Chloroform

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Chloroform has three chlorine atoms in every molecule. Shake a pure sample with silver nitrate solution, and nothing happens, no precipitate at all. Now do the same test on an old, badly stored bottle, and this time you get a precipitate! Same liquid, so where did that chloride come from? Keep that question in mind. Today we see how chloroform is made, what it is used for, how to store it, and how it reacts. Questions on chloroform come up often in JEE, so let's begin.

0:37What chloroform is, and how it is made

So, first, what is chloroform? Its proper name is trichloromethane, C H Cl three. The small number written low counts the atoms, so one carbon, one hydrogen and three chlorines. Clear? Now, how do we make it? Treat ethanol or propanone with bleaching powder, where the plus sign means together with. The name in brackets is the haloform reaction, and you will meet it again with iodoform. Okay? There are two more ways. Carbon tetrachloride, C Cl four, is partially reduced by iron, F e, and water, shown in brackets. One chlorine is replaced by a hydrogen, and we get C H Cl three. And methane can be chlorinated. With me?

1:20What is chloroform used for?

Now the uses. Chloroform is a solvent for fats, alkaloids and iodine, and it was once used as an anaesthetic. But neither is its main use today. So, what do you think it is? Did you guess it? It is used mainly to make the refrigerant freon R twenty two, C H Cl F two. So that carbon carries one hydrogen, one chlorine and two fluorines. Got it?

1:47Storage: why the dark, full bottle?

Now the most important part, storage. In air and light, chloroform is slowly oxidised. Two C H Cl three plus O two, with light written over the arrow as the condition, gives two C O Cl two, plus two H Cl. With me? And C O Cl two is phosgene, also called carbonyl chloride, as the brackets show. It is a very poisonous gas. Scary, right? So, how should we store chloroform? What must we keep out? Yes, exactly! Keep out the light, and keep out the air. So chloroform is kept in dark bottles, completely filled and closed, which leaves no room for air. Clear? And for extra safety, about one percent ethanol is added, with the percent sign you see. It turns any phosgene that does form into harmless diethyl carbonate. Smart, isn't it?

2:40Back to our puzzle

Now, back to our puzzle. Pure chloroform is covalent, so it gives no Cl minus ions, the chloride ions, with the small minus sign written high up. No chloride ions, no precipitate with silver nitrate. So why did the old bottle give one? Remember the storage equation? Oxidation also gives H Cl! The old sample contains H Cl from its oxidation, and that gives the precipitate. And that solves our puzzle. Got it?

3:10Reactions with KOH

Now the reactions. First, boil chloroform with KOH, aqueous potassium hydroxide. The hydroxide replaces all three chlorines, giving three O H groups on one carbon, a triol. That is the product, right? Not quite, and this is the trap, so be careful! Three O H groups on one carbon lose water, giving methanoic acid, which the alkali neutralises. So C H Cl three plus four KOH gives HCOOK, potassium methanoate, plus three K Cl and two water molecules. Why four KOH? Three replace the chlorines, and one neutralises the acid. Okay? Next, warm RNH two, a primary amine, with chloroform and alcoholic KOH, as shown on screen. We get RNC, plus three K Cl and three water molecules. Right? And RNC is an isocyanide, with a really offensive smell. The name in brackets says it all: this is the carbylamine test, a test for primary amines. Clear?

4:19More reactions, and dichlorocarbene

Now heat chloroform with A g, silver powder. Silver takes all the chlorine as six A g Cl, silver chloride, and the two C H pieces join as H C triple bond C H, which is ethyne. The three short lines mean a triple bond. With me? With concentrated nitric acid, HNO three, chloroform gives chloropicrin, C Cl three N O two, which is a tear gas. Okay? And a strong base removes H Cl, with both atoms taken from the same carbon, leaving C Cl two with two dots on the carbon. Those dots are a lone pair of electrons. This is dichlorocarbene. Right? It is very reactive, and it is the reactive species in the carbylamine reaction we just saw, and in the Reimer Tiemann reaction. Keep it in mind for our second example. Got it?

5:10Example 1: mass of phosgene from 11.95 g of chloroform

Now let's use all this. Find the mass of phosgene that can form from eleven point nine five grams of chloroform, where g means grams. The atomic masses are given: carbon is twelve, hydrogen is one, oxygen is sixteen, and chlorine is thirty five point five. Pause the video and try it first. Done? Okay, let's check. First, capital M, the molar mass of chloroform. We add twelve, plus one, plus three times thirty five point five, which gives one hundred and nineteen point five. Okay? So n, the number of moles, is eleven point nine five over one hundred and nineteen point five, which gives zero point one mole. Right? Now, remember the storage equation. How many moles of phosgene come from each mole of chloroform? Two moles of chloroform give two moles of phosgene, so the ratio is one to one, with the colon read as to. Oh no, careful, do not double the answer just because both have a two in front! So we get zero point one mole of C O Cl two. Clear? The molar mass of phosgene is twelve, plus sixteen, plus two times thirty five point five, which gives ninety nine. With me? So the mass is zero point one times ninety nine, which gives nine point nine grams. Under twelve grams of chloroform can give almost ten grams of a very poisonous gas. Now you see why storage matters!

6:41Example 2: chloroform acidity and dichlorocarbene

Now a harder one. Chloroform is more acidic than methane, and with a strong base it gives a reactive intermediate. Explain why, and name the intermediate. Pause the video and try it first. Done? Okay, let's check. Acidic means it can lose H plus, a proton, with the small plus sign written high up. So a strong base pulls H plus off chloroform and leaves C Cl three minus, an anion with the negative charge on carbon. Okay? Now the real question. Why is this easier for chloroform than for methane? Yes, the three chlorine atoms! They are electron withdrawing, so they stabilise C Cl three minus. Methane has no such chlorines, so it is far less acidic. Got it? Next, C Cl three minus loses a chloride ion, Cl minus, from the same carbon. Both the hydrogen and the chlorine leave from one carbon, so this is called alpha elimination, written with the Greek letter alpha. Right? And what is left behind is dichlorocarbene, C Cl two, which you met a few minutes ago, with its lone pair shown by the two dots. That is our reactive intermediate. Very good if you got it!

7:58Quick revision: preparation, phosgene, KOH, dichlorocarbene

So, quick revision. Chloroform is made from ethanol or propanone with bleaching powder. Air and light give phosgene, so we use dark, full bottles with about one percent ethanol. Aqueous KOH gives HCOOK, and primary amines give isocyanides. And a strong base gives dichlorocarbene. Now go and try the practice problems on this page. You can do them, all the best!