Question

You are given a light brown solid sample with the information that it is an impure sample of a phenol – you are tasked with its purification. You ask for information about the suspected impurities, but it seems no one really knows. So you ask for an IR spectra of the impure sample. Upon scrutinizing the IR spectrum carefully, you find that there is a small, but significant, signal at about 1700 cm^-1, a small broad peak at about 3000 cm^-1 and a much stronger broad peak centered at about 3560 cm^-1. That gives you an idea about the nature of the impurity, but you know you need to confirm it before finalizing your strategy.

a. What can you say about the nature of the impurities, based on the IR spectrum? Explain your reasoning, clarifying which IR signal(s) provided the clue(s).

b. What simple chemical test(s) could you perform to determine the correctness of your conclusions regarding the impurities? Explain your reasoning.

c. If you find that your conclusions are indeed correct, how would you plan and execute your purification task? Provide sufficient detail regarding your experimental strategy.

d. After recovering your product, you observe it closely: it looks like shiny, plate-like crystals that are light brown in color. You record its melting point, and believe you have completed the purification task. Explain how you might reach this conclusion from your observations.

Answer 1

a) From the IR spectrum of impure sample we can say impure sample is aromatic carboxylic acid.

   Explanation: Carboxylic acid contains C=O group which is responsable for peak at 1700 cm-1, acid contains -O-H group which responsable for peak at 3560 cm-1, and also aromatic C-H bond is responsable for peak at 3000 cm-1.

Hence the impure sample is aromatic carboxylic acid.

b) Using bicarbonate test we can confirm the presence of carboxylic acid.

Explanation: Phenol is weak acid so it can't react with sodium bicarbonate where as carboxylic acid is strong enough to dissociate bicarbonate. Carboxylic acid reacts with bicarbonate and CO2 will be liberated.

c) Purification can be done as follows.

Add Sodium bicarbonate to impure phenol then carboxylic acid reacts with bicarbonate and it forms sodium salt of carboxylic acid which is polar and it can be soluble in aqueous solution. Then add water to the mixture, now salt of carboxylic acid come into aqueous layer and phenol remains in organic layer. If you evaporate the organic solvent then you will get pure phenol.

d) Phenol is crystal at room temperature and it has brown color. From melting point determination we can say the compound is phenol as it has melting point 40.5 degrees C.