The Sodium Fusion Extract

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Chloroform has three chlorine atoms in every molecule, and silver nitrate is what we use to test for chlorine, right? Yet silver nitrate cannot be added directly to chloroform to test for chlorine. So what is going on here? Keep that question in mind. The answer lies in the sodium fusion extract, also called Lassaigne's extract. The tests for nitrogen, sulphur and the halogens in this chapter are all done on it, and it comes up often in JEE practical chemistry. So, let's begin.

0:37Why fuse the compound with sodium?

So, here is the root of the problem. In an organic compound, nitrogen, written N, sulphur, written S, and the halogens are held by covalent bonds. They are not present as ions, so they give no ionic tests. Okay? But our tests, like the one with silver nitrate, need ions. So how can we turn these covalently bound atoms into ions? We fuse the compound with sodium metal, yes, exactly. Sodium converts nitrogen, sulphur and the halogens into sodium salts. Sodium salts are ionic, so they give ionic tests. With me? And now our puzzle is solved. In chloroform, the chlorine is covalently bound, so it gives no chloride ions, the C l with a minus sign written high on screen. So the chlorine must first be converted into sodium chloride, N a C l, by fusion with sodium.

1:32Making the extract, step by step

Clear? Now let us make the extract. The chart on the right is from the course, and the box at the top is the extract we make today. First, a small piece of sodium is dried between filter papers, to remove the kerosene it is stored in, which would char and interfere. The sodium then goes into a fusion tube, with the organic compound. Okay? Now we heat the tube. Suppose you heat it strongly right from the start. What can go wrong with a volatile compound, like chloroform? The compound simply escapes, yes, exactly. It evaporates before the sodium has melted and reacted with it. So we heat gently at first, and only then until the tube is red-hot. Got it? Next, the red-hot tube is plunged into distilled water. Notice the word distilled. Why not ordinary tap water? The reason is that tap water contains chloride ions, the C l minus on screen again. With silver nitrate, they would give a false test for chlorine. So the water must be distilled. Clear? Finally, we boil the mixture and filter it. Boiling also destroys any unreacted sodium, and filtering removes glass pieces and carbon. The filtrate is our sodium fusion extract. In the chart, the four boxes below it are the tests on this extract, for nitrogen, sulphur, both together, and the halogens, with their results under them. We take them up in the next lessons. Okay?

3:07What the fusion produces

Now let us see what forms in the tube. For nitrogen, sodium, carbon and nitrogen together give sodium cyanide, N a C N, and the arrow means gives. The carbon comes from the organic compound itself. Remember this carbon, because it matters in Example two. Got it? Sulphur takes two sodium atoms. Two N a plus S gives N a two S, sodium sulphide, where the small two written low means two sodium atoms. Right? Now for the halogens, sodium plus X gives N a X, where the capital X stands for chlorine, bromine or iodine, as the bracket shows. That is a sodium halide, an ionic salt that dissolves in water. With me?

3:46Nitrogen and sulphur together

Now here comes a trap. Suppose the compound has both nitrogen and sulphur. Then we simply get sodium cyanide and sodium sulphide, right? That is true only when there is enough sodium, so be careful here. If the sodium is insufficient, sodium, carbon, nitrogen and sulphur combine into one salt, N a SCN, which is sodium thiocyanate. Okay? Excess sodium destroys the thiocyanate. N a SCN plus two N a gives N a C N plus N a two S, cyanide plus sulphide. There are three sodium atoms on each side, so it is balanced, right? So excess sodium is recommended. Then nitrogen appears as cyanide and sulphur as sulphide, and each can be tested separately. Clear?

4:34Alkaline extract, and why sodium

Now one more property of the extract. Remember, some sodium is left unused in the tube. So tell me, is the extract acidic, neutral or alkaline? It is alkaline, very good. The unused sodium gives sodium hydroxide, N a O H, with water. Okay? And this matters in the tests that follow. Most of them need the extract acidified with the proper acid, at the right stage. Otherwise metal hydroxides or oxides precipitate instead of the expected product. One test for sulphur, which we meet later, works directly in the alkaline extract. Got it? Finally, here is why sodium is chosen for the fusion. It melts easily, it reacts readily with organic matter, and it gives salts that dissolve in water. Right? Compare the other metals. Potassium reacts too violently, and magnesium gives salts that are not all soluble. So sodium is the best choice. With me?

5:38Example 1: sodium salts from urea, thiophene, bromobenzene

Now Example one, an easy one to warm up. Name the sodium salts formed when urea, thiophene and bromobenzene are fused with sodium. Here is a hint: first ask which of nitrogen, sulphur and the halogens each compound contains. Pause the video and try it first. Have you finished? Okay, let's check. Urea, whose formula on screen has two N H two groups, contains nitrogen, and carbon too. So it gives sodium cyanide, N a C N, and the double arrow means so we get. Okay? Now thiophene. It has carbon, four carbon atoms in fact. So does thiophene also give sodium cyanide? No, it does not, and that is the trap. Sodium cyanide needs nitrogen as well as carbon, and thiophene, C four H four S, has no nitrogen at all. It has sulphur, so it gives N a two S, sodium sulphide. Clear? And bromobenzene, C six H five B r, has bromine, so it gives sodium bromide, N a B r. So the answers are sodium cyanide, sodium sulphide and sodium bromide. That was easy, right?

6:47Example 2: moles of sodium used by one mole of thiourea

Now Example two, a little harder, with numbers. How many moles of sodium are used up in converting one mole of thiourea completely into sodium cyanide and sodium sulphide? Its formula is on screen: two N H two groups, then one carbon and one sulphur. Pause the video and try it first. Have you got an answer? Okay, let's check. First, count the atoms in one molecule: one carbon, two nitrogens and one sulphur. With me? Now, two nitrogens means two sodium cyanides, right? That is two moles of sodium, plus two more for the sulphide, so four moles in all? That is exactly the trap, so be careful here. Each sodium cyanide needs one carbon as well as one nitrogen, and thiourea has only one carbon. So the second nitrogen has no carbon to form cyanide, and only one N a C N forms. That uses one mole of sodium, and mol on screen is short for moles. Okay? Next comes the sulphur. One sulphur gives one N a two S, and that takes two moles of sodium. Right? So the total is one plus two, which gives three moles of sodium. The answer is three, not four. See, that carbon in the fusion equation decided the answer. Nice, isn't it?

8:11Quick revision: sodium salts, NaSCN, alkaline extract

So, quick revision. Fusion with sodium turns the covalent nitrogen, sulphur and halogens into ionic sodium salts. We get sodium cyanide, sodium sulphide and sodium halide, and with too little sodium, sodium thiocyanate. Heat gently first, use distilled water and excess sodium, and remember that the extract is alkaline. Now go and try the practice problems on this page. All the best!