The catalyst concentration was varied over a range of 5–25
mol% iron oxide nanoparticles on the basis of the total volume
of the reaction mixture. Table 1 shows the effect of catalyst
concentration on the reaction of benzaldehyde, ethylacetoacetate,
and urea or thiourea. The yield of the corresponding dihydropyrimidinone
or dihydropyrimidinthione increased with
an increase in the catalyst concentration from 5 to 20 mol%. A
further addition of catalyst had no noticeable effect on the yield.
This was due to the fact that beyond a certain concentration
more catalyst sites exist than that required by the reactant
molecules and, hence, the additional amount of catalyst does not increase the rate of the reaction. Therefore, in all further
reactions 20 mol% of the catalyst was used.