This question is about a biodegradable plastic

The recent campaign to end the use of plastic

straws has turned attention to biodegradable

plastics and paper as alternative straw materials.

Poly$($lactic acid$),($PLA$),$ was initially developed for

use in biomedical implants. PLA can be used as

biodegradable packaging. A section of PLA is

shmim halnus

Note: no stereochemistry is required in skeletal

formulae of organic structures in your answers to

this question.

$($a$)$ Draw the structure of lactic acid $($the monomer that forms PLA$).$ It has the molecular

formula $C_3{H}_6{O}_3.$ Mark any chiral centres on the structure you have drawn with an

asterisk $(*).$

$($b$)$ Which term$($s$)$ describe the polymerisation reaction of the monomer to form PLA?

Tick the correct option in the answer booklet:

addition

condensation

neutralisation

oxidation

reduction

PLA can also be formed from compound $A,$ which has the molecular formula $C_6{H}_8{O}_4.$

$($c$)$ Draw the structure of compound $A.$

The average number of lactic acid monomers per polymer chain in a sample of PLA can be

determined using end$-$group analysis. End$-$group analysis is a procedure whereby reactive end

groups of the polymer are used to determine the polymer's molecular weight.

$0\mathrm{.}1619g$ of PLA was dissolved in $25c{m}^3$ of benzyl alcohol. The mixture was titrated with

$0\mathrm{.}0400mold{m}^{-3}$KOH solution. The titre was $6\mathrm{.}81c{m}^3.$

$($d$)$ Calculate the average molar mass of polymer chains in this sample of PLA.

$($e$)$ Calculate the average number of monomer units in each polymer chain of this sample.

If you did not get an answer to part $($d$),$ use the incorrect value of $306gmo{l}^{-1}$ as the

average molar mass.

$286,000$ tonnes of PLA are manufactured each year.

$($f$)$ Calculate the mass of NaOH needed to completely degrade all the PLA manufactured in

one year to sodium lactate. Assume the PLA is pure and ignore the contribution from any

end groups.

In fact, PLA can be broken down by enzyme degradation. Assume an enzyme degrades PLA

into a mixture of the lactic acid monomer and compound $B($a dimer of lactic acid$).$ Commercial

plastics actually contain other compounds in addition to PLA. Assume these other compounds

in the plastic are unreactive.

A plastic was degraded, resulting in a sample of mass $1\mathrm{.}044g,$ and titrations were used to

determine the composition of this sample. The sample was dissolved in $100c{m}^3$ of water to

make a stock solution. $19\mathrm{.}40c{m}^3$ of $0\mathrm{.}100mold{m}^{-3}$NaOH solution was needed to neutralise

$20\mathrm{.}00c{m}^3$ of this stock solution.

$($g$)$ Calculate the amount of acid in the sample $($in moles$).$

Compound B was hydrolysed by boiling in NaOH solution to help determine the masses of

lactic acid and compound $B.$

This was done by taking a further $20\mathrm{.}00c{m}^3$ of the stock solution and mixing it with $40\mathrm{.}00c{m}^3$

of $0\mathrm{.}100mold{m}^{-3}$NaOH solution. This was boiled for $1h$ under reflux. The remaining NaOH

was titrated with $0\mathrm{.}100mold{m}^{-3}HCl.$ The titre was $18\mathrm{.}50c{m}^3.$

$($h$)$ Draw the structure of compound $B.$

$($i$)$ Calculate the mass of lactic acid and the mass of compound $B$ in the sample.

If you did not get an answer to part $(g),$ use the incorrect value of $8\mathrm{.}60{10}^{-3}$mol as the

amount of acid in the sample.