Semicarbazide ( 1 mole ) is added to a mixture of cyclohexanone ( 1 mole) and benzaldehyde ( 1 mole). If the product is isolated immediately, it consists almost entirely of the semicarbazone of cyclohexanone; if the product is isolated after several hours, it consists almost entirely of the semicarbazone of benzaldehyde. How do you account for these observations?

First, write down the reactions that take place between semicarbazide and cyclohexanone as well as between semicarbazide and benzaldehyde, forming their corresponding semicarbazones. Reaction 1: Semicarbazide + Cyclohexanone → Semicarbazone of Cyclohexanone Reaction 2: Semicarbazide + Benzaldehyde → Semicarbazone of Benzaldehyde

Now, we will investigate the reaction kinetics of the two reactions. Reaction kinetics tells us how fast a reaction proceeds. It is known that the reaction between semicarbazide and ketones (such as cyclohexanone) is generally faster than the reaction between semicarbazide and aldehydes (such as benzaldehyde).

With the understanding of reaction kinetics from Step 2, we can explain the given observations. The reaction between semicarbazide and cyclohexanone proceeds faster than the one between semicarbazide and benzaldehyde. Therefore, when the product is isolated immediately, most of the semicarbazide has reacted with cyclohexanone, forming the semicarbazone of cyclohexanone. Over time, as the reaction between semicarbazide and benzaldehyde continues, the semicarbazone of benzaldehyde will start forming, even though it happens at a slower rate. After several hours, there are no more cyclohexanone molecules left for semicarbazide as it has fully reacted, and hence semicarbazide can only react with benzaldehyde. Thus, the product isolated after several hours consists almost entirely of the semicarbazone of benzaldehyde.

In conclusion, the observed differences in the products isolated at different times can be accounted for by the reaction kinetics of the reactions between semicarbazide and cyclohexanone (a ketone) and semicarbazide and benzaldehyde (an aldehyde). The reaction with the ketone is faster, leading to the formation of semicarbazone of cyclohexanone when the product is isolated immediately. The reaction with the aldehyde is slower but continues to occur over time, leading to the formation of semicarbazone of benzaldehyde when the product is isolated after several hours.