Acyl Group: Reactivity & Transfer Reactions

Instructor: Korry Barnes

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

The purpose of this lesson will be to study a functional group of organic chemistry called an acyl group. Our primary points of discussion will include the reactivity of acyl groups and the specific kind of reaction they undergo.

The Transfer of the Baton

Have you ever watched a relay race during the Summer Olympics as part of the track and field events? The whole point of a relay race is to run the fastest overall time over the required distance and it usually involves four runners that make up each team. The interesting thing about a relay race is that you not only have to have fast runners, you also have to work together as a team and successfully transfer what's called a baton from one runner to the next without dropping it along the way.

Just like the runners of a relay team transfer a small tube (baton) between one another over the course of the race, did you know that organic compounds can also undergo transfer-like events? Although they certainly aren't racing with one another or passing off a baton, we can still use that analogy to help us understand how they transfer groups of atoms between one another. In this lesson, we are going to be learning about what's called an acyl group. Specifically, we are going to be talking about their unique reactivity and the transfer reactions they can undergo. Let's get the ''race'' started!

What is an Acyl Group?

Let's kick things off by first getting acquainted with what an acyl group is. In organic chemistry, an acyl group is defined as a carbon atom double bonded to an oxygen atom, with the carbon atom also being bonded to a -CH3 group. Acyl groups can easily be added to organic compounds in what's called an acylation reaction. Consider as an example the addition of an acyl group to the organic substrate below. In general, the form of an acylation reaction has the form: organic substrate + acyl group = acylated product.

A general acylation reaction between an organic substrate and an acyl group
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Reactivity of an Acyl Group

Acyl groups have what some would call a unique chemical reactivity profile. It turns out that the identity of the atom or group of atoms bonded to the acyl group has a direct effect on how readily the group undergoes a reaction. Let's consider as an example the following series of acyl groups below and note how they're ordered in terms of reactivity.

Reactivity hierarchy of acyl groups
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First off, notice how in the reactivity hierarchy the acyl group that contains the chlorine atom is the most reactive and the one that contains the -OH group is the least reactive. Why might that be the case? The reactivity of an acyl group depends on the nature of the leaving group that is attached to the carbon atom, and the better the leaving group is, the more reactive the acyl group becomes. Chlorine is an excellent leaving group because it's very stable on its own apart from the acyl group, but the nitrogen and oxygen groups aren't very stable at all on their own, which makes them bad leaving groups. Those groups would be considered bad runners in the relay race!

The reason we call it a leaving group is because when a reaction takes place involving the acyl group, that particular atom or group of atoms doesn't get incorporated onto the final product. Another way of stating this would be it 'leaves' from the acyl group so that the reaction product can be formed.

Acyl Transfer Reactions

Acyl groups can undergo reactions that are broadly referred to as acyl transfer reactions, mainly because the name implies what's going on with the reaction itself. When an acyl transfer reaction takes place, we are simply transferring an atom or group of atoms to the acyl group while simultaneously 'kicking off' a leaving group of some sort. The idea is very similar to transferring the baton in a relay race from one runner to another.

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