Can anyone explain why the acylation of ferrocene produces the following results:

Ferrocene + acetic anhydride + AlCl3 ==> mono-acylated ferrocene


Ferrocene + acetyl chloride + AlCl3 ==> di-acylated ferrocene (an acyl group on each ring).

I have seen various statements confirming this, but not a single explanation for it! My current thoughts are that it must be something to do with the carboxylic acid produced from the reaction with the anhydride somehow hydrogen bonding/blocking the second site, but not really sure how it would work.

Any explanations greatly appreciated!

  • $\begingroup$ Could it also maybe be that the HCl produced by the first acetyl chloride-catalyzed acylation is a stronger acid than the HOAc produced by the acetic anhydride acylation, and that the resulting acid could promote the 2nd acylation? (Honestly not sure, just a thought...) $\endgroup$
    – Curt F.
    Feb 14, 2020 at 2:02

1 Answer 1


One factor: since ferrocene is covalently bonded, the rings "communicate" with each other via the molecular orbitals passing through the iron atom. Thus the acetyl group placed on one ring not only deactivates that ring, but to some extent deactivates the second ring as well. The second ring would still be attacked preferentially in the second stage but may require a stronger electrophile, which acetyl chloride is compared with acetic anhydride.

  • 1
    $\begingroup$ Thank you - this seems to make sense! $\endgroup$
    – gd99
    Feb 14, 2020 at 11:28

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