Nitrations

Introduction

Nitration is a common lab reaction. The theory behind it is a bit complex and I’m going to use this reaction in various synthesis. Because of it, I decided to write a post just about the theory behind Nitrations. Thus, any time I need to use it I can reference this post for a deeper explanation of the process.

General Basis

Nitration is used to introduce a Nitro Group (NO2) into an organic molecule. Here is the structure of the Nitro Group (image from http://upload.wikimedia.org/wikipedia/commons/6/62/Nitro-group-2D.png):

To introduce the group, a mixture of Sulfuric Acid and Nitric Acid is used. This mixture is commonly referred to as “mixed acid” but its name is Sulfonitric mixture.

Theory

When added together, the Sulfuric Acid protonates the Nitric Acid, forming the very unstable Nitrooxonium species:

H2SO4(aq) + HNO3(aq) <-> HSO4(aq) + H2NO3(aq)

Here is the structure of this Nitrooxonium species (image from http://www.chemspider.com/Chemical-Structure.3001494.html):

NitrooxoniumNitrooxonium is extremely unstable and only exists briefly. It readily decomposes to form the Nitronium ion:

H2NO3(aq) <-> NO2+(aq) + H2O(l)

Here is the structure of the Nitronium ion (image from http://en.wikipedia.org/wiki/Nitronium_ion#/media/File:Nitronium-2D-dimensions.png):

These steps can be summarized by the following equation:

H2SO4(aq) + HNO3(aq) <-> HSO4(aq) + NO2+(aq) + H2O(l)

As you may have already figured, that reaction is an equilibrium. The left side is greatly favored and only small amounts of the Nitronium Ion species exist at any given time. What makes the equilibrium shift to the right is the constant reaction of the Nitronium Ion with the organic molecule of choice.

The Nitronium Ion is the species responsible for the actual Nitration. It reacts with the organic molecule to be nitrated, forming the Nitro compound.

Organic Molecule + NO2+(aq) –> Nitro Compound + H+(aq)

Note: the H+ doesn’t actually exist, as it readily “joins” with water to form the Hydronium ion, H3O+.

Finally, the H+ ion just mentioned reacts with the HSO4ion (which emerged from the first equilibrium) to regenerate the Sulfuric Acid.

HSO4(aq) + H+(aq) → H2SO4(aq)

Recap:

  1. H2SO4(aq) + HNO3(aq) <-> HSO4(aq) + NO2+(aq) + H2O(l)
  2. Organic Molecule + NO2+(aq) –> Nitro Compound + H+(aq)
  3. HSO4(aq) + H+(aq) → H2SO4(aq)

The entire thing can be over simplified by this equation:

HNO3(aq) + Organic Molecule –> Nitro Compound + H2O(l)

Note: Sulfuric Acid is normally mentioned as both a catalyst and a dehydrating agent. Sulfuric Acid “absorbs” some of the water formed in the equilibrium, making it shift to the right, forming more Nitronium ions.

Run Away

Nitrations are always very susceptible to Run Away. “Run Away” is the name given to the moment when a reaction goes “out of control” forming undesirable by-products. In the case of Nitrations, when a Run Away starts, copious amounts of Nitrogen Oxides are formed, which are toxic, accompanied by an increase of the reaction temperature.

Generally, Nitrations are done with careful control of the temperature, keeping it low, in order to avoid  a Run Away.

Final Notes

As always, if you have any doubts feel free to comment. Please, leave some feedback.

2 thoughts on “Nitrations

  1. I’m interested in making nitroglicerin (for medical purposes, as a vasodilator ofc !) i think i need this step but i’m not sure when :/ , could you advise me on how to do it properly (including whole recepie please) ?

    Like

    • Hey thedoc,

      Nitroglycerin is, in fact, a potent vasodilator. However, I advise against consuming anything made in the lab unless you are absolutely sure about what kind of contaminants your products may have and how pure your product is.

      To make Nitroglycerine you simply need to nitrate it. Keep in mind, though, that Nitroglycerine is also an explosive. If you wish to make it use very small quantities of reagents. Unfortunately, I’m not going to post a recipe here in this comment.

      Thanks for your comment,
      HgDinis25

      Like

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