Polyamides and polyesters are polymers.
Polyamides can occur naturally or can be manufactured.
(a) Part of the structure of a polyamide is shown in Fig. 6.1.
(i) On Fig. 6.1, draw a circle around one amide linkage.
(ii) Complete Fig. 6.2 to show the structures of the two monomers needed to make the polymer in Fig. 6.1.
Show all of the atoms and all of the bonds in the functional groups.
(iii) Name the other product formed in this polymerisation.
(iv) State the term given to natural polyamides.
(v) Name the type of monomers which are used to make natural polyamides.
(vi) One of the monomers which forms part of a natural polyamide has three carbon atoms.
Complete Fig. 6.3 to show the displayed formula of this monomer.
(b) PET is a polyester.
(i) Name the two types of monomer molecules needed to make polyesters.
(ii) Draw part of the structure of PET which shows two repeat units.
Show all of the atoms and all of the bonds in the linkages.
▶️ Answer/Explanation
(a)(i) One amide linkage circled (the -CO-NH- group)
(a)(ii) The two monomers are:
1. A dicarboxylic acid with structure HOOC-R-COOH (where R is the carbon chain)
2. A diamine with structure H₂N-R’-NH₂ (where R’ is the carbon chain)
Both must show all atoms and bonds in the functional groups.
(a)(iii) Water (H₂O) is the other product formed in this condensation polymerisation.
(a)(iv) Natural polyamides are called proteins.
(a)(v) The monomers used to make natural polyamides are amino acids.
(a)(vi) The displayed formula should show 2-aminopropanoic acid:
H₂N-CH(CH₃)-COOH or fully displayed with all bonds shown.
(b)(i) The two types of monomers needed to make polyesters are:
1. Dicarboxylic acids
2. Diols
(b)(ii)
The structure of PET showing two repeat units should include:
– The ester linkage (-COO-) between units
– The benzene rings in the main chain
– The continuation bonds at both ends to show it’s part of a larger polymer
Example: [-CO-C₆H₄-CO-O-CH₂-CH₂-O-] repeated twice with proper bonds shown
Natural polyamides are polymers made from amino acid monomers.
(a) State the type of polymerisation reaction that occurs when natural polyamides form.
(b) State the term given to natural polyamides.
(c) An amino acid is represented as shown in Fig. 6.1.
Complete Fig. 6.2 to show the general structure of an amino acid.
Show all of the atoms and all of the bonds in the functional groups.
(d) Three different amino acids are represented as shown in Fig. 6.3.
Complete the diagram in Fig. 6.4 to show the part of the structure of the natural polyamide that forms when the three amino acids, A, B and C, combine. Show all of the atoms and all of the bonds in the linkages.
(e) A mixture of the three amino acids, A, B and C, can be separated and the amino acids identified using paper chromatography.
Complete the equation for \( R_f \).
\[R_f =\]
(f) A sample of the mixture of the three amino acids, A, B and C, is placed onto the baseline and a chromatogram is allowed to develop as shown in Fig. 6.5.
The finished chromatogram is shown in Fig. 6.6.
The amino acids, A, B and C, are colourless. Water is used as the solvent.
(i) Explain why the baseline is drawn in pencil.
(ii) State the type of substance used to make the colourless amino acids visible on the chromatogram in Fig. 6.6.
(iii) Explain why in Fig. 6.6 only two spots are seen from the mixture of three amino acids.
(iv) Suggest how the experiment can be changed to separate all three amino acids.
▶️ Answer/Explanation
(a) condensation
Natural polyamides form through condensation polymerization where amino acids join together, releasing water molecules in the process.
(b) proteins
Natural polyamides are commonly known as proteins, which are essential biological macromolecules made from amino acid monomers.
(c) The general structure of an amino acid is:
This shows the amino group (NH2), carboxyl group (COOH), hydrogen atom, and R group (variable side chain) all connected to a central carbon atom.
(d) The polymer structure would show:
This shows the amide linkages (peptide bonds) between the amino acids, with the R groups extending from the central carbon atoms.
(e) \[R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}\]
The Rf value is a ratio used in chromatography to identify substances based on how far they travel compared to the solvent front.
(f)(i) pencil is insoluble (in solvent)
Pencil marks won’t dissolve in the solvent and interfere with the chromatography results, unlike ink which might run.
(f)(ii) locating agent
A chemical like ninhydrin is used to react with amino acids and produce colored spots that can be seen.
(f)(iii) same \( R_f \) value
Two of the amino acids must have identical or very similar Rf values in this solvent system, causing them to appear as a single spot.
(f)(iv) use a different solvent
Changing the solvent system would alter the relative solubilities and migration rates of the amino acids, potentially separating all three.