Methyl acetate iodide

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

Water with ethanol, n-propanol and isopropanol, tcrt-butanol, propionic acid, butyric acid, pyridine, methanol with methyl iodide, methyl acetate, chloroform,... 

The present procedure was developed from those of Wallach and Freylon, based upon the general method discovered by Leuckart. a-Phenylethylamine also can be prepared satisfactorily by the reduction of acetophenone oxime with sodium and absolute alcohol or sodium amalgam, but the reagents are more expensive and the processes less convenient. The amine has been obtained by reducing acetophenone oxime electro-lytically, by reducing acetophenone phenylhydrazone with sodium amalgam and acetic acid, from a-phenylethyl bromide and hexamethylenetetramine, and by the action of methyl-magnesium iodide upon hydrobenzamide, as well as by other methods of no preparative value. 

General Procedure for Batch Carbonylation of Methanol in the Absence of Methyl Iodide. A complete set of procedures appears in ref. 5 bnt the following procedure is representative of a methanol carbonylation. To a 300 mL Hastelloy C-276 autoclave was added 0.396 g (1.5 mmol) of RhCl3 3H20, 112.0 g (0.507 mol) of N-methyl pyridinium iodide, 30.0 g (0.5 mol) of acetic acid, and 64.0 g (2.0 mol) of methanol. The mixture was heated to 190°C under 250 psi (1.72 MPa) of 5% hydrogen in carbon monoxide. Upon reaching temperatnre the gas feed was switched... 

Methyl acetate Methyl acrylate Methyl r-butyrate Methyl w-butyrate Methyl chloride Methyl ethyl ketone Methyl formae Methyl iodide Methyl propionate Mehyl propyl ketone Methyl sulfide Naphthalene Nitric acid Nitric acid, 60% Nitrobenzene Nitrogen dioxide Nitrotoluene Octane Octyl alcohol Pentachloroethane Pentane Phenol... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

X-ray structure analyses of Rh(COCH3)(I)2(dppp) (14) and [Rh(I I)(I)(//-I)(dppp)]2 (15), where dppp l,3-bis(diphenylphosphino) propane, were reported. Unsaturated complex (14) possesses a distorted five-coordinate geometry that is intermediate between sbp and tbp structures.69 Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. By using labeled methyl formate (H13C02CH3) it was shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate.70... 

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

The only dependencies noted in the kinetic studies were first-order dependencies on iodide promoter and rhodium concentrations. Thus there was no observed effect of varying methanol concentration, and the partial pressure of carbon monoxide had no effect on the reaction rate. Similarly, the concentration of the products, methyl acetate and acetic acid, has no effect on the reaction rate. Thus we have the unusual situation of a reaction, CH3OH + CO — CH3COzH, in which the concentrations of the reactants and product have no kinetic influence. 

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. 

In a working catalytic system, however, the principal solvent component is acetic acid, so esterification (Eq. 2) leads to substantial conversion of the substrate into methyl acetate. Methyl acetate is activated by reaction with the iodide co-catalyst (Eq. 3) ... 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

Evidence has been presented that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2l2]"> the rate-determining step in the Rh cycle [12]. The precise mechanism of this promotion remains unclear formation of a highly nucleophilic dianion, [Rh(CO)2l3]2 , has been suggested, although there is no direct spectroscopic evidence for its detection. Possible participation of this dianion has been considered in a theoretical study [23]. An alternative nucleophilic dianion, [Rh(CO)2l2(OAc)]2 , has also been proposed [31,32] on the basis that acetate salts (either added or generated in situ via Eq. 7) can promote carbonylation. Iodide salts have also been found to be effective promoters for the anhydrous carbonylation of methyl acetate to acetic anhydride [33]. In the absence of water, the catalyst cannot be maintained in its active form ([Rh(CO)2l2]") by addition of Lil alone, and some H2 is added to the gas feed to reduce the inactive [Rh(CO)2l4]. ... 

The iodide (HI(aq)) concentration is also influenced by the water and methyl acetate concentrations (according to Eq. 11) ... 

Scheme 16 Acid catalysis of conversion of methyl acetate into methyl iodide...

The two catalyst components are rhodium and iodide, which can be added in many forms. A large excess of iodide may be present. Rhodium is present as the anionic species RhI2(CO)2. Typically the rhodium concentration is 10 mM and the iodide concentration is 1.5 M, of which 20% occurs in the form of salts. The temperature is about 180 °C and the pressure is 50 bar. The methyl iodide formation from methanol is almost complete, which makes the reaction rate also practically independent of the methanol concentration. In other words, at any conversion level (except for very low methanol levels) the production rate is the same. For a continuous reactor this has the advantage that it can be operated at a high conversion level. As a result the required separation of methanol, methyl acetate, methyl iodide, and rhodium iodide from the product acetic acid is much easier. 

In many applications acetic acid is used as the anhydride and the synthesis of the latter is therefore equally important. In the 1970 s Halcon (now Eastman) and Hoechst (now Celanese) developed a process for the conversion of methyl acetate and carbon monoxide to acetic anhydride. The process has been on stream since 1983 and with an annual production of several 100,000 tons, together with some 10-20% acetic acid. The reaction is carried out under similar conditions as the Monsanto process, and also uses methyl iodide as the "activator" for the methyl group. 

In a kinetic study of the esterification of acetic acid with methanol in the presence of hydrogen iodide, iodimethane was identified as a by-product. The authors propose that this derives from iodide ion attack on protonated methanol. However, attack by iodide ion on protonated methyl acetate (10) is more likely, since acetic acid is a better leaving group than ethanol. 

Methyl acetylphosphoramidothioate, see Acephate Methylacrylamide, Acrylamide Methyl alcohol, see Methyl acetate, Methyl acrylate. Methyl iodide... 

With a ruthenium promoter (added as [Ru(CO)4l2]), r(CO) bands due to Ru iodo-carbonyls dominated the spectrum, precluding the easy observation of iridium species. Before injection of the Ir catalyst, absorptions due to [Ru(CO)2l2(sol)2], [Ru(CO)3l2(sol)] and [Ru(CO)3l3] are present. After injection of the iridium catalyst (Ru Ir = 2 1), [Ru(CO)3l3] becomes the dominant Ru species (Figure 3.11(b)). The observations indicate that the Ru(II) promoter has a high affinity for iodide and scavenges Hl(aq) as H30 [Ru(CO)3l3] . An indium promoter is believed to behave in a similar manner to form H30 [Inl4] . These promoter species also catalyse the reaction of Hlj q) with methyl acetate (Eq. (3)), which is an important organic step in the overall process. 

Reppe reaction involves carbonylation of methanol to acetic acid and methyl acetate and subsequent carbonylation of the product methyl acetate to acetic anhydride. The reaction is carried out at 600 atm and 230°C in the presence of iodide-promoted cobalt catalyst to form acetic acid at over 90% yield. In the presence of rhodium catalyst the reaction occurs at milder conditions at 30 to 60 atm and 150-200°C. Carbon monoxide can combine with higher alcohols, however, at a much slower reaction rate. 

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). 

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