What is the main reason that it takes 2 to 3 minutes for the reaction to occur between sucrose and sulfuric acid in the dehydration of sucrose?

 There is a time delay of almost two to three minutes before the reaction proceeds because of the following processes:

1)     The acid starts to go yellow as the dehydration begins.

2)     The rate of dehydration then accelerates as the acid heats up because the reaction is exothermic/chemical (releases energy by light or heat).  As the sugar molecules are stripped of water, the heat generated turns the water into steam which then expands the remaining carbon into a porous, smoking, black column.

3)    This expands out of the reaction vessel, producing a choking acrid vapor and the smell of burned sugar.

 At this stage, sulfuric acid is highly corrosive and will burn skin so one must avoid contact with it. 

level 4 to level 2 : ))