Alkenes heats of formation

For many strained hydrocarbons and alkenes, heats of formation are not known. In these cases, computational methodology may be used. For a number of molecules, the total energies determined by ab initio calculations and transformed into heats of formation by special group equivalents may be used (81b). Strain energies have also been obtained via isodesmic reactions, which measure deviations from the additivity of bond energies (224). 

Derivatives and Isomers of Alkenes Nomenclature The Cahn-Ingold-Prelog Priority System Relative Stability of Alkenes Heats of Formation Double Bonds in Rings Physical Properties of Alkenes Alkynes Structure and Bonding... 

Part B of Table 1.5 gives heats of formation for the C4, C5, and some of the Cg alkenes. A general relationship is also observed for the alkenes. The more highly substituted the double bond, the more stable is the compound. There are also other factors that enter into alkene stability. trans-Alkenes are usually more stable than cis-alkenes, probably largely because of increased nonbonded repulsion in the cis isomer. ... 

Hartree-Fock calculations, peracid alkene epoxidation, 48-50 Hazardous materials commercial codes, 621 emergency response, 746-7 environmental hazards, 747, 751-3 labels, 751-3 NIOSH Pocket Guide, 749 occupational hazards, 747-9 safety issues, 744-9 HDL see High-density lipoprotein Heat of formation see Enthalpy of formation HEHP (1-hydroxyethyl hydroperoxide), 605, 638... 

The heats of formation of alkenes may sometimes be obtained by combining heats of hydrogenation with the heat of formation of the saturated product. This procedure has been used to obtain heats of formation of many strained alkenes. The heats of hydrogenation are usually measured in solution, and may differ somewhat from the value appropriate for the gas phase. Thus, the heats of formation of alkenes obtained in this way have a somewhat higher uncertainty than those obtained from heats of combustion. 

Table 11. Comparison of calculated and observed heats of formation of alkenes and cycloalkenes... 

In Tables 10 to 12 we show the heats of formation calculated by the various methods, together with their deviation from the experimentally observed values for alkanes and cycloalkanes, alkenes and cydoalkenes, and acetylenes and aromatic compounds. Table 13 shows a comparison of heats of formation of hydrocarbon radicals calculated by the MINDO methods. Finally, in Tables 14 and 15 we show the results of MINDO/1 calculations on a selection of oxygen- and nitrogen-containing compounds. 

Semi-empirical AMI calculations for alkyl-substituted 1,2,3-trioxolanes show that the products derived from trans-alkenes are systematically more stable than those arising from r -alkenes. The absolute values of the heat of formation increase as the substituents become more bulky <1997JOC2757>. 

With the alkenes the difference is greater wbutene-butene 1 3.5 kcal, trans-cis butene 2 1 kcal, the first-mentioned with the lower energy content (higher heat of formation or lower heat of combustion). Thus the equilibrium is displaced at higher temperatures towards the side of the less branched and normal isomers15. 

The mechanistic aspects of aromatic and alkene radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy. A review of the reactivity of trivalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics. " The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms. ... 

The same type of calculation can be continued for larger hydrocarbons. The enthalpy for any alkane can be obtained from rel. (2.2.30), and a similar correlation relation can be obtained from experimental heats of formation for alkenes (see Figure 2.2.6) ... 

Calculations suggested that the preferred elimination of 3-substituted andro-stanes to give A2- rather than A3-compounds could not be explained alone by differences in calculated heats of formation. Differences in the energies of the respective transition states appeared to be important.15 Dehydration of the cyclobutanol (5) gave largely the exo-alkene (7) but the tetradeuterio-compound (6)... 

Why do the adduct carbocations 46 not react with a nucleophile to give an adduct of type 48, as happens in electrophilic addition to double bonds The main reason is probably thermodynamic. The hypothetical reaction (5.32) is very exothermic, based on experimental heats of formation. If we attribute this exothermicity to the stabilization of the benzene nucleus, we obtain a value of 150 kJ mol-1 for this stabilization. This stabilization would be lost in reaction (5.31) if the adduct 48 were formed, but is regained if the substituted product 47 is produced. This stabilization does not apply to reaction with alkenes, so addition can take place if the reaction is favourable energetically. 

Heats of formation are calculated as a sum of the bond energies and other stabilizing and destabilizing (e.g., strain) increments for the structure. MM4 calculations include terms for contributions of higher-energy conformations. For a set of hydrocarbons ranging from methane and ethane to adamantane and bicyclo[2.2.2]octane, the heats of formation are calculated with a standard deviation of 0.353 kcal/mol. The MM4 system has also been applied to alkenes, ° aldehydes, and ketones. ... 

Proceeding as before, data for the alkynes were fitted (A. Y. Meyer, unpublished results) using parameters for structure and heat of formation (Table 8) based on experimental information which is even more limited than in the case of alkenes. 

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