You are given a light brown solid sample with the information that it is an impu

Question

You are given a light brown solid sample with the information that it is an impure sample of an aromatic hydrocarbon– you are tasked with its purification. You ask for information about the suspected impurities, but it seems no one really knows exactly. So you ask for an IR spectra of the impure sample. Upon scrutinizing the IR spectrum carefully, you find several medium intensity signals at 3020 - 3040 cm-1 and 1510 cm-1, a small but distinct signal at 1730 cm-1, and a small broad peak centered at about 2700 cm-1. This confirms the presence of the aromatic hydrocarbon, and also gives you an idea about the nature of the impurity, but you know you need to further confirm it before finalizing your purification strategy.

(a) What can you deduce about the nature of the impurity, 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 yellow in color. You carefully record its melting point, and believe you have completed the purification task. Explain how you might reach this conclusion from your observations.

(e) When you give the purified sample to the synthetic chemists planning to use this substance in a synthesis, they tell you it cannot be pure, because the pure substance is supposed to be white, not light yellow. You bring it back to your lab and set to work again: what might you do to purify it further? Explain clearly.

Explanation / Answer

Signal at 1730 cm-1 tells us there is a C=O group present in impurity, and a small broad peak centered at about 2700 cm-1 further confirms it's an Aldehyde.

It should show positive tollen's test / Silver mirror test. The aldehyde groups are easily oxidisable to carboxylic acids(salts).

Most aldehydes will be water soluble when it's reacted with the sodium bisulphite or sodium metabisulphite. So, the insoluble content will be dried and washed with water.

Pure compound should show the sharp melting point range. If sharp melting points are observed then the compound expected to be pure.

Recrystallisation might help us to purify the impure sample.

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