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Formation of nitrate esters

Formation of a nitrate ester from reaction of an alcohol with nitric acid. Created by Jay.

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Video transcript

Here's the general reaction to form nitrate esters from alcohol. So we have our alcohol over here on the left. And we react that with some concentrated nitric acid and concentrated sulfuric acid as our catalyst. And I believe this reaction is reversible. So we could think about that. We would form our nitrate ester over here on the right. We would also form water in the process. So the water molecule's going to come from this hydrogen on the alcohol and this OH on our nitric acid. Let's take a look at the mechanism to form nitrate esters. And so I'll start by redrawing our nitric acid molecule here. And I think this is the correct mechanism, although I tried to look up the mechanisms in some textbooks, and I could not find this mechanism anywhere. So I hope that the one that I show you is the correct one. All right. Well, if I think about nitric acid and sulfuric acid, sulfuric acid is actually the stronger acid. So sulfuric acid is going to donate protons, and the nitric acid is going to accept those protons. The nitric acid is actually going to function as a base. So a lone pair of electrons on the oxygen are going to pick up that proton. So now, oxygen has three bonds. Right? Two to hydrogens and one still to this nitrogen here. It still has a lone pair of electrons on it, which give it a plus 1 of formal charge. This nitrogen over here is still double-bonded to one oxygen. And it's still bonded to this oxygen over here-- a negative 1 formal charge and a plus 1 of formal charge, like that. So that was our first step of our mechanism-- an acid/base reaction. In the next step, if we look closely, we can see that we kind of have water as a leaving group. If a lone pair of electrons in our oxygen move in here to form a new pi bond, that can kick these electrons in here off onto the oxygen. And water has now left. So let's go ahead and draw the results of that. All right. So now we have H20 as a leaving group. So H20 has left. We now have nitrogen- which was double-bonded to an oxygen up here. It's now double-bonded to another oxygen down here. And it still has a plus 1 formal charge now. So this is called the nitronium ion. And since the nitrogen is positively charged, it wants electrons. So it's going to function as an electrophile in the next step of the mechanism. When an alcohol molecule comes along, alcohols have these lone pairs of electrons here. We know that negatively-charged electrons can function as nucleophiles. So the negatively-charged electron is attracted to the positively-charged nitrogen. And nucleophilic attack will kick these electrons off onto your oxygen. Let's go ahead and draw the product of that nucleophilic attack. All right. So now, we have our R group bonded to an oxygen. And our oxygen is now bonded to this nitrogen here. And this oxygen is also still bonded to a hydrogen, giving it a plus 1 formal charge. And our nitrogen still has a double-bond to the top oxygen here. And it now has a single-bond to this oxygen, giving this a negative 1 formal charge, giving this nitrogen a positive formal charge. Like that. So we've almost formed our nitrate, ester. If we look at this, all we have to do now is an acid/base reaction to take this proton off of our oxygen. So water is a decent base. And a lone pair of electrons on our oxygen can take this proton, leaving these electrons behind on this oxygen here. And we would form our nitrate ester. All right. So now, we have R with an oxygen, and then, N02, like that. So once again, I think this is the correct mechanism, but I'm not 100% sure. The formation of nitrate esters has a few very famous examples. Let's look at the most famous reaction where a nitrate ester is formed. Let's look at what would happen if we started with this alcohol as our reactant. This is called glycerol or glycerine. It has three OH groups on it. And if you react glycerine with excess nitrate acid and also sulfuric acid-- all concentrated-- well, we would form a nitrate ester at each one of our OH groups. So we're going to form a nitrate ester at each of our OH groups to form this as our product. Right? We put nitro groups onto glycerine, so this is called nitroglycerin. So of course, everyone knows about nitroglycerin, a very famous high explosive. It's a liquid. It's extremely shock-sensitive, so it's extremely dangerous. Nitroglycerin is not something that anyone should attempt to make at home. So it's not a good demonstration for chemistry instructors. A much better demonstration for chemistry instructors would be to form another nitrate ester starting from cotton or cellulose. So here we have the cellulose polymer. It's made up of a bunch of glucose molecules. So here's a glucose molecule. Here's a glucose molecule. And if you stick a whole bunch of glucose molecules together in some giant polymer, you form cellulose, otherwise known as cotton. So if you take a bunch of cotton balls and you mix them with concentrated nitric acid and concentrated sulfuric acid, you can put nitro groups in each one of those alcohol groups. You can form nitrate esters. So each of these alcohol groups gets turned into a nitrate ester. So I can go ahead and put my nitrate esters in. You notice there are three nitrate esters for each glucose molecule. So we form nitrocellulose as our product, otherwise known as guncotton. Guncotton is much more stable than nitroglycerin is. You can store guncotton overnight. And if you ignite it, it will give you a nice little fireball. So it's an excellent demonstration for chemistry students.