Chitosan is a linear polysaccharide composed of β-(1-4)-linked D-glucosamine (deacetylated unit) and — Organic Chemistry Chemistry Question
Natural Chelator from Shrimp Shell
Chitosan is a linear polysaccharide composed of β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shells of shrimp or other crustaceans with a strong base, like sodium hydroxide. The deacetylation of chitin by a strong base yields partially-acetylated chitosan as shown below:
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Chitosan can be used for a variety of purposes like helping plants to fight off fungal infections, preventing spoilage in wine, helping deliver drugs through the skin, being applied to reduce bleeding, and as an antibacterial agent. In the environmental aspect, chitosan can be used as an effective metal ion adsorbent. The adsorption capacities of chitosan for Cu(II), Hg(II), Pb(II) and Zn(II) are 79.94, 109.55, 37.2, 47.15 mg (g chitosan-1, respectively.
23.1 Suggest the preferred binding sites in completely-deacetylated chitosan for metal ion.
Model Answer
The preferred binding sites are indicated in the circles as follows:
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The selectivity of chitosan towards Pb2+ can be increased by modification with CS2 yielding partially modified chitosan (PMCS) as in the structure below:
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The absorption capacity toward Pb2+ was increased from 37.2 to 156.0 mg g-1 chitosan (Wang, N.; Zheng, P.; Ma, X. Powder Technol. 2016, 301, 1-9.)
23.2 Suggest the most preferred site(s) in chitosan for Pb2+ and draw the bond(s) between that preferred site(s) and Pb2+. Explain the increase in absorption capacity of PMCS toward Pb2+.
Model Answer
Pb2+ is a soft metal ion so it prefers to form bond with sulfur. This helps PMCS adsorb Pb2+ better.
To increase the sensitivity of the chitosan toward metal ion, the dye like meso-tetra(p-carboxyphenyl)porphyrin is chemically bonded with chitosan matrix. After the two acidic protons are removed, the four nitrogen atoms will bind with metal ion. The two axial sites of the metal ion normally bind with water molecules yielding the octahedral complex. Each metal ion gives characteristic absorption maximum in the visible range.
23.3 Suggest the bonds between meso-tetra(p-carboxyphenyl)porphyrin and chitosan if a weak protonic acid is used as a catalyst.
Model Answer
meso-tetra(p-carboxyphenyl)porphyrin can form ester bond with chitosan as shown in the proposed structure below:
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Or amide bonds are formed as below:
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23.4 If Fe2+ is being adsorbed by the chitosan-porphyrin adsorbent, draw the structure of the complex and predict the approximate d-orbital splitting in the proposed complex.
Model Answer
One of the structures in question 23.3 was chosen to bond with Fe2+ as the proposed structure below.
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The two vacant axial sites are bonding with water molecules.
Assuming that the crystal fields are equivalent along the x, y, and z axes, the splitting diagram of Fe2+ d orbitals is shown below:
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