Acetyl iodide equilibrium

Reaction (9) generates methyl iodide for the oxidative addition, and reaction (10) converts the reductive elimination product acetyl iodide into the product and it regenerates hydrogen iodide. There are, however, a few distinct differences [2,9] between the two processes. The thermodynamics of the acetic anhydride formation are less favourable and the process is operated much closer to equilibrium. (Thus, before studying the catalysis of carbonylations and carboxylations it is always worthwhile to look up the thermodynamic data ) Under standard conditions the AG values are approximately ... 

Reaction (12) ensures that acetyl iodide is converted to the product, because in the case that M=H the equilibrium lies to the left. The second reaction (13) is slow, and the equilibrium shifts to the right with decreasing size of the cation. With lithium as the cation, this reaction has the highest rate and it is most complete. (Li+, K=0.388, k=8 l.mol. h Na+, K=0.04, k=2.6). Hence, this combination of reactions necessitates the use of Lil instead of HI, and it adds a third cycle to the reaction scheme, namely the lithium cycle, which must generate Mel. (In Figure 6.5 the acid cycle and the salt cycle are drawn as two coinciding cycles). At low concentrations this cycle may be rate-determining. 

A nucleophilic attack by 4.7 on CH3I produces 4.8 and I. Conversion of 4.8 to 4.9 is an example of a carbonyl insertion into a metal alkyl bond. Another CO group adds onto the 16-electron species 4.9 to give 4.10, which in turn reacts with I to eliminate acetyl iodide. Formation of acetic acid and recycling of water occur by reactions already discussed for the rhodium cycle. Apart from these basic reactions there are a few other reactions that lead to product and by-product formations. As shown in Fig. 4.4, both 4.9 and 4.10 react with water to give acetic acid. The hydrido cobalt carbonyl 4.11 produced in these reactions catalyzes Fischer-Tropsch-type reactions and the formation of byproducts. Reactions 4.6 and 4.7 ensure that there is equilibrium between 4.7 and 4.11. 

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