Acetylation of Ferrocene: Mechanism & Explanation

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

Have you ever wondered what interesting compounds iron can form? In this lesson we will be learning about how an iron-containing compound called ferrocene undergoes a reaction called acetylation by talking about the reaction mechanism and explaining the individual steps.

Sandwiches Made out of Iron?

What's your favorite kind of sandwich? Perhaps you're old-school and go for the tried-and-true peanut butter and jelly, or maybe you're more of the saucy type and really enjoys a good meatball-marinara sub. Sandwiches are a staple of almost every type of cuisine and can usually be found in all kinds of cultures around the world.

What if someone offered you a sandwich that was made out of iron? Probably doesn't sound that tasty or enticing! As it turns out though, iron can be used for 'sandwiches' in chemistry. You see one of the earliest discovered organometallic complexes called ferrocene was made out of iron and carbon atoms and after it was made, it was discovered to be what's called a sandwich complex. Our goal for this lesson will be to gain an understanding of a chemical reaction that ferrocene undergoes by carefully studying how it happens via the reaction mechanism. Let's make a sandwich!

What is Ferrocene?

Let's get started by getting introduced to ferrocene before we talk about our reaction of interest. Ferrocene is an organometallic compound made of iron, carbon, and hydrogen that's considered the 'prototypical' metallocene. A metallocene is a type of compound that contains a metal atom of some sort bound in between two cyclopentadienyl rings. The cyclopentadienyl anion is a very important ligand for metal atoms and is a five-membered carbon ring that contains a negative charge that is delocalized across the entire ring system (note the circle inside the ring indicating the charge delocalization).


The cyclopentadienyl anion contains five carbon atoms and a negative charge that is delocalized across the entire ring system
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In order to make ferrocene, iron is reacted with excess of the cyclopentadienyl anion and what comes out is ferrocene! Ferrocene and compounds like it are often referred to as sandwich compounds because quite literally, the iron atom is sandwiched in between the two cyclopentadienyl rings. Think of the iron atom as the meat of the sandwich and the two rings as the bread.


Ferrocene is an iron atom sandwiched in between two cyclopentadienyl rings
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Acetylation of Ferrocene

When ferrocene is acetylated, it means that an acetate group has been added to one of the cyclopentadienyl rings. An acetyl group is simply a methyl group that's bonded to a carbonyl group (carbon-oxygen double bond). The squiggly line here just indicated that the group could be bonded to any kind of compound.


Example of an acetyl group
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In order to add an acetyl group to ferrocene all we need to do is react it with acetic anhydride (the source of the acetyl group) and some phosphoric acid. When these three components are mixed together the reaction happens quite smoothly and gives acetylferrocene in high yield.


Ferrocene reacts with acetic anhydride and phosphoric acid to give acetylferrocene
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Reaction Mechanism

Now that we're familiar with ferrocene itself and the general form of the acetylation reaction let's see how the reaction happens in terms of the mechanism. Fortunately for us the reaction mechanism can be broken down into two simple steps.

Step 1

In the first step of the mechanism acetic anhydride reacts with phosphoric acid to give acetic acid plus a very important intermediate called an acylium ion, which is the activated version of the acetyl group that ultimately ends up on the cyclopentadienyl ring.


Acetic acid reacts with phosphoric acid to produce acetic acid and the acylium ion
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