Parachor: Application to Structure Determination

Parachor:

Macleod in 1923 gave the following relation between the surface tension (γ) and density (D) for a normal liquid-
C=γ^1/4/(D −d)
where d is vapour density of the liquid at given temperature and C is constant.
In 1924, Sugden modified the above equation as-
γ^1/4/(D −d)=MC=P
where M is the molecular weight of the liquid and P is the parachor.
At ordinary temperature, d is very small in comparision to D then-
M γ^1/4/D=P
If γ=1 at a particular temperature, then-
M/D=P
Thus, at a particular temperature, the molar volume of a liquid having surface tension unity is called Parachor.
If two liquids having the same surface tension are taken whose molecular weights are M₁ and M₂ and their densities are D₁ and D₂ respectively, then-

M₁ γ^1/4/D₁=P₁
and
M₂ γ^1/4/D₂=P₂
P₁/P₂=(M₁/D₁)/ (M₂/D₂)
So, the ratio of parachors of two liquids having the same surface tension is equal to the ratio of molar volumes. Parachor is both an additive and constitutive property.

Parachor Value of some Elements and Groups:

Element	P Value	Group	P Value
C	8.6	C=O	44.4
H	15.7	OH	30.2
N	12.5	COOH	73.7
O	19.8	NO2	73.8
Cl	55.2	Double Bond	19.9
Br	68.8	Triple Bond	40.6
I	90.3	Six Membered Ring	1.4

Application of Parachor:

Parachor data are used to determine the structure of molecules and the nature of bonds.
Example:
Two structure proposed for QUINONE-

Structure-1
6C=6 X 8.6=51.6
4H=4 X 15.7=62.8
2O=2 X 19.8=39.6
Four double bonds=4 X 19.9=79.6
one 6 membered ring=1 X 1.4=1.4
So, Calculated Parachor=235

Structure-2
6C=6 X 8.6=51.6
4H=4 X 15.7=62.8
2O=2 X 19.8=39.6
Three double bonds=3 X 19.9=59.7
Two 6 membered ring=2 X 1.4=2.8
So, Calculated Parachor=216.5
Since the experimental Parachor is 236.8. Hence structure-1 is correct for QUINONE.