Nitration: Reaction & Products

Instructor: Korry Barnes

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

In this lesson we will learn about an important reaction in organic chemistry known as nitration. In addition to exploring the reaction mechanism, we will get some practical application of these building blocks and explore what they can be used for.

How Important is Nitrogen?

In the context of organic compounds, we could potentially make the case that nitrogen just might be the most important atom other than carbon and hydrogen. Nitrogen finds many applications in things that we depend on everyday including things like pharmaceuticals, textiles (such as nylon), plastics, and even biological polymers like proteins.

Since organic compounds that contain nitrogen atoms are so important, it stands to reason that understanding chemical reactions that introduce nitrogen atoms to other substrates are extremely vital. One of the most important methods of adding a nitrogen atom to an already existing organic substrate is a reaction called nitration. Nitration is the process of adding a nitro group (NO2) to a benzene ring.

An easy way to think about nitration might be like playing building blocks or Legos. When we add a nitro group to an aromatic ring, we take two 'blocks' and just click them together and the result is something more complex than the two individual building blocks. Come along and let's see how this important process actually works!

A Specific Type of Reaction

As mentioned above, nitration simply means that we are adding a nitro group to an aromatic (or benzene) ring. In technical terms, nitration is actually part of a reaction type known as electrophilic aromatic substitution (EAS). That sounds like a lot of words and can be confusing, so let's break each part down briefly.

Electrophilic

In order for the reaction to take place, we need two things: an electrophile (electron acceptor) and a nucleophile (electron donor). In the context of EAS, the nucleophile is always the aromatic ring itself, and in this case the electrophile is actually our nitro group.

Aromatic

As the word implies, an aromatic ring is needed for this reaction, and the pi electrons inside the ring will be acting as our nucleophile.

Substitution

In this reaction, we will be substituting a hydrogen atom on the aromatic ring for another functional group. In this case, it's the nitro group that we are adding (or substituting) in place of one of the hydrogens.

An Electrophile is Needed!

Fundamentally, we have to have an electrophile for nitration to be able to happen. Let's talk about how we are going to generate our nitro-group electrophile.

A nitrogen electrophile is generated by taking nitric acid and sulfuric acid and adding them together. When we mix these two reagents with one another, a series of events takes place that provides us with our electrophile. Let's explore each step in the process and see how this happens.

In the first step, the hydroxyl (OH) group on a molecule of nitric acid is protonated by a hydrogen ion, which is provided by sulfuric acid.


Protonation of a molecule of nitric acid
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As we can see, a water molecule is generated (labeled in red) which is a VERY good leaving group. The water molecule falls off, which provides what's called a nitronium ion, which is just a fancy term for a nitro group that is ready to react with our aromatic ring.


Generation of the nitronium ion
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What to do with the Electrophile

Now that we've successfully made our electrophile, let's talk about what's next. The next step that happens in our reaction is a set of pi electrons from our aromatic ring comes out and makes a bond with our nitronium ion. Since nitrogen can't have more than 4 bonds, we need to break an existing nitrogen-oxygen bond. Also notice that when we break our carbon-carbon double bond, one of the carbons is short 2 electrons, which is why it has a positive charge.


First step in nitration: attack of the nitronium ion by aromatic ring
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The intermediate shown above is extremely reactive, mostly because it is no longer aromatic. It doesn't stick around long, and in this case a water molecule is sufficient enough of a base to come in and pluck off a hydrogen atom, which re-generates aromaticity in the ring system and gives our final nitrated product!


Water acting as a base to pull of hydrogen and generate final nitrated product
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What are Nitrated Compounds Good for?

Aromatic rings that contain nitro groups can be very useful, so let's take a look at some examples of how they can be utilized.

Use as explosives

One of the most common explosives which we've all heard of is actually a product of nitration. It's called trinitrotoluene or TNT for short. TNT is made by taking toluene and nitrating it three times in the exact same fashion we detailed previously.


Formation of TNT by nitration of toluene
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