Universal methylene increments

We take here twice the universal methylene increment (cf Reference 1) as found in the liquid state, i.e. 20.6 + 4.7 where 20.6 is the usually proposed (gas phase) value and 4.7 is (within a sign) the enthalpy of vaporization or condensation per carbon for an arbittary organic compound as suggested in Reference 4. 

While there is some dispute about how universal the universal methylene increment really is (cf Reference 1), it is nonetheless generally conceded that a methylene group affixed to two carbons usually contributes ca — 21 kJ mol-1 to the gas phase enthalpy of formation. 

If there were ever a simple comparison of enthalpies of formation to be investigated as part of this chapter it would be the oximes of the aliphatic aldoximes, RCH=NOH, as R proceeds through Me, Et, n-Pr,. Ideally, we would have the values for at least R = Me and Et and then employ the universal methylene increment to derive the remaining values by addition of —20.6 kJmoR for gaseous species and a related ca —26.0 kJ moR for... 

G. Geiseler, M. Ratzsch, K. Ebster and E. Ziegel, Ber. Bunsenges. Phys. Chem., 70, 221 (1966). J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, London, 1970. In addition, the universal methylene increment has been a major tool in our analyses of literature thermochemical data in many of our earlier Patai/Rappoport chapters, e.g. References 8 and 9. 

The linear correlation of enthalpies of formation with the number of carbon atoms is a useful and well-known feature of homologous series of functionalized organic compounds. The slope of the regression line for the gaseous n-alkanes (CH3—(CH2)j —H), —20.6 kJmol-, and the similar values of the slopes for other CH3—(CHi) —Z series is often called the universal methylene increment . In the liquid phase, the increment for the n-alkanes is —25.6 0.1 klmoD. The most accurate determination of the increment... 

The slopes, ag, for various n-alkyl substituted homologous series are commonly compared to the universal slope (methylene increment) of —20.6 kJmol-1 calculated for the n-alkanes9. An unanswered question is whether for Targe enough nc the methylene increment should be identical for all functionalized alkanes. In previous studies it is shown that the slopes vary, although not too widely (ca 2 kJ mol-1), and there is no discernible relationship between the functional group and a. With three exceptions, the slopes reported in Table 1 are in line with those calculated for other functional group series. 

Insertion of a methylene group into a molecule causes a decrease in aqueous solubility, however not with a universally applicable constant increment, as available GCMs might suggests. The odd-even effect (see Section 11.3) and the chain length have to be considered for accurate, quantitative estimations. In addition polar groups in the molecule affect the methylene contribution, as the following rule illustrates ... 

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