Joback method: Difference between revisions

Revision as of 17:30, 22 October 2012

Revision as of 18:49, 5 December 2012

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==Model Strengths and Weaknesses==

===Strengths===

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The data base Joback and Reid used for obtaining the group parameters was rather small and covered only a limited number of different molecules. The best coverage has been achieved for normal boiling points (438 components) and the worst for heat of fusion (155 components). Current developments that can use data banks like the [[Dortmund Data Bank]] or the DIPPR data base have a much broader coverage.

The formula used for the prediction of the normal boiling point shows another problem. Joback assumed a constant contribution of added groups in homologous series like the [[alkane]]s. This doesn't describe the real behavior of the normal boiling points correctly.<ref>Stein S.E., Brown R.L., "Estimation of Normal Boiling Points from Group Contributions", ''J. Chem. Inf. Comput. Sci.'' 34, 581–587 (1994)</ref>. Instead of the constant contribution a decrease of the contribution with increasing number of groups must be applied. The chosen formula of the Joback method leads to high deviations for large and small molecules and an acceptable good estimation only for mid-sized components.

== Formulas ==

In the following formulas G_i denotes a group contribution. G_i are counted for every single available group. If a group is present multiple times each occurrence is counted separately.

===Normal Boiling Point===

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[[Image:AcetonGruppen.PNG]]

[[Acetone]] (Propanone) is the simplest [[ketone]] and is separated into three groups in the Joback method: two [[methyl group|methyl groups]]s (-CH₃) and one ketone group (C=O). Since the methyl group is present twice, its contributions have to be added twice.

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