Unfavorable entropy change

In a reaction in which the number of product molecules is equal to the number of reactant molecules, (e.g., A + B —> C + D), entropy effects are usually small, but if the number of molecules is increased (e.g., A —> B + C), there is a large gain in entropy because more arrangements in space are possible when more molecules are present. Reactions in which a molecule is cleaved into two or more parts are therefore thermodynamically favored by the entropy factor. Conversely, reactions in which the number of product molecules is less than the number of reactant molecules show entropy decreases, and in such cases there must be a sizable decrease in enthalpy to overcome the unfavorable entropy change. 

Decreases Decreases Only if unfavorable entropy change is offset by favorable enthalpy change... 

Regardless of the relative importance of polar and nonpolar interactions in stabilizing the cyclohexaamylose-DFP inclusion complex, the results derived for this system cannot, with any confidence, be extrapolated to the chiral analogs. DFP is peculiar in the sense that the dissociation constant of the cyclohexaamylose-DFP complex exceeds the dissociation constants of related cyclohexaamylose-substrate inclusion complexes by an order of magnitude. This is probably a direct result of the unfavorable entropy change associated with the formation of the DFP complex. Thus, worthwhile speculation about the attractive forces that lead to enantiomeric specificity must await the measurement of thermodynamic parameters for the chiral substrates. 

For the polymerization to proceed spontaneously, AG < 0. But AS < 0, because the system evolves to a more ordered state (the number of configurations in which free monomers may be placed in space decreases by the introduction of covalent bonds among themselves) thus, the entropy change does not favor polymerization. Then, the only possibility of getting AG < 0 is to have a significantly exothermic reaction (AH < 0) to counterbalance the unfavorable entropy change. 

The low stabilities of the oligo (ethylene glycol) diphenyl ether complexes are accounted for by unfavorable entropy changes upon complexation which are no longer important after attachment of aromatic donor end groups to the ligand... 

We noted earher that the reaction of an aldehyde with an alcohol to give an acetal is not favorable entropically. However, the simple expedient of using a diol to make the acetal ehminates this unfavorable entropy change. The most common alcohol used is for making cychc acetals 1,2-ethanediol (ethylene glycol). In this reaction, two molecules of reactant yield two molecules of product, and the entropy change is approximately zero. The acid catalyst is tolunesulfonic acid (TsOH), and benzene is the solvent. 

The [3 + 2] cycloaddition of nitronate 401 requires a very high temperature, presumably because of the unfavorable entropy change during the formation of the seven-membered ring [164]. Moreover, the reaction is not selective and formation of at least eight isomers has been observed. It has been proposed that the [3 + 2] cycloaddition is especially slow also because of steric interactions between the... 

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