Heat of isomerization

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

Wiener H (1947b) Correlation of heats of isomerization, and differences in heats of vaporization of isomers, among the paraffin hydrocarbons. J. Am. Chem. Soc. 69 2636-2638. 

Ray, J. D. (1962). Heat of isomerization of peroxynitrite to nitrate and kinetics of isomerization of peroxynitrous acid to nitric acid. J. Inorg. Nttcleotide Chem. 24, 1159-1162. 

Amett and co-workers,75 76 in a series of investigations, have determined heats of ionization (AHi) of secondary and tertiary chlorides and alcohols in SbF5-S02ClF and HS03F-SbF5-S02ClF solutions, respectively, at low temperatures. They have also measured heats of isomerizations of secondary to tertiary carbocations in the superacid media. These measured thermochemical data have been useful to determine the intrinsic thermodynamic stability of secondary and tertiary carbocations. 

Phenylamino-l,2,3,4-thiatriazole 87 in principle can be prepared by heating of isomeric l-phenyltetrazole-5-thiol 170 (Equation 18) <1967JOC3580>. The formation of the thiatriazole 87 is accompanied by significant formation of phenylaminonitrile. Therefore, we recommend the use of other synthetic methods (see Section 6.09.8), for instance, the nitrosation of 4-phenyl-3-thiosemicarbazide < 1971JIC843, 1981JIG1087 >. 

One of the crucial points in the debate about the norbornyl cation has been the extra stability gained from the presumed o-bridging in comprison with a classical carbocation. This stabilization was recently determined calorimetric-ally from the heats of isomerization of the 4-methyl-2-norbornyl cation [212] to the tertiary 2-methyl-2-norbornyl cation [213] in SbFj-SOgClF by Arnett et al. (1980), thereby avoiding the large initial state contributions that follow the use of different neutral precursors. As a classical reference system the... 

In general chemistry, in particular in discussion of isomerization equilibria, we may well require accuracy a good deal higher than this. Our only information about heats of isomerization of 1 or 2 kcal rnnle t may well come from combustion experiments where the total heat quantities measured are perhaps some thousands of kcal mole b At present the greatest accuracy normally attained is about 0 02 per cent, but Parks has pointed out that in- .c uraries in thermal data have such a large effect on equilibrium ( instants that a tenfold improvement in accuracy is desirable. 

The "heat of isomerization" has been reported as 27.5 5.0 kcal mol" by Colburn et al. (9). The derivation of this value is unknown. The authors observed three values of the equilibrium constant for the isomerization reaction. Third law analysis of their data gives Aj.H (298.15 K) = 2.52 0.05 kcal mol" for the reaction cis NgFg(g) - trans NgFg(g) which is in good agreement with Armstrong and Sidney s data. 

As pointed out by Ettinger et al. (4), the trans N F is the less active isomer which is different from the active isomer by Its (1) vapor pressure (2) boiling point (3) heat of vaporization (4) critical temperature (5) melting point (6) mass spectral cracking pattern (7) NMR spectrum (8) infrared spectrum (9) heat of isomerization (10) reactivity with mercury and glass. 

Wiener, H. (1947a). Correlation of Heat of Isomerization, and Differences in Heats of Vaporization of Isomers, among the Paraffin Hydrocarbons. J.Am.Chem.Soc., 69,2636-2638. 

Further compelling evidence indicating additional stabilization of the 2-norbornyl cation comes from Arnett et al. s measured heats of isomerization of secondary cations into tertiary cations.The measured heat of isomerization (A// ) of 4-methyl-2-norbornyl cation (143) to the 2-methyl-2-norbornyl cation (142) is -6.6kcalmoF [Eq. (5.20)]. In contrast, the related isomerization of the 2-butyl cation (58) to the tert-butyl cation (50) involves a difference of -14.2kcalmoTi [Eq. (5.21)] ... 

The estimated value of four gauche for the skew-pentane interaction was confirmed experimentally by Allinger and Miller (iO), who obtained 3.70 kcal/mole for the heat of isomerization of cm-1,1,3,5-tetramethyl-cyclohexane to the trans isomer ... 

Experimentally, the enthalpy differences have been determined from heat-of-combustion data and from measurements of equilibrium ratios over a range of temperatures. From the heats of combustion of cis-decalin (1502.92 0.22) and liquid phase) and 3.09 0.77 kcal/mole (vapor phase). Equilibrium ratios were determined in the liquid phase by Allinger and Coke (48) and in the vapor phase by Frye (49). Within... 

Heats of isomerization can be calculated directly from the equilibrium constants for the reactions shown above because there are no changes in symmetry and the other conformational isomers (boat conformations) of sufficient stability to introduce significant entropy-of-mixing terms. The liquid-phase heats of isomerization in Table XX for aa ial-l-methyl-fm s-decalin (cis-anti isomer) to equa-anti isomer) decrease at lower temperature consistent with the greater volatility of the more stable isomer, appropriate to the 5° difference in boiling points. At the highest experimental temperature, the liquid-phase heats of isomerization closely approach the 2.10 kcal/mole observed in the vapor phase. The analytical data are in Table XXI. 

This problem concerns thcrmod Tiamic equilibrium and multiple reactions. It is usually assigned at th.e graduate level. The heat of isomerization must be calculated at temperature below lOO C where no other reactions are dominate alternate to F6-5. 

In 1980 Arnett determined the heat of isomerization of the secondary 4-methyl-2-norbornyl cation to the tertiary 2-methyl-2-norbomyl ion in SbFj— SOjFQ in such an experiment the stabilities of neutral molecules are of no significance. R rrangement of a 4-methyl-2-norbomyl cation into a 2-methyl-2-norbomyl releases 6.57 0.41 kcal/mole in contrast, rearrangement of sec-butyl to tert-butyl ion releases 14.20 + 0.60 kcal/mole. Thus, the secondary 2-norbomyl ion is more stable than the usual s ondary ions by 7.5 kcal/mole the 4-methyl group is assumed to exert an insignificant effect on the charge at C. Also the ionization heats (AHj) of 2-exo-norbomyl chloride and 4-methyl-2-exo-norbornyl chloride into the respective secondary ions are very close to each other —23.16 + 0.43 and —22.20 0.49 kcal/mole at —100 °C. All these data indicate a specific stabilizing effect in the secondary 2-norbomyl ions. 

In each case the trans isomer has a higher heat of hydrogenation, and therefore a less negative heat of formation than the corresponding cis isomer. The heats of isomerization trans cis) calculated for the eight-, nine- and ten-membered cyclo-olefins are —9 2,... 

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