Sn Sanyal Organic Chemistry Pdf 93
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From the perspective of bond activation chemistry, it has been illustrated that the functional groups of biomass can be categorized into two major groups, i.e., the hydroxyl groups and carboxyl groups, and have a synergistic effect on the conversion of biomass and plastic. In this context, biomass-derived aliphatic monomer products (for example, alkyl-esters, carboxylic acids, and formyl-esters) are often converted into the corresponding aromatic monomers. The aliphatic and aromatic monomer products can be further converted into the corresponding chemicals via Fischer-Tropsch and other catalytic reactions. For example, polyester oligomer hydrolyses into aromatic diamine monomers, and the amine monomers can be further functionalized into pharmaceuticals or other industrially important chemicals. With regard to the conversion of biomass and plastic, the industrial catalytic conversion of methyl esters or carbonate esters of alcohols can lead to the production of aliphatic ethers or esters. This type of transformation is a C-C bond formation under the catalysis of bases in aqueous media, which is an important step in the chemical industry. In this regard, the ester linkage is thought to be the least stable of the bonds in biomass and plastic, and is usually cleaved with difficulty. As such, the ester linkage often represents a bottleneck in the conversion of biomass and plastic, and bond activation approaches and catalyst designs for biomass and plastic have in some cases cross-contributed to each other. For example, phosphoric acid, that is, H3PO4, is an acidic catalyst for cleaving ester and amide bonds, and is frequently used in the industry as a catalyst for the saponification of vegetable oils and the hydrolysis of polymers205,206. This is because the phosphorus center is a good nucleophile, and can be considered as a potent weak base with high acidity207. In a similar fashion, basic catalysts (NaOH, KOH, and so on) have been widely used in solvolysis and transesterification processes for the conversion of biomass and plastic. This is because the OH group of nucleophiles can easily be deprotonated by these basic reagents, to form an active and stable transition state where the covalent bonds are cleaved207,208. Interestingly, the basicity of catalysts can be tuned via the introduction of amide and ether groups209.
The amide linkage can also be cleaved through solvolysis. The use of organic solvent provides convenient access to the acid, amine, and anhydride by-products, whereas in an aqueous phase, water will generate the carboxylic acid, ammonia, and possibly salts. This process is now being used in industry to transform different PAs into their respective amino acids and other valuable chemicals205,206,207. The selectivity of the products is much higher in aqueous phase than in organic phase (Fig. 5c). A typical example is the production of L-glutamic acid through hydrolysis of polyglutamic acid (PGA). In the presence of strong acid, the polymeric glutamates are broken down to glutamic acid (or hydrolysed to the free carboxylic acid), ammonia, and water. However, in the presence of base or neutral solution, the polymeric glutamates are depolymerized into glutamic acid and L-glutamate (or hydrolysed to their free form) through a series of acid catalyzed reactions. In terms of harshness, the processes conducted in organic solvents and aqueous phase are comparable. However, the selectivity of the carboxylic acid and amino acid are much higher in aqueous phase. It is noteworthy that the aqueous phase has the advantage of recovering the used base in the form of ammonium hydrogen carbonate. 827ec27edc