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I was taught that molecules on top of arrows are reactants and below arrow are solvent.

But I've come across a series of molecules written above and below arrows with numbers and commas. Do these numbers mean that all these reagents above and below arrows are in separate containers and are added one by one?

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    $\begingroup$ When you have numbers it means that these are sequentially added. So, in this case, you'd add Hg(OAc)2 and HNuc to your starting material, let it react for a certain period of time, then after that add NaBH4. $\endgroup$ – orthocresol Dec 4 '16 at 19:39
  • $\begingroup$ Is that for all cases where u see the numbers? My TA was saying sometime, it could also mean that all these molecules above and below arrows are dumped together in one container and let to react on their own? I don't know if he was right in saying that? $\endgroup$ – TLo Dec 4 '16 at 19:42
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    $\begingroup$ If there are numbers present, I would say they are always sequentially added. If there are no numbers present, then it's usually added together. $\endgroup$ – orthocresol Dec 4 '16 at 19:43
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The numbers indicate steps. The only problem is that the definition of a "step" can be slightly different. Sometimes, separate steps mean that you do something, wait some time, and then do the next thing. Other times, you need to work-up or destroy the excess reagents that are left and isolate the desired product before you can proceed with the next step.

In your example, you need to keep the mercury and the borohydride separate. If you used any excess in the first step, you might need to destroy it before you add the borohydride.

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