Question

benzylidene protection of methyl alpha D-glucoside. Compounds in lab: methyl alpha-D-glucoside, benzaldehyde dimethyl acetal, sulfuric acid, 1,4-dioxane. ( Experimental procedure: Suspend the glucoside in 1,4-dioxane. Add the benzaldehyde followed by a catalytic amount (i.e. a small drop) of sulfuric acid to the stirred solution. Heat to ca 85 o C, leave the reaction flask open but make sure that it is not left to run dry. Follow the reaction by TLC until it is complete, after which you can allow the RBF to cool.)

Question: Why is it beneficial for the reaction to heat the mixture and leave the RBF open?

Answer 1

1,4-dioxane is an organic solvent with boiling point of 101.1 deg C. When it is heated to 85 deg C, it has significant vapor pressure. Hence, if the RBF is not left open, then significant pressure will be developed in the closed system which may even break the RBF. Hence, it should be heated in open RBF.

When reaction mixture is heated to 85 deg C, the energy of reactant molecules can overcome activation energy barrier and there is significant rate of reaction. At room temperature, very few reactant molecules have energy equal to or more than activation energy.