AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Polyesters and polyamides can be broken down by hydrolysis and are…
Full specification wording
Biodegradability and disposal of polymers (A-level only): Polyesters and polyamides can be broken down by hydrolysis and are biodegradable.
The backbone of a polyester or a polyamide is interrupted, at every link, by a carbonyl carbon bonded to an oxygen or a nitrogen.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The backbone of a polyester or a polyamide is interrupted, at every link, by a carbonyl carbon bonded to an oxygen or a nitrogen. Both of those are more electronegative than carbon, so that carbon carries a δ⁺ charge, which leaves it open to attack by a nucleophile. Water, especially with an acid or an alkali present, attacks it and cuts the chain, reversing the condensation and returning the monomers. Acid hydrolysis of a polyamide gives the dicarboxylic acid and the diamine as its salt, because excess acid protonates the amine; alkaline hydrolysis gives the free diamine and the carboxylate salt. Micro-organisms use enzymes to do the same chemistry slowly in soil and water, which is what makes these polymers biodegradable.
What earns the marks
- The carbonyl carbon of the link carrying a δ⁺ charge, so a nucleophile can attack it.
- Water or the hydroxide ion identified as the attacking nucleophile.
- The monomers named as the products of hydrolysis.
- Those products being obtained as salts under acidic or alkaline conditions.
Common mistakes and how to put them right
Giving the free diamine as the product of acid hydrolysis, when the excess acid protonates it.
Giving the free dicarboxylic acid as the product of alkaline hydrolysis rather than its salt.
Describing biodegradation as the polymer melting or dissolving.
Claiming polyamides hydrolyse but polyesters do not, when both links are attacked.
Exam tips
- Decide first whether the conditions are acidic or alkaline, then adjust each product for it before you write anything down.
- Number the products: a polymer of n repeating units gives n of each monomer, so say 'the dicarboxylic acid and the diamine', not 'the monomer'.
Try the lesson
The start of the Biodegradability and disposal of polymers (A-level only): Polyesters and polyamides can be broken down by hydrolysis and are biodegradable. lesson, made from checked teaching notes for this specification point.
Have a go first
Why are condensation polymers such as polyesters and polyamides considered to be biodegradable?
Listen
The backbone of a polyester or a polyamide is interrupted, at every link, by a carbonyl carbon bonded to an oxygen or a nitrogen. Both of those are more electronegative than carbon, so that carbon carries a δ⁺ charge, which leaves it open to attack by a nucleophile. Water, especially with an acid or an alkali present, attacks it and cuts the chain, reversing the condensation and returning the monomers. Acid hydrolysis of a polyamide gives the dicarboxylic acid and the diamine as its salt, because excess acid protonates the amine; alkaline hydrolysis gives the free diamine and the carboxylate salt. Micro-organisms use enzymes to do the same chemistry slowly in soil and water, which is what makes these polymers biodegradable. A strong answer earns marks for these points. the carbonyl carbon of the link carrying a δ⁺ charge, so a nucleophile can attack it. water or the hydroxide ion identified as the attacking nucleophile. the monomers named as the products of hydrolysis. those products being obtained as salts under acidic or alkaline conditions. Exam tip: Decide first whether the conditions are acidic or alkaline, then adjust each product for it before you write anything down.
Key words
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hydrolysis of links
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nucleophilic attack
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biodegradability
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See every definition
- hydrolysis of links
- Water, especially with an acid or an alkali present, attacks the carbonyl carbon and cuts the chain, reversing the condensation and returning the monomers.
- nucleophilic attack
- The δ⁺ charge on the carbonyl carbon leaves it open to attack by a nucleophile, such as water or the hydroxide ion.
- biodegradability
- Micro-organisms use enzymes to do the same chemistry slowly in soil and water, which is what makes these polymers biodegradable.
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