Question

I was conducting an acid-base extraction experiment with the objective of identifying an unknown benzoic acid from a mixture. My group recrystallized the aqueous layer and obtained its melting point to compare with standard compounds. In the end, our melting range for the recrystallized product was 136.0ºC - 142.2ºC which was only similar to the range of the standard 2-chloromethyl benzoic acid (139.4ºC - 143.6ºC) out of all the provided standards. However, when testing the mixed melting point of the two, we got a questionable range of 112.9ºC - 114.2ºC. Our unknown still can't be the other standards because their mixed melting points were out of range as well.

My overall question is if we're still assuming that the unknown was 2-chloromethyl benzoic acid, is there a possible reason as to why the mixed melting range was completely different?

Answer 1

If mixed melting point range is not coming as that of a standard, it is coming low,

that implies the compound is not 2 - chloro methyl benzoic acid.

Because in case of mixed melting point range the compound should show a sharp range of melting point as that of standard.

but, still if there is no other option left, one thing you could do is recrystallise again the compound, so that if there any small impurities remaining it will be eliminated From your compound.

In this manner, you will be having your recrystallized compound and again, you can check melting point confirm.

Hope this helps in clearing your doubt. feel free to ask for any further queries.