A polyester used in biodegradable packaging film is formed from propane-1,3-diol, HOCH2CH2CH2OH, and butanedioic acid, HOOC(CH2)2COOH, and has the repeat unit -O(CH2)3OCO(CH2)2CO-. (a) The polyester is hydrolysed by heating under reflux with an excess of aqueous sodium hydroxide…

Answer: ((a)) Ester bond broken; products per repeat unit are propane-1,3-diol, HO(CH2)3OH, and disodium butanedioate, NaOOC(CH2)2COONa. ((b)) Products per repeat unit are propane-1,3-diol, HO(CH2)3OH, and butanedioic acid, HOOC(CH2)2COOH (neutral form); with no base present, the diacid is not deprotonated and so is not converted to its sodium salt as in (a). ((c)) No, poly(ethene) is not hydrolysed; its backbone consists entirely of non-polar C-C and C-H bonds with no polarised carbon atom for a nucleophile such as OH^- to attack, unlike the polarised carbonyl carbon of the ester linkage in the polyester.. Part (a): Alkaline hydrolysis of the polyester Refluxing with excess aqueous sodium hydroxide hydrolyses the ester bonds (-O-CO-) in the polymer backbone. Hydroxide ions attack the polarised carbonyl carbon of each ester link, breaking the…

A2 Chemistry 9701: Alkaline vs Acidic Hydrolysis of a Polyester, and Poly(ethene) Resistance

A polyester used in biodegradable packaging film is formed from propane-1,3-diol, $\text{HOCH}_2\text{CH}_2\text{CH}_2\text{OH}$ , and butanedioic acid, $\text{HOOC}(\text{CH}_2)_2\text{COOH}$ , and has the repeat unit $-\text{O}(\text{CH}_2)_3\text{OCO}(\text{CH}_2)_2\text{CO}-$ .

(a) The polyester is hydrolysed by heating under reflux with an excess of aqueous sodium hydroxide. Name the type of bond broken, and give the formula(e) of the organic product(s) formed per repeat unit under these alkaline conditions. [3]

(b) The same polyester is instead hydrolysed by heating under reflux with an excess of dilute hydrochloric acid. Give the formula(e) of the organic product(s) formed per repeat unit under these acidic conditions, and explain why one of the products differs in form from the corresponding product in (a). [3]

(c) A sample of poly(ethene) is also treated under the same alkaline reflux conditions as in (a). State, with a reason, whether the poly(ethene) is hydrolysed under these conditions. [2]

Part (a): Alkaline hydrolysis of the polyester

Refluxing with excess aqueous sodium hydroxide hydrolyses the ester bonds ( $-\text{O-CO}-$ ) in the polymer backbone. Hydroxide ions attack the polarised carbonyl carbon of each ester link, breaking the chain and regenerating the diol and the diacid:

$-\text{O-CO}- + \text{OH}^- \;\longrightarrow\; -\text{OH} + {}^-\text{OOC}-$

The diol product, propane-1,3-diol, $\text{HO}(\text{CH}_2)_3\text{OH}$ , is unaffected by the base and stays neutral. The carboxylic acid released from the diacid portion, however, is immediately deprotonated by the excess $\text{NaOH}$ present, so the diacid is recovered as its sodium salt, disodium butanedioate:

$\text{NaOOC}(\text{CH}_2)_2\text{COONa}$

Part (b): Acidic hydrolysis of the same polyester

Under reflux with excess dilute hydrochloric acid, the same ester bonds are hydrolysed, again regenerating the diol and diacid. This time, however, there is no base present to remove a proton from the carboxylic acid product, so it is recovered in its neutral form:

$\text{HO}(\text{CH}_2)_3\text{OH} \quad \text{and} \quad \text{HOOC}(\text{CH}_2)_2\text{COOH}$

The diol product is identical in both cases (alcohols are neither protonated by dilute HCl nor deprotonated by dilute NaOH under these conditions); only the diacid’s ionisation state differs between the two sets of conditions.

Part (c): Why poly(ethene) resists the same conditions

Poly(ethene)‘s backbone is made entirely of $\text{C-C}$ (and $\text{C-H}$ ) single bonds between atoms of very similar electronegativity, so these bonds are essentially non-polar. Hydroxide ions attack esters by acting as a nucleophile toward a partially-positive ( $\delta+$ ) carbonyl carbon, but poly(ethene) has no such polarised carbon anywhere in its structure. With no site for $\text{OH}^-$ to attack, poly(ethene) is not hydrolysed even under prolonged alkaline reflux, which is why poly(alkene)s remain non-biodegradable while polyesters can be broken down by hydrolysis.

Final answers

(a) Ester bond broken; products: $\text{HO}(\text{CH}_2)_3\text{OH}$ and $\text{NaOOC}(\text{CH}_2)_2\text{COONa}$ .
(b) Products: $\text{HO}(\text{CH}_2)_3\text{OH}$ and $\text{HOOC}(\text{CH}_2)_2\text{COOH}$ (neutral diacid, since no base is present to deprotonate it).
(c) Not hydrolysed. The non-polar $\text{C-C}$ backbone of poly(ethene) has no polarised carbon for $\text{OH}^-$ to attack.