Acetyl iodide

Acetyl iodide [507-02-8] M 170.0, b 108 /760mm. Purified by fractional distn. 

Acetyl cyclohexane sulfonyl peroxide, 7 Acetyl Iodide, 7 Acetyl thiourea, 8 Acetylaminofluorene, 7 Acetylene, 7... 

The final step is the reaction hetween acetyl iodide and methyl alcohol, yielding acetic acid and the promotor ... 

It is important that in the two organic equilibria involving iodide reaction (Equation (2)) shows complete conversion of methanol to methyl iodide, whereas the reaction with acetyl iodide shows complete conversion to acetic acid and hydrogen iodide (Equation (3)) ... 

The acetyl iodide that is released in the last step of the process reacts with water to produce acetic acid,... 

In an alternate procedure, the acetyl iodide is converted to methyl acetate if methanol is the solvent for the process. 

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

Under the reaction conditions for methanol carbonylation in which hydroxylic solvents are present, acetyl iodide would be solvolyzed very rapidly, giving either acetic acid or methyl acetate together with hydrogen iodide. The hydrogen iodide can rapidly react with more methanol to give methyl iodide to complete the iodide cycle. 

Transition metal catalysed carbonylation of methyl iodide then gives acetyl iodide (Eq. 4), which is rapidly hydrolysed to the product acetic acid (Eq. 5). The net result of the reactions described in Eqs. 2-5 is the carbonylation of methanol (Eq. 1) ... 

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. 

For instance, the CROP of EtOx using four different acetyl halide type of initiators showed that the rate of polymerization increases with the decreased basicity of the counter ion acetyl iodide < acetyl bromide < acetyl chloride. The apparent rates of polymerization of EtOx with different initiators are listed in Table 2. 

Table 2 Polymerization rates (in 10 L mol s ) of CROP of EtOx with different initiators at various temperatures - acetyl chloride (AcCl), acetyl bromide (AcBr), acetyl iodide (Acl), and 2-bromo-2-methylpropanoyl bromide (BrEB/B)...

Water is essential, since acetic acid is formed by the reaction between water and acetyl iodide or the Ni-acetyl complex. Acetic acid is also formed via the hydrolysis of any methyl acetate that is formed by methanol attack on acetyl iodide or the Ni-acetyl complex. 

The four-coordinate alkyl complex, LNiI(C0)CH3, may coordinate with carbon monoxide to regenerate the five coordinate alkyl species, and this leads to insertion to form Ni-acyl complex. This complex, LNil (CO)(COCH3), can be cleaved either by water yielding acetic acid or by methanol to give methyl acetate. However, in the presence of high iodide concentration formation of acetyl iodide may predominate (29). This step is reversible and can lead to decarbonylation under low carbon monoxide partial pressure. Similar decarbonylations of acyl halides by nickel complexes are known (34). 

Acetyl iodide is very reactive and it reacts efficiently with water or methanol leading to acetate compounds. Hydrolysis of acetyl iodide along with the subsequent conversion of methanol to methyl iodide are very rapid under the reaction conditions leading to a complete mechanistic cycle. 

Figure 2. Simplified Scheme of Rhodium Catalyzed Generation of Acetaldehyde and Acetyl Iodide.

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