Question

You have completed a reaction and take the melting point of your resulting solid product and the melting point you measured on the melt-temp is 130 – 142°C. The product you were trying to make was chloramphenicol which has a literature melting point of 150 - 151.5°C.

(A) What are two possibilities for why your melting point is different from the literature melting point?

(B) How would you figure out which of your two possibilities actually happened?

Answer 1

Presence of impurities can consderably decrease melting point. Usually organic compounds have sharp melting points. A difference of +2 degree celcius and -2 degree celcius is acceptable.

In the present experiment, as mentioned, it was attempted to prepare chloramphenicol. Its expected melting point is 150-151.5 degC. However the observed melting point (reported melting poinnt) is 130-142 degC. There is difference of about 10 degC which is not at all acceptable.

There can be two possibilities

1. The product chloramphenicol contains lot of impurities (starting material, byproduct, moisture etc)
2. The product formed is not chloramphencol. Accidently some other product is formed which has melting point in the range 130-142 degC.

How to check these possibilties?

1. MP can be lowered by presence of impurities. These impurities can be unreacted reactants, byproducts, sometimes moisture etc. Take the GC (gas chromatograph) of the sample. If you observe multiple peaks that means your compound is impure. If you assign the peaks, well and good. A thumb rule in GC is that compound whose boiling point is more will have longer retention time on cloumn and will be detected at last. Lower boiling compounds will low retention time.
2. Another way to confirm the product is chloramphenicol, take its IR or NMR. Interpret it. Compare the spectrum with known spectrum of chloramphenicol available in literature. If it exactly matches, it is chloramphenicol Additional peaks that cannot be assigned correspond to impurities. In order to take NMR, the compound has to be soluble in CDCl3 solvent. IR and NMR are used to confirm structure of a compound.