Hydrogenation calorimetry

For somewhat over seventy years, the structural proscription against bridgehead double bonds known as Bredt s rule has provided a guiding principle for strained molecules. More precisely, it is suggested that bicyclo m.n./ alk-l-ones are destabilized for small, but nonzero, m, n and p. The enthalpy of formation of three such species is available54 from hydrogenation calorimetry. These are the isomeric bicyclo[3.3.1]non-l-ene (22a), bicyclo[4.2.1]non-l-ene (22b) and bicyclo[4.2.1]non-l(8)-ene (22c), bicyclo[4.2.1]non-A18-ene) with gas-phase enthalpies of formation of 31, 74 and 50 kJ mol-1, respectively. These three numbers alone inadequately address the issue of bridgehead destabilization because the alicyclic carbon skeleton is not the same for all three olefins. 

In turn, our archive shows that 3-methylcyclopentene has been reported to be 7.2 2.1 kJmol-1 more stable than the 4-isomer. Interestingly, hydrogenation calorimetry (cf Reference 67) shows the 1,3-isomer difference to be 14.1 Id mol 1 but no data is available to compare the 3- and 4-isomers. We also note that 1-methylcyclohexene (as liquid) is found to be 6 4 kJ mol-1 more stable than its 4-isomer as shown by the archival enthalpy of formation of the former and the measurement of B. Lebedev and H. Smirnova, Macromol. Chem. Phys., 195, 35 (1994). 

Examples of the first type are to be found in the determination of the heats of formation of olefins. For example. Prosen and Rossini give the heats of formation of a number of olefines in the ideal gas state, calculated from the known heats of combustion of the paraffins and the known heats of hydrogenation of the olefins 3. 27 q It is possible to compare some of these results with heats of combustion measured directly. For instance, Coops et al. give for - A//R( z-heptene-l 1.) 1,111-5 kcal (corrected to the more recent benzoic acid result) whereas Prosen and Rossini s value is 1,113 6 kcal (corrected to the liquid state using the value given for the latent heat of vaporization by Bent et al. ) a difference of 2 kcal, or 0-2 per cent. The differences for the pentenes are less than this. On the other hand, the differences between the heats of formation of isomers are known better than this. The accuracy of these differences obtained by hydrogenation calorimetry has been confirmed by some recent hydrobromination calorimetry Lacher et have measured the vapour phase heats of hydro-... 

What about the reaction enthalpy in the C4 case While there are no measured enthalpies of formation for the product butatriene, this quantity may be simply estimated. The enthalpy of formation of its 1,4-dimethyl derivative (for both the CIS- and rrans-isomers) has been determined by hydrogenation calorimetry [56] to be ca. 265 kJ mor. Demethylation of propene, 1,2-butadiene, ( )-2-butene,... 

The modified NBR samples were characterized by differential scanning calorimetry [11,78-80,98]. The glass-transition temperature (T ) decreased with the level of hydrogenation. In the case of HFNBR, Tg increased with an increase in the addition of aldehyde groups to the polymer chain. Thermogravimetric analysis of the modified polymers have also been carried out [15]. 

ADMET polymers are easily characterized using common analysis techniques, including nuclear magnetic resonance ( H and 13C NMR), infrared (IR) spectra, elemental analysis, gel permeation chromatography (GPC), vapor pressure osmometry (VPO), membrane osmometry (MO), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The preparation of poly(l-octenylene) (10) via the metathesis of 1,9-decadiene (9) is an excellent model polymerization to study ADMET, since the monomer is readily available and the polymer is well known.21 The NMR characterization data (Fig. 8.9) for the hydrogenated versions of poly(l-octenylene) illustrate the clean and selective nature of ADMET. 

The desired enthalpy of formation of fulvene and of its 6-methyl derivative were determined by Roth by measurement of the appropriate enthalpy of hydrogenation. The facile polymerization of this compound precludes conventional bomb calorimetry. 

In addition to the above-mentioned possibilities for the in-situ monitoring of hydrogenations, there are, of course, also techniques involving calorimetry and IR spectroscopy [34, 35c, 39, 41, 53, 54]. 

P. M. Nunes, C. F. Correia, R. M. Borges dos Santos, J. A. Martinho Simoes. Time-resolved Photoacoustic Calorimetry as a Tool to Determine Rate Constants of Hydrogen Abstraction Reactions. Int. J. Chem. Kinet. 2006, 38, 357-363. 

The amount of heat released during a reaction is proportional to the amount of substance involved but the relationship is complicated in enzyme studies by secondary reactions. Although the use of entropy constants means that calorimetry theoretically does not require standardization, in many instances this will be necessary. The initial energy change can often be enhanced, giving an increase in the sensitivity of the method. Hydrogen ions released during a reaction, for instance, will protonate a buffer with an evolution of more heat. 

The photoacoustic calorimetry technique employs photolysis by laser pulses of a mixture containing di-im-butyl peroxide, an appropriate metal hydride, and solvent. Photolysis of the peroxide gives i-BuO radicals that abstract a hydrogen atom from the hydride, and the measured photoacoustic signal is proportional to the overall reaction enthalpy. After calibration,... 

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

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