Referring to this webpage, Condensation polymerisation of glycine

A simple counting argument would tell us that:
2 glycine molecules react, releasing 1 water molecule
3 glycine molecules react, releasing 2 water molecules
4 glycine molecules react, releasing 3 water molecules

in other words, n glycine molecules react, releasing n-1 water molecules

So, Why does polymerisation of n glycine molecules give n water molecules?

Is it because the author assumes that the polyglycine molecule forms a loop (end-to-end) instead of a "straight chain"?

  • 5
    $\begingroup$ n/(n-1) converges to 1 for large n. If you do not know n, but you know it is large, you can safely count with 1:1 glycine:water approximated ratio. $\endgroup$
    – Poutnik
    Aug 5 at 5:56
  • 3
    $\begingroup$ Don't call it polymerisation, but polycondensation. $\endgroup$
    – Karl
    Aug 6 at 6:39

Because the product as pictured is not the complete polymer; it includes only $n$ units (called "mers") in the middle of the chain and not the complete polymer which has more than $n$ mers. The missing mers include the ends that retain extra hydrogen and oxygen atoms and would make the number of eliminated water molecules one less. Think of the case $n=10$ but the actual polymer has 30 units mers with only the middle 10 mers shown in the quoted product. The 10 mers shown eliminate 10 out of the 29 water molecules that emerge from the complete 30-mer chain including the proper ends.

Textbooks often do this because, supposedly, the ends are negligibly small portions of the long chain. Whether they truly are negligible in a chemical sense is something I will leave to users specializing in polymers or especially in proteins.


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