We tried the synthesis of methyl benzoate in the lab via the Fischer Esterification reaction with excess starting methanol. After 1 hour of refluxing the reactants we immediately set up a simple distillation procedure to remove the excess methanol and here lies my question. By closely monitoring the temperature of the thermometer the mixture started boiling at 70+ oC and continued to rise slowly as expected. At one point the temperature started dropping slowly and after asking the professor she said that the distillation was over. I fail to understand why the temperature dropped. Can someone provide insight as to why it dropped and furthermore what should the thermometer show from start to finish and why? To my knowledge, the solution before the distillation contains methanol, some benzoic acid, methyl benzoate, sulfuric acid and water. Thank you in advance.
1 Answer
The thermometer that records the temperature is in the air high above the solution (usually above a Vigreux column). Your distillation was under ambient conditions, so there was air in contact with everything.
The thermometer can only records a rising temperature if the temperature around it rises. During the distillation, this is due to methanol vapour (around $\pu{70 °C}$) which can also condense on the thermometer to transfer heat.
Once all methanol has been distilled away, there is nothing coming from below that could transfer any additional heat: water, sulphuric acid and benzoic acid all have far too high boiling points and low vapour pressures. Thus, the thermometer is again in contact with the surrounding air and cools down.
While I have described this with air, basically the same thing will happen in a vacuum distillation, too.
