstrom, and that of Chlorine is ~ 1.7, the exposed non-polar surface
 area of chlorine is much larger than that of hydrogen.
 Best regards
 Andreas
 Am 23.01.2013 15:36, schrieb Mannan K malie_03(0)yahoo.co.in:
>  Sent to CCL by: "Mannan  K" [malie_03|*|yahoo.co.in]
>  Hi CCLers,
>  Chlorine/Fluorine atoms are commonly used to influence the hydrophobic contact
>  in small molecule design.
>  But how the hydrophobicity is being measured for halogen atoms with reference to
>  Hydrogen atom?
>  How Molar refractivity and steric paramemters are connected to hydrophobicity of
>  an atom?
>  Thanks in Advance,
>  Mannan>
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