How are organic molecules built in steps, and what are polymers?
Multi-step organic synthesis and reaction pathways, choosing reagents and conditions and improving purity and yield, addition and condensation polymers including polyesters and polyamides, and the disposal and biodegradability of polymers.
A CCEA A-Level Chemistry answer on organic synthesis and polymers, covering multi-step reaction pathways and the choice of reagents and conditions, methods of improving purity and yield, addition and condensation polymers including polyesters and polyamides, and the disposal and biodegradability of polymers.
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What this dot point is asking
CCEA wants you to plan multi-step organic syntheses and reaction pathways, choose reagents and conditions, improve purity and yield, describe addition and condensation polymers (polyesters and polyamides), and discuss the disposal and biodegradability of polymers.
Multi-step synthesis
Planning a route means knowing the standard one-step conversions between functional groups and stringing them together. The common moves CCEA expects include: alkene to alcohol (hydration, or addition of then hydrolysis); haloalkane to alcohol (aqueous , reflux); alcohol to aldehyde (dichromate, distil off) or to carboxylic acid (dichromate, reflux); alcohol to alkene (dehydration with concentrated ); and acid to ester (esterification with an alcohol and concentrated catalyst). A good strategy is retrosynthesis: start from the target, ask what could have made its functional group in one step, and work backwards until you reach the starting material, then write the route forwards with the correct reagent and condition on each arrow.
Improving purity and yield
The percentage yield is . Yields fall short of because of side reactions, incomplete reactions (especially reversible ones such as esterification), and losses during transfer and purification. Reversible reactions are pushed towards the product by removing a product as it forms (for example distilling off an ester) or by using an excess of the cheaper reactant. A separate idea is atom economy, the proportion of the reactant mass that ends up in the desired product, which is high for addition reactions and low when a large by-product is lost.
Addition and condensation polymers
In addition polymerisation the double bond of each monomer opens and the monomers link end to end, so the empirical formula of the polymer matches that of the monomer and nothing is given off; the repeat unit is drawn by opening the double bond and showing the two free bonds in brackets, as in for poly(ethene). In condensation polymerisation each new link forms between two functional groups with a small molecule (usually water, or if an acyl chloride is used) eliminated, so a polyester has repeating ester links along its backbone and a polyamide (such as nylon) has repeating amide links . Each monomer in a condensation polymer must have two reactive groups so that the chain can grow at both ends.
Disposal and biodegradability
Examples in context
Condensation polymers dominate the textile and packaging industries: PET (poly(ethylene terephthalate)) is a polyester used for drinks bottles and clothing fibres, while nylon-6,6 is a polyamide made from a diamine and a diacid. Because their ester and amide links can be hydrolysed, these polymers can be chemically recycled back to monomers, and PLA (polylactic acid), another polyester, is compostable. Addition polymers such as poly(ethene), poly(propene) and PVC are cheap and durable but their inert backbones mean they persist for centuries in landfill or as microplastics, which is why disposal routes (recycling, energy recovery by careful incineration with flue-gas cleaning, or substitution by biodegradable polymers) are an active environmental concern, mirroring the CCEA emphasis on sustainability.
Try this
Q1. State the two types of monomer needed to make a polyamide. [2 marks]
- Cue. A diamine and a dicarboxylic acid.
Q2. Explain why condensation polymers are more biodegradable than addition polymers. [2 marks]
- Cue. They contain ester or amide links that can be hydrolysed, unlike the inert C-C backbone of addition polymers.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20185 marksDescribe how a polyester is formed from a diol and a dicarboxylic acid, write the repeat unit in general terms, and explain why polyesters are more biodegradable than poly(ethene).Show worked answer →
A polyester forms by condensation polymerisation: each of the dicarboxylic acid reacts with an of the diol, forming an ester link and releasing a molecule of water each time a bond is made. Many monomers join alternately to build a long chain.
The repeat unit is , where comes from the diol and from the diacid, with ester () links along the backbone.
Polyesters are more biodegradable because their ester links can be hydrolysed (by water, acid, base or enzymes), breaking the chain. Poly(ethene) has an unreactive saturated backbone with no such links, so it resists hydrolysis and persists.
Markers reward (1) condensation with loss of water, (2) ester link formed between and , (3) a correct repeat unit, (4) ester links can be hydrolysed, (5) the inert backbone of poly(ethene) cannot.
CCEA 20214 marksSuggest a two-step synthesis of propanoic acid from 1-bromopropane, giving the reagents and conditions for each step, and state how the purity of the final product would be checked.Show worked answer →
Step 1: convert 1-bromopropane to propan-1-ol by warming with aqueous sodium hydroxide under reflux (nucleophilic substitution): .
Step 2: oxidise propan-1-ol to propanoic acid by heating under reflux with acidified potassium dichromate(VI) (excess oxidant, reflux to take it all the way to the acid): , orange to green.
Purity is checked by measuring the boiling point (or melting point of a solid derivative) and comparing with the literature value, a pure compound boiling sharply at the expected temperature.
Markers reward (1) aqueous NaOH, reflux for the substitution, (2) acidified dichromate, reflux for the oxidation, (3) recognising reflux gives the acid not the aldehyde, (4) a purity check against a literature value.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)