In a lab this week at uni we made ferrocene (I'm a second year Chemistry student). This was done by reacting nBuLi with cyclopentadiene (kept in freezer) to create the Cp anion in-situ.

To this mixture I mistakenly added iron(+3) chloride hexahydrate instead of anhydrous iron(+2) chloride, which was supposed to have been used later on to observe the oxidation of ferrocene to ferrocenium. I was told to continue with my synthesis, and I did make some ferrocene but only in mg amounts. I ask what was it in the mixture that reduced the iron?

In terms of my observations - as the hydrated salt was added a sludgy black precipitate began to form (solvent was THF) I cleaned the Schlenk tube at the end with washes of 6M HCl and small amounts of this solid dissolved with each wash and the solution turned green each time. Could someone tell me what this solid was?

Also could you suggest some of the effects of water and oxygen on the mixture? Why does the reaction need to be performed under inert atmosphere considering ferrocene is stable to the air? What are some of the unwanted reactions that would occur?

  • 2
    $\begingroup$ Butyllithium is an immensely powerful base. Any water present will instantly react with it so it will not be available to react with the cyclopentadiene. There is water in the air which is why you keep the system under dry nitrogen. $\endgroup$
    – Waylander
    Commented Dec 17, 2021 at 18:39
  • $\begingroup$ Okay, thanks! What effect would adding the hydrated salt in the second step have had, instead of something anhydrous - assuming the LiCp had already formed? $\endgroup$
    – Tilly
    Commented Dec 17, 2021 at 18:52
  • $\begingroup$ @Tilly you were supposed to add Iron (Ii), by the way... $\endgroup$
    – Alchimista
    Commented Dec 17, 2021 at 19:42
  • $\begingroup$ The effect of adding the hydrated salt would be to regenerate the cyclopentadiene as the anion of cyclopentadiene will steal a proton from the water $\endgroup$
    – Waylander
    Commented Dec 17, 2021 at 20:59
  • $\begingroup$ The fact that the final product is stable to air and water doesn't matter. All the key ingredients are highly sensitive to water and air and will disappear in side reactions if you don't exclude the air and water. $\endgroup$
    – matt_black
    Commented Dec 17, 2021 at 21:49


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