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Chitin is usually insoluble in water. How does its chemical structure explain this?

Furthermore, when chitin undergoes deacetylation, it becomes soluble and forms hydrogen bonds with water. How is this so?

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Chitin is a homopolymer of β(1,4)-linked N-acetyl-D-glucosamine residue .

enter image description here

The structure of chitin, an extended ribbon, is identical to that of cellulose, except that the $\ce{-OH}$ group on each C-2 is replaced by $\ce{-NHCOCH3}$, so the repeating units are N-acetyl-D-glucosamines in β (1→4) linkage.

enter image description here

The acetyl group can contribute to the formation of hydrogen bonds that can stabilise the crystalline structure. The chains of chitin form extended ribbons and pack side by side in a crystalline, strongly hydrogen-bonded form. The acetyl groups contribute to the formation of stable H-bonds along the inter sheet direction, fracture along this direction requires more energy to break these high occupancy H-bonds (which prevent association with water molecules).

As a result of the hydrophobic property of acetyl group, chitin is ordinarily insoluble in water. This explains the insolubility properties of chitin.

When deacetylated, the solubility apparently increase due to increased hydrophilicity of the introduced amino groups (as well as the weakened crystalline structure allowing formation on H-bonds with water molecules).

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  • $\begingroup$ hydrophobic/philic argument isn't good argument here $\endgroup$ – Mithoron Sep 16 '17 at 12:36
  • $\begingroup$ @Mithoron From most research articles I reviewed, this was often the case, at first I was tempted to conclude that there is no free -OH groups available to H-bond with water molecules, but then again looking at cellulose (which has free -OH instead of glucosamine on C2) it isn't soluble either. After a bit of digging thats when I realized that when chitin is deacetylated (with a strong base), it becomes more soluble woth the properties coming from the more hydrophilic amines hence the answer. $\endgroup$ – xavier_fakerat Sep 16 '17 at 12:43

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