Hydrocarbons free energy change

Table 3 Free energy changes in the ionic dissociation of carbon-carbon cr bonds in some hydrocarbons at 25°C. ...

This expression makes possible the examination of extrathermodynamic correlation between the free energy changes for two dissociation equilibria, i.e. AGhet(R-R ) values of the hydrocarbons and AGhet(ROH + HsO" ) values of the alcohols. 

Measuring enthalpy changes for the dissolution of hydrocarbons, such as alkanes, in water shows that heat is evolved, i.e., A/f is negative and energetically water and alkanes attract each other. However, such attraction does not make alkanes soluble in water to any appreciable extent. This is because the free energy change AGsomtion opposes the process and is positive. 

FIGURE 10.5 Standard free energy change, in various liquid hydrocarbons, versus temperature upon electron trapping from the quasi-free state according to the quasi-ballistic model. Reproduced from Mozumder, (1996), with the permission of Am. Chem. Soc. ... 

Since other possible transformations, such as, formation of dimethyl ether, higher alcohols, and hydrocarbons, are accompanied with higher negative free-energy change, methanol is thermodynamically a less probable product. Therefore, solely on a thermodynamic basis, these compounds as well as methane should be formed in preference to methanol. To avoid the formation of the former compounds, the synthesis of methanol requires selective catalysts and suitable reaction conditions. Under such conditions, methanol is the predominant product. This indicates that the transformations leading to the formation of the other compounds are kinetically controlled. In the methanol-to-hydrocarbon conversion, dimethyl ether generally is converted similarly to methanol. 

Method of Amidon and Anik The method of Amidon and Anik [9] applicable to hydrocarbons and is based on the group additivity of the surface area. The approach is to model the Gibbs free energy change for the vaporization process, AGV, as an additive parameter according to the following equation ... 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

Fig. 10. Dependence of log k-eI of return electron transfer within ion pairs (A- D+ ) in acetonitrile on the free energy change (solid line experimental) — according to Ref. [70]. A = 9,10-dicyanoanthracene, 2,6,9,10-tetracya-noanthracene D = aromatic hydrocarbons...

It appears that there are two temperatures of a universal nature that describe the thermodynamic properties for the dissolution of liquid hydrocarbons into water. The first of these, 7h is the temperature at which the heat of solution is zero and has a value of approximately 20°C for a variety of liquids. The second universal temperature is Ts, where the standard-state entropy change is zero and, as noted, Ts is about 140°C. The standard-state free energy change can be expressed in terms of these two temperatures, requiring knowledge only of the heat capacity change for an individual substance... 

The term mb — ma is the standard free-energy change for the transfer of an amphiphile molecule from the aqueous medium to the bulk amphiphilar phase. Since no information is available for this term at the present time, it is set equal to the standard free-energy change for transferring a hydrocarbon chain from an aqueous solution of monomer to a pure hydrocarbon phase.3a Consequently... 

The hydrocarbon core of a membrane is typically 30 A wide, a length that can be traversed by an a helix consisting of 20 residues. We can take the amino acid sequence of a protein and estimate the free-energy change that takes place when a hypothetical a helix formed of residues 1 through 20 is transferred from the membrane interior to water. The same calculation can be made for residues 2 through 21,3 through 22, and so forth, until we reach the end of the sequence. 

---