Question

Procedure

IMPORTANT NOTE: Approximately 2 g of PET plastic cut into small pieces will be provided for you. This is approximately ¼ to ½ of a small soda bottle. Please make sure that the bottle is cut into ½ inch squares before proceeding.

1.Hydrolysis of the PET Samples: Charge a 50 mL RB flask with 15mL of 1‐pentanol, 2.0 g of PET (cut into small pieces) and 1.8 g of KOH. Equip the flask with a magnetic stir bar and a reflux condenser. The mixture does not dissolve, but a thick white suspension is formed.

2.Cover the flask with foil for insulation, heat the mixture to reflux and continue to heat for 60 minutes (when should you begin the timer?) If the stirring stops due to the thickening of the suspension, add more solvent.

3.Allow the reaction mixture to cool to room temperature, add 10mL of water to the reaction flask, and stir to dissolve the white terephthalate salt that is present.Can you draw the structure of this salt?

4. Vacuum filter the mixture and add the filtrate to a separatory funnel. Drain the aqueous layer from the separatory funnel into a 125 Erlenmeyer flask.Which layer should be the aqueous layer?

5.Extract the alcohol layer with an additional 10 mL of water; and combine the aqueous extracts in the 125mL Erlenmeyer flask.

6.Slowly and with stirring, add dilute HCl to acidify the aqueous extracts. (If HCl is added too quickly, the terephthalic acid will form fine crystals that may pass through or clog the filter paper.)What is happening here?

7.Cool the acidified mixture in an ice bath. Collect the white terephthalic acid via vacuum filtration and allow it to dry. Obtain a mass of the crystals.Why are you not obtaining a melting point for this compound?

8. Analyze your product by IR,1H‐NMR and 13C‐NMR.Can you predict what you expect to observe in the NMRs by now?

9. Add your crystals to the container in the common hood so that it may be used as a starting material in future semester

a) Show the calculation for the limiting reagent below.

b) Provide a separation scheme for your experiment:  Include a scheme beginning with PET, KOH, 1‐pentanol and ending with terephthalic acid.   ​

Answer 1

(3)

PET on reaction with KOH produces dipotassium terephthalate salt as follows:

(4)

As water been added to dissolve the organic compound. Thus, the layer containing the salt is the aqueous layer. The salt is soluble due to the presence of potassium ion.

(6)

The potassium of salt combines with chloride from HCl to produce KCl and hydrated salt of terephthalate. This hydrated salt having larger size to get separated through filteration. If the acid is added rigorously then most of the salt not get hydrogen attached with it and gets smaller size in comparison to hydrated salt and get passes through filter paper.

(7)

On obtaining the melting point will not provide the percentage yield which is useful for determining the progress of reaction and its efficiency such as how efficiently reactants reacted to produce product.

(8)

NMR usually informs about the different environment of carbon or hydrogen present in the structure of compound.

The number of signals present in graph indicates about the different types of carbon or hydrogen present which further split according to the carbon or hydrogen present adjacent to it.

a.

Limiting reagent is the chemical substance which is reacting in the reaction completely to give the desired product.

It can be determined by dividing the number of moles by its stoitiometric coefficient.

Polyethylene terephthalate is the limiting reagent as its molar mass is variable and quite high in comparison to KOH having molar mass of 56 g /mol. Thus, PET having lower number of mole and thus having lower value of mole divided by coefficient. Hence, PET is limiting reacgent and uses completely to give terephtallic acid.

b.

Thus, cooling of acidified mixture done in ice bath and crystal of terephthalic acid collected through filteration.