Reaction between acrolein and hydrazine When acrolein is heated with hydrazine we get dihydropyrazole. But I can't see how. First I think hydrazine should add on acrolein to form corresponding hydrazone, but what will happen next? Also is my first step correct?
Your suggestion of resulting corresponding hydrazone by the reaction between acrolein and hydrazine is correct. It is normal hydrazine-hydrozone reaction you perform in idetifocation of aldehyde and ketone carbonyl groups. Usually, E-isomer is preferred (Ref.1). E- to Z-isomerization can be occurred in presence of trace amount of acid (Ref.1). I expect that may be the case during constant heating. As a result, Z-isomer can undergo intramolecular Michael addition as depicted in following mechanism to give dihydropyrazole:





References:


Shigehisa Uchiyama, Masanori Ando, Shohei Aoyagagi, "Isomerization of aldehyde-2,4-dinitrophenylhydrazone derivativesand validation of high-performance liquid chromatographic analysis," Journal of Chromatography A 2003, 996(1-2), 95–102 (https://doi.org/10.1016/S0021-9673(03)00542-9).