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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Step 2
Once the acylium ion is generated, it goes on to react with one of the carbon atoms of a cyclopentadienyl ring, which gives us our acetylated ferrocene product. An important thing to realize is that the acetylation reaction is a substitution reaction, one in which a hydrogen atom on the cyclopentadienyl ring is substituted for the acetyl group.
Lesson Summary
All right, let's take a moment or two to review.
Ferrocene is an organometallic compound made of iron, carbon, and hydrogen that's considered the prototypical metallocene. We saw that, in general, a metallocene is a type of compound that contains a metal atom of some sort bound in between two cyclopentadienyl rings.
The synthesis of ferrocene is accomplished by reacting iron with an excess of the cyclopentadienyl ion, which gives 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.
In the acetylation of ferrocene, an acetate group is added to one of the cyclopentadienyl rings, with an acetyl group being a methyl group that's bonded to a carbonyl group (carbon-oxygen double bond). The acetylation reaction involves combining ferrocene with acetic anhydride and phosphoric acid to produce acetylferrocene.
The mechanism of the acetylation reaction can be broken down into two simple steps:
- 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.
- In the second step, the acylium ion reacts with one of the carbon atoms of a cyclopentadienyl ring, which gives acetylferrocene. The acetylation reaction is a substitution reaction, one in which a hydrogen atom on the cyclopentadienyl ring is substituted for the acetyl group.
