Question

What Lewis acid is used in the acetylation reaction of ferrocene?

In the experiment of acetylation of ferrocene, what will be the ordering of elution? How will you explain the observed ordering?

A mixture of cis and trans isomers of [Cr(CO)₄(PPh₃)₂] is loaded onto a column and eluted with 10:90 ethyl acetate : petroleum ether solvent mixture. Which isomer will elute first? Why?

What two acetylated products are isolated from the acetylation of ferrocene? Which of the products is the minor product and how will you explain the poor yield of the minor product?

Answer 1

The Lewis acid, phosphoric acid (H₃PO₄) is used in the acetylation reaction of ferrocene.

If the reaction is not complete, then there may be traces of unreacted ferrocene. Monoacetylated and diacetylated ferrocenes are isolated from the acetylation of ferrocene. The poor yield of the minor product, i.e. the diacetylated product, is due to the use of milder conditions, i.e. weak Lewis acid, (H₃PO₄).

Order of elution: Unreacted ferrocene (if any) < monoacetylated ferrocene < diacetylated ferrocene (if any)

Explanation: Least polar compound will elute first and the most polar one at the last. The reason is that you have to start the isolation by using pure hexane as eluent (which elutes the least polar compounds, i.e. unreacted ferrocene if any), followed by a mixture of major hexanes + minor ethyl acetates as the eluent (which elutes the more polar like monoacetylated product, then most polar compound, i.e. diacetylated product if any).

The trans isomer of [Cr(CO)₄(PPh₃)₂] elutes first by the use of less polar 10: 90 (ethyl acetate: petroleum ether) because the trans isomer is less polar due to the trans-arrangement of both PPh₃ moieties, whereas the more polar cis isomer elutes last.