I Lab Services 

These problems have already been tackled, and to a good degree of success, by two ACD/Labs 

algorithms:  

(i)  The acid base ionization coefficient, pK

a

, can be predicted normally to within 

0.2 pK

a

 units 

for most functional groups by ACD/pK

a

. 

(ii) The octanol water partition coefficients can be predicted normally to within 

0.3 logP units 

by ACD/LogP DB. 

Thus, it only makes sense to combine the capabilities of both  prediction engines  in one simple 

to use prediction of logD: ACD/LogD Suite. 

4.18.3  Definition of LogD 

The partition coefficient for dissociative mixtures, logD, is defined as follows: 

a

org

log D

log

i

  ( I ) 

2

a

H O

i

where 

a

H O

2

i

 the concentration of the i th microspecies in water 

a

org

i

 the concentration of the i th microspecies in organic phase 

For correct solution of a proposed scheme of equilibrium we use the master equation approach.  

We construct a system of equations where the concentrations of all microspecies (both in organic 

and aqueous phase) are represented by a set of independent linear equations. 

To calculate the concentrations all microspecies at any pH we need to solve the following system 

of equations: 

a

H O

2

H

[

]

a

H O

j

2

( II ) 

i

K

i

a

a

H O

2

a

org

i

i

i

a

1

i

a

org

a

H O

2

P

i

i

i

where 

K

 the ionization constant of the i th microspecies 

K

pK

a

10

i

i

P

 the partition coefficient of the i th microspecies 

i

a

 the total concentration of the i th microspecies having one more H

+

 than the j th 

i

a

 the total concentration of the j th microspecies in both H

j

2

O and octanol 

Once the concentrations are known, the ratio in equation (I) and thus the logD value can easily 

be determined. 

ACD/I Lab via ChemSketch  

User's Guide 

48