Free enthalpy difference

Numerous disperse dyes are marketed in a metastable crystalline form that gives significantly higher uptake than the corresponding more stable modification. The molar free enthalpy difference can be used as a criterion of the relative thermodynamic stabilities of two different modifications [53]. Certain dyes can be isolated in several different morphological forms. For example, an azopyrazole yellow disperse dye (3.52) was prepared in five different crystal forms and applied to cellulose acetate fibres. Each form exhibited a different saturation limit, the less stable modifications giving the higher values [54]. 

The condition 1) states that unequal amounts of stereoisomers are produced or destroyed by the reaction together conditions 2) and 3) assure that the increase in chiral genus is due to a stereoselectivity which is caused by chiral influences alone. Condition 1) is an essential part of the definition it is interrelated with condition 3), and together they restrict the type of chirality increasing stereoselective reactions that will be called an asymmetric synthesis34. In fact, a stereoselective reaction is never infinitely selective , because this would require an infinite free enthalpy difference of stereoisomers, stereoisomeric transition states respectively (with thermodynamic control, or with productive selectivity) in the case of an idealized destructive selectivity infinite selectivity would be reached at infinite reaction time, when none of the considered stereoisomers would be left over3S ... 

The selectivity of a productive reaction refers to the relative amounts of P, P at the time of observation. The ratio of the amounts of P and P which are formed is the ratio of the corresponding rate constants, if the stereoselective is a pair of corresponding reactions53. If, however, the productive stereoselective reaction is a more complex kinetic scheme, then the ratio of the amounts of any two stereoisomeric products, P and P , which depends on time and pairs of the appropriate kinetic constants, has a positive lower bound and a finite upper bound. Both of these bounds are the ratios of two rate constants54. However, since the free enthalpy difference of stereoisomeric transition states is due to different non-bonded interaction and does not, as a rule, exceed 3 kcal/mole, and since the rate constant ratio depends on the free enthalpy difference, this ratio has a rather low upper bound. Accordingly, the stereoselectivity of productive reactions is generally low (50—90% relative yield of the preferred product in most cases). 

A good linear correlation was obtained (82MI1) between the free enthalpy differences for the open-chain and cyclic tautomers of 1 (R1 = Me R2 = alkyl) and the constants a and Es of substituent R2 ... 

Curtin-Hammett In stereogenic reactions leading from a substrate to two diastereomers, it is irrelevant Principle which conformer of the substrate is preferred if conformational isomerization is much faster than the reaction. The favored reaction is the one that takes place via the transition state with the lowest energy and may or may not derive from the preferred conformation of the substrate. The extent of diastereoselectivity encountered in the stereogenic reaction under scrutiny results from the competition between the favored and the disfavored reaction paths it depends exclusively on the free enthalpy difference between the competing, diastereomeric transition states. 

If we now replace the anomeric halogen by an acetoxy group, the equilibrium position is inverted, the weak anomeric effect not being able to compensate for two diaxial interactions at room temperature. The per-O-benzoylated derivative, however, leads to a 1 1 equilibrium. Here we recognize the limits of these analyses, as they only take into account the oxane part. With benzoate substituents, the essential point is no doubt elsewhere. Another aspect of the problem is that the conformational energy differences can appear weak as compared to the stacking forces in crystals. In crystalline form, per-(9-acetylated -D-xylo chloride adopts an all-equatorial conformation, just as the peracetylated fluoride, 85% tetraaxial in solution, crystallizes in the tetraequatorial form. In these cases where there is equilibrium in solution, conesponding to free enthalpy differences close to... 

The greatest advantage of enzymes is their often unsurpassed selectivity, especially in the differentiation between enantiomeric substrates when a pair of substrates has Gibbs free enthalpy differences AGrs between the R- and the 5-enantiomer of around 1-3 kJ/mol. With enzymes sometimes enantioselectivities of >99% e.e. can sometimes be achieved. 

The same type of molecular forces are involved as those in GC, except that, as the solutes no longer need to be volatile, ionic interactions can now be used to control retention, in addition to dispersive and polar interactions, as in GC. It will be seen that temperature can also be used to control retention in LC, in a somewhat similar manner to GC. The distribution coefficient of a solute between the two phases in LC will always result from both standard free entropy and enthalpy changes during distribution, as in GC. In addition, the separation of enantiomers will also depend primarily on a difference in the standard free entropy between the two isomers, that results from spatial variations and which are then augmented by standard free enthalpy differences. 

By the determination of the melting point (T ) and latent heat (Q), the melting entropy (S ) and Gibbs free enthalpy difference of the transition... 

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