Dimerization, of acrylic acids

A variety of substrates have been catalytically hydrogenated at room temperature and 1 -atm. hydrogen pressure by pentacyanocobaltate(ll) anion. Conjugation is required for the reduction of C=C bonds The effects of detailed molecular structure on reducibility and of cyanide-cobalt ratio on mode of reduction have been noted Poisoning and reactivation of the catalyst as well as the effect of alkali are described, and mechanisms are tentatively proposed for these phenomena It is concluded that the aging reaction of pentacyanocobaltate(ll) is reversible A dimerization of acrylic acids at elevated temperatures was found ... 

The dimerization of acrylonitrile is a cheaper route to the synthesis of highly valuable hexamethylenediamine, which is one component of the starting materials for nylon-6,6 [ 16,35] In some cases of the dimerizations of acrylic acid... 

The bottoms from the foreruns column are fed to the product column where the glacial acrylic acid of commerce is taken overhead Bottoms from the product column are stripped to recover acrylic acid values and the high boilers are burned The principal losses of acrylic acid in this process are to the aqueous raf finate and to the aqueous layer from the dehydration column and to dimerization of acrylic acid to 3-acryloxypropionic acid If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack tliis dimer back to acrylic acid (60). 

Dimerization of acrylonitrile is a cheaper way to synthesize highly valuable hexamethylenediamine [30]. In some cases of dimerization of acrylic acid esters, acrylonitriles, and acroleins, the direct C-H bond cleavage step seems to be involved in the catalytic reaction. At an early stage of catalytic dimerization of acrylonitrile, czs-l,4-dicyanobut-l-ene is formed as the major product, not trans-isomer [31]. This high czs-selectivity is suggested to indicate selective cleavage of C-H cis to CN by the metal coordinated to the nitrile group in side-on fashion. 

At approximately 20°C, the rate of formation of this acyloxypropionic acid is said to be negligible. The problem appears to be acute primarily during out-of-doors storage of acrylic acid in the summer months. Acryloxy-propionic acid does not seem to inhibit or interfere otherwise with the polymerization of acrylic acid, but, of course, its presence represents a variable impurity in the monomer. There appears to be no method of inhibiting the dimerization of acrylic acid. 

The relative abundance of each of these species can be followed for instance by infrared analysis (4, 5). Figure 3 shows the I.R. spectrum of acrylic acid in the 17 O O cm l region. The main peak at 1705 cm-1 is due to the vibration of the carbonyl group in the cyclodimeric form, whereas the shoulders at 1730 and 1740 cm-l correspond to "open dimers" and "linear oligomers". 

Figure 4. Ratios of optical densities at 1730 cm1 (linear oligomers)/1705 cm"1 (cyclic dimers) as a function of mole fraction of acrylic acid in various solvents (1) dioxane (2) methanol (3) acetic acid (4) chloroform (5) toluene (6) CClk (4).

The reduction of acrylic acid was attempted at elevated temperatures. Surprisingly, the reaction was found to yield not only propionic acid, but also the dimer, a-methylglutaric acid. When the reaction was conducted in the absence of hydrogen, the product obtained was 3-methylglutaconic acid, which apparently is the precursor of the saturated dimer formed in a hydrogen atmosphere. Similarly, methacrylic acid yielded a-methylene-y,y-dimethylglutaric acid when heated with cyanocobaltate (II) in the absence of hydrogen. Its structure was established via ozonolysis. Similar dimerizations have been reported for acrylic acid (I, 14), methacrylate ester (7, 11), crotonic acid (13), and its diethylamide (15). 

Figure 22. Spectra in the gas phase at 323 K of hydrogenated (right part) and deuterated (left part) of acrylic acid dimer. Grayed Experimental line shape according to ref. [99]. Thick line Theoretical line shape according to Eq. (279).

Acidity is determined by glc or titration, and the dimer content of acrylic acid by glc or a saponification procedure. The total acidity7 is corrected for the dimer acid content to give the value for acrylic acid. 

The hydrodimerization of acrylic acid to adipic acid is a tail-to-tail addition hydrodimerization of methacrylic acid and crotonic acid produces mainly the head-to-head and head-to-tail dimers [48]. 

Methyl acrylate can be dimerized to give a molecule that can be hydrogenated to dimethyl adipate the latter, in turn, can be hydrolyzed to yield AA. Methyl acrylate is synthesized by esterification of acrylic acid, which is obtained by the two-step oxidation of propylene. However, the overall scheme requires several reaction steps, and investment requirements may be large. 

Dimer formation, which is favored by increasing temperature, generally does not reduce the quality of acrylic acid for most applications. The term dimer includes higher oligomers formed by further addition reactions and present in low concentrations relative to the amount of dimer (3-acryloxypropionic acid). Glacial acrylic acid should be stored at 16—29°C to maintain high quality. 

Chiral samarium (II) complexes have also been applied towards the hydrodimerization of acrylic acid amides [16]. Such reactions involve the ligand-controlled dimerization of conjugated ketyl radicals in the enantioselective formation of 3,4-tra .y-disubstituted adipamides (Eq. 11). Yields were mainly low, often under 40% and enantiocontrol was modest with selectivities ranging from around 50-85% ee. A nine-membered chelated transition state 37 is used to rationalize the stereoselectivity of the dimerization where the ligand-bound conjugated ketyl radicals are oriented cis to each other on the metal assuming an octahedral geometry. 

These nickel compounds have high reactivities and high selectivities to substrates and are easily handled in experimental operations. Then, these compounds are widely available as reagents and catalysts for organic syntheses [61-72]. In particular, the production of acrylic acid by Reppe reactions, the production of butene by the dimerization of ethylene, and the synthesis of 1,5-cyclooctadiene or 1,5,9-cyclododecatriene by the dimerization or trimerization of butadiene, are well known as reactions using nickel catalysts, shown in eqs. (19.35)-(i9-38) [61,65,72-77]. 

The dimeric structure of acrylic acid may be represented by structure I. 

Water-soluble polymers obtained through a radical polymerization [e.g., poly(acrylic acid) PAA] often contain sodium sulfate Na2S04 as a decomposition product of the initiator. The peak of Na2S04 is eluted before the dimer. In the interpretation of the chromatogram, a typical GPC program has to be truncated before the Na2S04 peak, or at a Mpaa value of about 200. The calibration curve in this region can be flattened by an additive small pore column as well, but the principle problem remains unsolved. 

On the other hand, a good correlation was established between auto-acceleration and the type of molecular association involving the monomer in the system. Pure acrylic acid associates by hydrogen bonds to form "cyclic dimers" and "linear oligomers". The two species are in equilibrium. 

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

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