Which stretch, present on the IR spectrum of your product, indicates that the reaction was successful?

To answer this question, we need to know what the product of your reaction is, as different compounds will have different characteristic IR stretches.

In general, the IR spectrum can provide information about the functional groups present in a molecule. For example, a broad stretch around 3200-3600 cm-1 often indicates the presence of an OH group, a sharp peak around 1700 cm-1 can indicate a carbonyl group (C=O), and a stretch around 3300 cm-1 can indicate an NH group.

So, if your reaction was supposed to produce a compound with a new OH group, for example, you would look for the appearance of a broad stretch around 3200-3600 cm-1 on the IR spectrum of your product. If this stretch is present, it would indicate that the reaction was successful.

However, without knowing the specifics of your reaction and the expected product, it's impossible to give a more precise answer.