Chemistry 5070/21 — May/June 2016

Walkthrough

The question asks for the polymer made from propene. Propene is an alkene with the formula $\text{CH}_2=\text{CHCH}_3$. When propene undergoes addition polymerisation, the double bond opens up to form a carbon chain with methyl groups ($\text{CH}_3$) attached to alternating carbon atoms. Looking at Fig. 1.1, the structure labelled poly(propene) shows a carbon backbone with $\text{CH}_3$ and $\text{H}$ groups attached, matching the monomer propene.

Key Takeaways

Addition polymers are formed from alkene monomers. The repeat unit of the polymer contains the same atoms as the monomer, just with the double bond changed to a single bond in the chain.

Common Mistakes

Confusing poly(propene) with poly(ethene). Poly(ethene) has only hydrogen atoms attached to the carbon backbone, whereas poly(propene) has methyl groups ($\text{CH}_3$).

Things to Be Careful About

Ensure you match the name of the polymer in the figure to the monomer name. Propene gives poly(propene), not poly(ethene) or PVC.

Walkthrough

Hydrogen chloride ($\text{HCl}$) contains hydrogen and chlorine. For a polymer to produce $\text{HCl}$ on combustion, it must contain chlorine atoms in its structure. Looking at the figures:

• PVC (poly(chloroethene)) in Fig. 1.1 has chlorine atoms ($\text{Cl}$) attached to the carbon backbone. Combustion of PVC releases $\text{HCl}$ gas.
• PTFE contains fluorine ($\text{F}$), which would produce $\text{HF}$, not $\text{HCl}$.
• The other polymers (Terylene, nylon, starch, poly(propene), protein, polyester, poly(ethene)) do not contain chlorine.

Key Takeaways

Polymers containing halogens can produce hydrogen halides when burned. PVC is poly(chloroethene) and contains chlorine.

Common Mistakes

Choosing PTFE because it is a halogen-containing polymer. PTFE contains fluorine, producing hydrogen fluoride ($\text{HF}$), not hydrogen chloride ($\text{HCl}$).

Things to Be Careful About

Read the chemical symbols in the structures carefully. $\text{Cl}$ is chlorine, $\text{F}$ is fluorine.

Walkthrough

Amino acids are the monomers of proteins. Proteins are natural polyamides (polypeptides) linked by peptide bonds, which are amide linkages ($-\text{CO}-\text{NH}-$). In Fig. 1.2, protein is shown as a chain with amide linkages connecting different blocks (representing amino acid residues). Hydrolysis of the amide bonds in proteins breaks them down into individual amino acids.

Note: Nylon is also a polyamide, but it is a synthetic polymer made from diamines and dicarboxylic acids, not amino acids. Therefore, protein is the correct answer for hydrolysis to amino acids.

Key Takeaways

Proteins are natural polyamides that hydrolyse to amino acids. The linkage is the amide (peptide) bond, $-\text{CO}-\text{NH}-$.

Common Mistakes

Choosing nylon. While nylon has amide linkages, it does not hydrolyse to amino acids; it hydrolyses to diamines and dicarboxylic acids.

Things to Be Careful About

Distinguish between natural polyamides (proteins) and synthetic polyamides (nylon) based on the hydrolysis products.

Walkthrough

Simple sugars (monosaccharides like glucose) are the monomers of polysaccharides. Starch, shown in Fig. 1.1, is a polysaccharide made of many glucose units linked together. Hydrolysis (breaking down with water) of starch breaks the bonds between the glucose units, producing simple sugars (glucose).

Key Takeaways

Starch is a natural polymer (polysaccharide) of glucose. Hydrolysis yields simple sugars.

Common Mistakes

Confusing starch with proteins or polyesters. Starch is the carbohydrate polymer.

Things to Be Careful About

The taxonomy classifies carbohydrates under a legacy topic, but starch hydrolysis is a standard concept. Ensure you recognise the structure of starch (repeating sugar units linked by oxygen atoms) in Fig. 1.1.

Walkthrough

Fats are esters formed from glycerol and fatty acids. They contain ester linkages ($-\text{COO}-$).

• In Fig. 1.1, Terylene is a polyester. Its structure shows alternating blocks linked by $-\text{O}-\text{CO}-$ groups, which are ester linkages.
• In Fig. 1.2, polyester also shows the same ester linkage ($-\text{COO}-$).
  Both Terylene and the generic polyester contain the ester linkage found in fats. The mark scheme accepts either.

Key Takeaways

Fats contain ester linkages. Polyesters (like Terylene) also contain ester linkages. This is the link between condensation polymerisation of dicarboxylic acids/diols and the chemistry of fats.

Common Mistakes

Choosing nylon or protein. These contain amide linkages ($-\text{CO}-\text{NH}-$), not ester linkages.

Things to Be Careful About

The ester linkage is $-\text{COO}-$ (or $-\text{C}(=\text{O})-\text{O}-$). Ensure you can spot this in the displayed formulae. In Terylene, it is $-\text{O}-\text{C}(=\text{O})-$.