Group contribution methods allow one to estimate some physical and chemical properties of molecules. We do this in a screening effort when we don’t have time or resources to make measurements. In this column, I want to review this technology and describe some recent improvements.
THE BASIC IDEA
It’s simple — that equilibrium properties of a chemical are related to the atoms of which it is composed, and how they are arranged in the chemical molecule.
Equilibrium properties include critical (temperature, pressure, volume),1 thermodynamic (heats of formation and vaporization),2 physical (boiling and melting points as well as surface tension),3 safety or environmental (flash point, and log KOW), and chemical (Hansen Solubility Parameters).4,5
Not included are transport properties such as viscosity and thermal diffusivity, or other properties which are path functions instead of functions of state.
HOW IT’S DONE
What one does is to subdivide a molecule, whose atomic structure must be known, into functional groups. Common ones might be CH3, CH2, -OCH2O-, -OCH2CH2O-, cyclic -CH2O- (Tetrahydrofuran), -OH, CH3COO- (glycol ether acetates), AcH (benzene), and HCOO- (formates).
- Then one counts the number of times each group is found by inspection in the atomic structure.
- Next, one notes the value assigned to each group. These individual group contribution values are determined by the developer of the technology from various methods of regression analyses.
The general equation for estimation of any property is:
Property = A +ƒ(Σ Gi)
where A is an constant determined by regression, and ΣGi is the sum of the individual group contribution values times the number each occurs in the molecule.
There are two: 1) in complex molecules it can be difficult to “correctly” identify to which group an atom belongs, and 2) this approach may not well describe mixtures where the environment around a molecule includes not only others of the same type, but those of other mixture components.
I am certain that readers of this column use this technology. It’s buried within complex computer programs which enable some to evaluate feasibility of using molecules which haven’t been yet synthesized as new pharmaceutical or biomedical intermediates.
SMILES (Simplified Molecular Input Line Entry Specification) is a tool for unambiguously describing the structure of chemical molecules using short ASCII strings.6 Researchers are working on translating the SMILES nomenclature for thousands of molecules into a table of specific groups and the number of them found within each molecule. This would allow the first weakness noted above to be overcome.
Other researchers are working to relate SMILES to other environmental properties, such as Maximum Incremental Reactivity (MIR) of chemicals with UV light to form ozone in support of efforts to enhance solvent substitution, as well as the European REACH program.
In summary, you should know about group contribution methods. They are producing increasing value in our efforts to select and manage chemicals, for cleaning and other uses.
- Joback K.G. and Reid R.C., "Estimation of Pure-component Properties from Group-contributions", Chemical Engineering Communications, Vol. 57, 233-243, 1987.
- Constantinou, L. and Gani, R. ("New Group Contribution Method for Estimating Properties of Pure Compounds," AIChE Journal, Vol. 40, No. 10, October 1994, pages 1697 to 1710)
- Panday, J.D., et. al., “Estimation of Surface Tension of Ternary Liquid Systems by Corresponding-states Group-contributions Method and Flory Theory,” Fluid Phase Equilibria, Vol. 273, Issues 1-2, 25 November 2008, Pages 40 to -51
- Stefanis, E. And Panayiotou, C., Prediction of Hansen Solubility Parameters with a New Group-Contribution Method," International Journal of Thermophysics, Vol. 29, No. 2, 2008, pages 568 to 585.
- Abbott, S. and Hansen, C.H., Hansen Solubility Parameters in Practice, 2009; available for purchase including an e-handbook at http://www.hansen-solubility.com.
- For example, CCO, OCC and C(O)C all specify the structure of ethanol.