Thermodynamics of Addition-Elimination Equilibria

We have seen that both the forward and reverse reactions represented by the hydration-dehydration equilibrium are useful synthetic methods. 

We can prepare alcohols from alkenes, and alkenes from alcohols, but how do we control the position of equilibrium so as to maximize the yield of the compound we want  

The qualitative reasoning expressed in Le Chatelier s principle is a helpful guide a system at equilibrium adjusts so as to minimize any stress applied to it. For hydration-dehydration equilibria, the key stress factor is the water concentration. Adding water to a hydration-dehydration equilibrium mixture causes the system to respond by consuming water. More alkene is converted to alcohol, and the position of equilibrium shifts to the right. When we prepare an alcohol from an alkene, we use a reaction medium in which the molar concentration of water is high—dilute sulfuric acid, for example. 

Le Chatelier s principle helps us predict qualitatively how an equilibrium will respond to changes in experimental conditions. For a quantitative understanding, we need to examine reactions from a thermodynamic point of view. 

At constant temperature and pressure, the direction in which a reaction proceeds— that is, the direction in which it is spontaneous—is the one that leads to a decrease in free energy (G) 

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The 02 -elimination reactions maybe divided into three groups. Those peroxyl radicals that have an -OH or -NH function in the a-position make up the first group. Such peroxyl radicals play a major role in nucleobase peroxyl radical chemistry [cf. reactions (12) and (13)]. Upon deprotonation at die heteroatom by OH" [reactions (10) and (12)], the peroxyl radical anion is formed (cf. the enhancement of the acidity of the functions a to the peroxyl group discussed above for the thermodynamics of the various equilibria that are involved in these reactions see Goldstein et al. 2002). As before, the driving force for the elimination reaction is the formation of a double bond [in addition to the energy gain by the formation of the stabilized 02- radical [cf. reactions (11) and (13)]. 

Reductive elimination is the reverse of oxidative addition. This class of reaction forms products from the coupling of two covalent ligands at a single transition metal center (Equation 8.1) or two ligands from two different metal centers (Equations 8.2 and 8.3). This reaction is the product-forming step of many catalytic processes. Because oxidative addition and reductive elimination are the same reaction occurring in opposite directiorrs, the formation of products from oxidative addition or reductive elimination depends on the thermodynamics of the two processes. In some cases, equilibria between the two reactions have been observed directly, but more often the thermodynamics favor addition or elimination to a sufficient extent that high yields of either the addition or elimination products are obtained. Mormation on the thermodynamics for several types of oxidative addition and reductive elimination were provided in Table 6.2. 

Additions [reactions (b) and (d)] are normally favored by thermodynamics (Section 12-1). For elimination to occur, conditions have to be established to drive the equilibria the opposite way. In (a) the water lost in the reversible El process is protonated by the concentrated H2SO4, removing it from the equilibrium. No good nucleophiles are present therefore, the carbocation undergoes loss of a proton to form the alkene. In (c) the strongly basic ethoxide ion induces bimolecular elimination and... 

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