Oxidation of Fluorene to Fluorenone Mechanism

Instructor: Korry Barnes

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

The focus of this lesson will be on a specific organic reaction in which fluorene is oxidized to fluorenone. Our goal will be to understand how this reaction happens by studying the reaction mechanism.

Can Air Really Do That?

Air. It's something that we can't live without, right? Air is a mixture of gases and is mostly nitrogen and oxygen. Did you know that air can be used in organic chemistry reactions? It actually can be! Some reactions in organic chemistry won't actually occur in the presence of air (due to oxygen and water interference) and have to be run in special containers, but others really like air as an additive and need it for the reaction to happen.

One such example of an organic reaction that needs oxygen and moisture (water) from the air is the oxidation of fluorene to fluorenone. Fluorene is a polycyclic aromatic hydrocarbon that contains three ring systems bonded together, and in going from fluorene to fluorenone, a carbon is oxidized to introduce a ketone functional group (carbon-oxygen double bond) onto the molecule. What we are going to be learning about in this lesson is how this reaction happens from a mechanistic perspective. Let's dive in!

General Reaction Form

Before we look at how the reaction mechanism works, let's see what the general reaction itself looks like. A lot of times looking at the reaction before you try to draw the mechanism is helpful because it gives you the big-picture of what to expect. It's kind of like planning a road trip from a map. You always start by looking at where you are and then find where you're ultimately trying to go on the map before figuring out what route you'll take.

The same is true for writing a good reaction mechanism. The reactants in a chemical equation could be likened to your starting point on the map, and the products could be thought of as where you're trying to go. The mechanism describes in detail how you get from start to finish. For the air oxidation of fluorene, the reaction needs some sodium hydroxide (NaOH) as well, which we'll talk about later. Notice how in going from start to finish, we've lost two carbon-hydrogen bonds and gained a carbon-oxygen bond (the ketone).

The oxidation of fluorene to fluorenone in the presence of sodium hydroxide in air

Reaction Mechanism

Now that we know the general form of the reaction, let's see how this transformation happens step-by-step.

Step 1

In the first step of the mechanism, the hydroxide ion from sodium hydroxide acts as a base to pull off one of the hydrogen atoms on fluorene. The resulting intermediate has a negative charge on the carbon atom, which will become important in the second step. Notice how the curved arrows show the direction of electron flow for the bond-making and bond-breaking events.

First step in the reaction mechanism in which the hydroxide ion pulls off a hydrogen atom from fluorene

Step 2

In the second step of the process, the negative charge on our carbon atom comes out and forms a bond with a molecule of oxygen from the air (this is where air plays a role in the reaction). Note that now we've formed a carbon-oxygen bond and there is a negative charge on the second oxygen atom on the chain.

Second step in the mechanism in which the negative charge on fluorene bonds to an oxygen molecule from the air

Step 3

Here's where the reaction gets interesting. When we're running the reaction there are lots of hydroxide ions floating around in solution. In the third step, a hydroxide ion comes in and coordinates with the intermediate we just formed. We use the term 'coordinate' here because it's really not formal chemical bonds at play but rather a weak interaction between the oxygen atom and a hydrogen on the hydroxide ion, and the hydrogen on fluorene with the oxygen on the hydroxide species.

Third step in the mechanism where a hydroxide ion coordinates to the oxygenated fluorene intermediate

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