Carbon and Hydrogen; the Sodium Fusion Extract

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Chlorobenzene surely contains chlorine, right? Now add aqueous silver nitrate to it. And you see no precipitate at all. Strange, isn't it? But fuse the same compound with a little sodium, put the product into water, and now silver nitrate does give a precipitate. So where was the chlorine hiding? Today we will find out, and we will detect carbon and hydrogen too. Questions on this come up often in JEE Main, so let's begin.

0:34Carbon and hydrogen: heat with copper(II) oxide

So, first the easy pair, carbon and hydrogen. We heat the compound with dry copper two oxide, written C u O on screen. The Roman two in brackets tells you copper is in its plus two state. Okay? Now let me ask you this. When carbon takes oxygen from the copper oxide, what does it become? Carbon dioxide, yes, exactly! C plus two C u O gives two C u plus C O two. The arrow means gives, and the small two written low is a subscript: two oxygen atoms. In the same way, hydrogen becomes water: two H plus C u O gives C u plus H two O, which is water. Clear? Now, to see the carbon dioxide, we pass it into lime water, which is calcium hydroxide. The brackets with a two outside mean two O H groups, okay? Calcium carbonate forms, and the little down arrow after it means it comes out as a solid, a precipitate. That is why the lime water turns milky. Right? And the water meets anhydrous copper sulphate. Anhydrous means without water. Five water molecules attach to each copper sulphate, which is what the dot on screen shows, and the compound turns blue. So, milky lime water means carbon, and blue copper sulphate means hydrogen. With me?

1:55Lassaigne's test: why fuse with sodium?

Now the other elements: nitrogen, sulphur, the halogens, and phosphorus, written N, S and P on screen. In an organic compound, these are all covalently bound. Okay? Now remember our chlorobenzene. Its chlorine is held by a covalent bond. So tell me, can it give a test that looks for chloride ions? The answer is no, it cannot. Covalent means no free ions, so there are no ionic tests. The double arrow on screen means so, or therefore. That is exactly why silver nitrate saw nothing. Right? So here is how we fix it. We fuse the compound with a small piece of sodium metal, which means we heat them strongly together. This is Lassaigne's test, also called the sodium fusion test. The elements become ionic sodium salts that dissolve in water, so the chlorine of chlorobenzene ends up as chloride ions, in sodium chloride. Now silver nitrate can find them, and that solves our puzzle. Got it?

2:57What sodium fusion makes

So, here is what sodium makes. N a is the symbol for sodium. Sodium plus carbon plus nitrogen gives sodium cyanide. And two sodium plus sulphur gives sodium sulphide, where the subscript two means two sodium atoms. Now look carefully at the cyanide: carbon comes in along with nitrogen. Keep that in mind. Okay? Then the halogens. On screen, the letter X stands for any halogen: chlorine, bromine or iodine. So sodium plus a halogen gives a sodium halide, like sodium chloride. Clear? And now one special case to remember. When nitrogen and sulphur are both present and there is only a little sodium, they stay together. Sodium plus carbon plus nitrogen plus sulphur gives sodium thiocyanate, the N a SCN on screen. Got it?

3:48Making the sodium fusion extract

Now we need these salts in a solution. We plunge the red-hot fusion tube straight into distilled water. Then we boil the mixture, and filter it. Okay? The clear liquid that comes through is called the filtrate, and this filtrate is our sodium fusion extract. With me? Now, a small detail that is often asked. Why distilled water, and not tap water? Because tap water contains chloride ions. They would give a false halogen test, even when the compound has no halogen. So we always use distilled water. Clear?

4:23Points to remember

Now some points to remember. We use sodium because it is reactive enough to break those covalent bonds, and the salts it forms are all soluble in water. Magnesium and zinc are not used. Okay? Now tell me something. Many organic compounds contain oxygen too, so can Lassaigne's test detect oxygen? The answer is no. Fusion gives no ion that is characteristic of oxygen. So oxygen is estimated by difference, from what is left once the other elements are measured. Right? Next, nitrogen shows up as the cyanide ion, C N minus, or, when sulphur is also present, as the thiocyanate ion, SCN minus. The minus sign written up high is the negative charge. Either way, the ion contains carbon, so the nitrogen needs carbon to be detected. Our first example needs this. Clear? And remember sodium thiocyanate, which forms with only a little sodium. With excess sodium, it breaks up, and then the separate tests for nitrogen and sulphur work. With me? Finally, look at diazonium salts. On heating, they lose their nitrogen as nitrogen gas, N two, before it can form cyanide. So they may fail the test, even though they contain nitrogen. Got it?

5:40Example 1: why hydrazine fails Lassaigne's nitrogen test

Now let's do Example one. Why does hydrazine, N H two N H two, not give Lassaigne's test for nitrogen, although urea and aniline do? Pause the video and try it first. Done? Okay, let's check. Look at hydrazine: two nitrogen atoms in every molecule. So it should pass the nitrogen test even more easily than aniline, right? No, no, careful, that is the trap here! Nitrogen is detected as cyanide here, and to make cyanide, sodium needs carbon along with the nitrogen. And that carbon must come from the same compound. Clear? Now hydrazine has nitrogen and hydrogen only, not a single carbon atom. So the answer: hydrazine contains no carbon, so sodium cyanide cannot form, and there is no nitrogen test. Urea and aniline both contain carbon, so they give the test. Okay? And the same holds for ammonium chloride, N H four C l, and for hydroxylamine, N H two O H, which have no carbon either. So neither gives the test. With me?

6:48Example 2: thiourea fused with little vs excess sodium

Now a slightly harder one, Example two. Write the product of fusing thiourea with, in part A, a small amount of sodium, and in part B, an excess of sodium. On screen, the brackets with a subscript two outside mean two N H two groups, then one carbon and one sulphur. Pause the video and try it first. Done? Okay, let's check. Thiourea contains carbon, nitrogen and sulphur, all in one molecule. So in part A, nitrogen gives sodium cyanide and sulphur gives sodium sulphide, as usual, right? No, no, wait, that is the trap here! With only a little sodium, nitrogen and sulphur stay together. Sodium plus carbon plus nitrogen plus sulphur gives sodium thiocyanate. Clear? Now part B, with excess sodium. The extra sodium breaks up the thiocyanate: sodium thiocyanate plus two sodium gives sodium cyanide plus sodium sulphide. Check the sodium: three atoms on each side. Got it? So the answer: part A gives sodium thiocyanate, and part B gives sodium cyanide and sodium sulphide. Remember, a little sodium keeps nitrogen and sulphur together, and excess sodium splits them apart. Okay?

8:10Quick revision: C and H test, sodium fusion, N needs carbon

So, quick revision. For carbon and hydrogen, heat with copper oxide: carbon dioxide turns lime water milky, and water turns anhydrous copper sulphate blue. Sodium fusion turns covalent nitrogen, sulphur and halogens into ionic sodium salts. And nitrogen shows up as cyanide or thiocyanate, so the compound must contain carbon. Now go and try the three practice problems on this page. All the best!