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What is AIBN? - Structure & Mechanism

Instructor: Korry Barnes

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

In this lesson, our focus will be on an organic compound known as azobisisobutyronitrile. Our primary points of discussion will include its structure followed by a detailed look at how this unique compound can be used to initiate free radical reactions.

Radical Organic Compounds

When you hear the word radical, what's the first thing that comes to your mind? Perhaps you think about someone you know who's a die-hard fan of some sort of sports team and you would say they are a radical fan of their team. Or maybe you liken the word radical to a person who seems to over emphasize and push a set of political beliefs they may feel strongly about. Whatever the case may be, you've likely heard the word before and have a general idea of what it represents.

Did you know that some reactions in organic chemistry involve radicals? In organic chemistry, a radical is defined as a compound in which one of the atoms has a single non-bonding electron, in contrast to a pair of non-bonding electrons, which is usually the case. The focus of our lesson is going to be on a compound called azobisisobutyronitrile, a specific molecule that is very good at promoting radical reactions. Our two topics of discussion will include the structure of azobisisobutyronitrile followed by a mechanistic description of how the compound breaks down to initiate radical reactions. Let's get radical!

Structure of Azobisisobutyronitrile

Let's get started by getting familiar with the definition and structure of our compound of interest. Azobisisobutyronitrile is an organic compound that is a white powder and contains four nitrogen atoms. Obviously the full name is quite a mouthful, so most of the time it's referred to using it's abbreviation, AIBN. It's soluble in a wide variety of organic solvents, including alcohol-based solvents.


Structure of AIBN
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There are several important structural features of AIBN that are worth making note of in our discussion. First and foremost, notice the two nitrogen atoms in the middle of the molecule that are connected to one another by a double bond. The presence of these atoms and that bonding pattern makes AIBN classified as an azo compound, which will become important when we get the to mechanism portion of our lesson.

Also notice the presence of a pair of carbon-nitrogen triple bonds on each end of the molecule. In organic chemistry anytime there is a carbon-nitrogen triple bond we call that particular functional group a nitrile. The nitrile groups will also become very important when we study the mechanistic aspect of AIBN. Probably something you've noticed as well is that AIBN is symmetrical, meaning that if we were to cut the molecule in half we would get two equal halves. Kind of like when you cut a sandwich in half you get two equal halves if you cut it just right.

How AIBN Initiates Radical Reactions

Now that we know what AIBN is and what its structure looks like, let's talk about its most important property—its ability to initiate radical reactions. When a radical reaction happens in organic chemistry, a radical initiator is always needed. As you probably guessed, a radical initiator is something that serves to 'initiate' or get a radical reaction started. It turns out AIBN is exceptionally good at serving this purpose.

Remember that nitrogen-nitrogen double bond we mentioned earlier? Here's where it comes into play. AIBN breaks down and forms a molecule of nitrogen gas and two carbon radicals. The nitrile functional groups serve a very important purpose in that they help to stabilize the carbon-centered radicals we've just formed. Liberating a molecule of nitrogen is a favorable process from a thermodynamics standpoint because anytime a gas is formed from a reaction, it represents a huge driving force in terms of entropy (the randomness or disorder of a process).


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