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. 

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. 

The secondary isopropyl and icc-butyl hthium compounds, for which there are liquid-phase enthalpy of formation values, are two in a homologous series. Their enthalpy of formation difference, representing one methylene group, is ca —13 or —30 kJmoP depending on which enthalpy of formation is chosen for isopropyl lithium. By comparison, the methylene increment for liquid-phase 2-methylaIkanes is ca —25 kJmoP. ... 

The enthalpy of formation of methyl n-pentyl ether is unavailable from experiment, but a value of —316 kJmoP is obtained from the linear regression analysis of the known enthalpies of formation of methyl n-alkyl ethers vs. the number of carbon atoms in the ethers . The methylene increment of —25.3 kJmoP for this homologous ether series is nearly identical to the methylene increment for n-alkanes. Because the experimental protodelithiation enthalpies for the two primary lithio ethers are identical, the methylene increment in that homologous series as calculated here is necessarily identical to that of the homologous methoxy ethers. As calculated from the methylene increment or from equation 16, the enthalpy of formation of 5-lithiopentyl methyl ether is ca —309 kJmol. ... 

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... 

Finally, assuming a linear relationship between the enthalpies of formation and carbon number for the 2-alkanone oximes, and a gas phase methylene increment of —20.7 kJ mol , the same as for the 2-alkanones, the enthalpies of formation of 2-butanone and 2-octanone oximes would be —82.5 and —166.3 kJmoH, respectively. The latter value is within 1 kJmoE of that derived from equation 23. 

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 liquid enthalpy of formation difference between 1-hexyl and 1-heptyl hydroperoxides is almost twice that of a normal enthalpy of formation methylene increment of about 25 kJmol . But which of these two, if either, is correct For hydrocarbon snb-stituents bonded to electronegative functional groups, the secondary isomers are more stable than the n-isomer. Accordingly, either the 1- or 4-heptyl hydroperoxide, or both, have an inaccurate enthalpy of formation because the primary isomer is reported to have the more negative enthalpy of formation. All of the enthalpies of formation for the Cg and C7 hydroperoxides cited in Reference 2 come from a single source. There is a reported value for the gas phase enthalpy of formation of fert-butyl hydroperoxide that is 11 kJ mol less negative than the value in Reference 2. 

There are much fewer data for the dialkyl peroxides. The gas phase enthalpy of formation difference between the diethyl and dibutyl peroxides of about 40 kJ moH per methylene group is about twice that of the normal methylene increment of ca 21.6 kJmoH. The 219 kJmoH enthalpy of formation difference between the di-fert-butyl and di-fert-amyl peroxide is so large as to be incredible. 

The enthalpy of isomerization of hquid 1-hexanol to either 2- or 3-hexanol is ca 15 kJ mol , and the enthalpy of isomerization of hquid 1 -heptanol to 2-, 3- or 4-heptanol is ca 13 kJ mol . From the experimental enthalpy of formation of 1 -hexyl hydroperoxide, the calculated enthalpy of formation of 2- and 3-hexyl hydroperoxide is ca —315 kJ moU , which is about 5-10 kJmoU more negative than their experimental values. As noted earlier, the measured enthalpies of formation of 2- and 3-heptyl hydroperoxide are about the same as that of 1-heptyl hydroperoxide, while that of 4-heptyl hydroperoxide is actually less negative than for its primary isomer. Using instead the above-derived enthalpy of formation of 1-heptyl hydroperoxide of —325 kJmoU , the enthalpy of formation of the secondary isomers would be ca —338 kJ moU. This value is very close to the experimental enthalpy of formation of 4-heptyl hydroperoxide, but 8 kJmoU less negative than the experimental values for 2- and 3-heptyl hydroperoxide. These latter enthalpies of formation are too negative compared to the experimental values for 2- and 3-hexyl hydroperoxide, with a methylene increment of ca 36 kJ mol . The derived values are more plausible. 

The three saturated long-chain tert-butyl peresters are members of a homologons series, and as such, the weighted least-squares regression analysis of the enthalpies of formation V5. number of carbons yields a methylene increment of —26.7 kJmol , a typical valne for liquids. The methylene increment for the terf-butyl esters of the Cg, Cjo, Cn and C14 acids is —28.0 kJmol. The closeness of these two values ensures that the enthalpies of formal reaction 16 will be nearly constant. For the three pairs from Table 3, the value is —70.3 8.1 kJmol. The standard deviation from the mean is quite large because the arithmetic difference for the C12 ester and perester, —79.5 kJmol, is quite a bit more negative than the differences for the Cio and C14 pairs, —64.4 and —66.9 kJmol, respectively. Unfortunately, the acids and esters are in different phases and so we are reluctant to attempt any comparison between them, such as a formal hydrolysis reaction or disproportionation with hydrogen peroxide. 

Upid hydroperoxide determination, 676 Methylenecyclohexene, final ozonide, 718 Methylenecyclopentene, final ozonide, 718 Methylene increment, peroxycarboxylic acids, 158... 

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... 

From the archival enthalpies of formation of the other species, the enthalpy of formation of cyclohexyl methyl amine is —145.4 kJmoH. From equation 11, the enthalpy of formation of the corresponding salt is —501.2 kJmoH. Attempts to estimate an enthalpy of formation for A-methyldodecanamide reveals a paucity of data to work with, primarily for unsubstituted and A-methylamides . There is much enthalpy of formation data for w-alkyl carboxylic acids, including dodecanoic acid. The methylene increment... 

A recent study combined quantum chemical calculations and electron diffrac-tion/photoelectron spectroscopy to derive the following dialkylzinc gas phase enthalpies of formation ethyl, 57 8 n-propyl, 10 8 isopropyl, 32 8 f-butyl, —17 8 neopentyl, — 117 8 kJmoF. The benchmark value of 53 1 kJmol was chosen for the gas phase enthalpy of formation of dimethylzinc. Compared to the experimental values, the diethyl and dineopentyl values are very close, but the w-propyl enthalpy of formation is just barely within the combined large error bars. The methylene increment from the theoretically derived values of diethylzinc and di-n-propylzinc is —23.5 kJ mol , a value that is consistent with other gas phase homologous series. Using this increment, the enthalpies of formation of gaseous di-w-butylzinc and di-n-pentylzinc are calculated to be —37 and —84 kJmoU, respectively. 

---