Question

Extra Credit viac a reasonable mechanism for the formation of pyrazole in the reaction below. Use curved arrows to show all atom and electron motion, and show formal charges where appropriate. U U NHANH2 PH~5 N-NH CH₂ H₃ CH₂ H₂C

Answer 1

In order to arrive at a plausible mechanism for organic reactions, one must understand the role of reagents and the effect of certain reaction conditions on the reaction flow.

In this case, as it is given that a pH of 5 is maintained, this means that reaction proceeds under acidic conditions. Now, for the sake of this reaction, let's assume that this acid is p-Toluenesulfonic acid (TsOH), which is an organic acid and appropriate for this reaction.

Now, since the reaction proceeds under acidic conditions, the oxygen atom in the given diketone reactant can be easily protonated (activated). The hydrazine molecule would act as a nucleophile and interact with the carbon of the carbonyl group.

Note: Someone might say that we have a diketone, and there are acidic hydrogen atoms present on the reactant, so hydrazine might behave as a base. This would be a mistake because, in order for hydrazine to behave as a base, the reaction conditions have to change. And in the given conditions hydrazine can behave as only a nucleophile.

The reaction mechanism along with the flow of electrons is in the image given below, one can also see what happens to the acid at every stage of the reaction.

CO ди, -Оон Унт. —ент -Сиз HNNK цс — не V-NH₂ CH + H₂O Nң,

N-NK 2 HN-N 'मन- _____ RAO c -NH te H HR