Question

Lab: Kinetic vs. Thermodynamic Control in Competing Reactions (2-furaldehyde and cyclohexanone reacted with semicarbazide hydrochloride to form semicarbazones)

What are 3 experimental errors that result in a melting point that is lower than expected and how can they be fixed?
1. Impurities-- how to prevent impurities during recrystallization, vacuum filtration, oven drying, melting point determination with apparatus?
2. ?
3. ?

Answer 1

1.Impurities-During recrystallization of the pdt obtained,one should take extensive care so that no genuine amount of impurity can go through the test tube which may inhibit recrydtallization process.During vaccum filtration and oven drying, one should be careful of cleaning the glasswares within which the sample is place to the oven.

2.Rapid Heating of the Sample under test- Heating the sample rapidly results imperfect reading in melting point as the thermal equilibrium is not established in this case.So heat the sample 1 to 2 degrees per minute but this generally takes a long time if the sample has a high mp(semicarbazones generally have mp beyond 200),so it is advised to repeat the experiment at least twice first heating rapidly to have an idea of the mp range then heating slowly to get the exact value.

3.Fitting the sample perfectly to the capillary tube-A very common mistake is to fill the capillary tube to the half with the sample which normally takes time to melt resulting wide melting point range.The sample in the bottom of the capillary has already started melting much earlier but often we report the MP point when at lest the entire sample starts to melt for which the temperature has already gone up.So take such amount of sample which is just enough to be seen in the tube.