Overview

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Picture a gas that is very stable, non toxic, non corrosive and easily liquefied. Perfect for refrigerators, right? Yet this same gas damages the ozone layer, high up in the stratosphere. So how can something so safe do so much harm? Strange, isn't it? That gas is a freon, a polyhalogen compound, that is, a carbon compound with more than one halogen atom. Today we meet the whole chapter, and solve the puzzle too. Questions on these compounds do appear in JEE papers. So, let's begin.

0:39Meet the family

So, here are four members of the family. First, chloroform, C H Cl three, where the small numbers written low count the atoms: three chlorines, so trichloromethane. Next, freon twelve, C Cl two F two, our puzzle gas, which is dichlorodifluoromethane. Okay? Then the famous insecticide, DDT, short for p p prime dichlorodiphenyltrichloroethane. The italic p means para, and the prime, the small mark, refers to the second ring. In the figure, two benzene rings, each with a para chlorine, join one C H carbon carrying a C Cl three group. With me? And last, BHC, benzene hexachloride, C six H six Cl six. In the figure you see six carbons in a ring, with one chlorine on every carbon, so its proper name is hexachlorocyclohexane. The label says gammaxene, and later we will see that gammaxene is one particular isomer of it.

1:43Dihalides and trihalides with aqueous alkali

Clear? Now the dihalides with aqueous alkali. In a vicinal dihalide, the halogens sit on neighbouring carbons, and each is replaced by O H, so Cl C H two, C H two Cl gives HOCH two, C H two O H, a diol. The dash is a bond, and the arrow means gives. Okay? In a geminal dihalide, both halogens sit on one carbon, so we get two O H groups on one carbon. Let me ask you this: is that a stable product? No, it is not, and that is the trap! Two O H groups on one carbon lose water at once. Right? So, with X for any halogen, a terminal CHX two gives an aldehyde. The sign that looks like greater than, before C X two, means two bonds to that carbon, so it sits inside the chain, and that gives a ketone. And a trihalide, C X three, gives a carboxylic acid, as its salt. Got it?

2:44Chloroform: why the dark, full bottle?

Next, chloroform, a solvent, mainly used today to make the refrigerant freon R twenty two, C H Cl F two. But why is it always kept in dark bottles, filled right to the top? Here is the reason. In air and light, chloroform is slowly oxidised. Two C H Cl three plus O two, with light written over the arrow, gives two C O Cl two, plus two H Cl. With me? And C O Cl two is phosgene, a very poisonous gas. So the dark bottle keeps out light, and a full, closed bottle keeps out air. Clear? And about one percent ethanol is added, which turns any phosgene into harmless diethyl carbonate. Okay?

3:29Iodoform and the haloform test

Now the haloform reaction. Look for C H three C O, joined to a carbon or a hydrogen, or for C H three CHOH, with O H in brackets as a side group, which can be oxidised to it. With iodine and sodium hydroxide, it gives CHI three, iodoform, a yellow solid. That is the iodoform test. Got it? So these pass: ethanal, the only aldehyde, methyl ketones, ethanol, the only primary alcohol, and secondary alcohols like C H three CHOHR, with R an alkyl or aryl group. Now, ethanoic acid contains this group too. Does it pass? No, it fails, and that is a classic trap! Its C O group is attached to oxygen, not to carbon or hydrogen. Methanol, propan one ol and pentan three one fail too. Right? And iodoform was once used as an antiseptic, through the iodine it liberates.

4:33Carbon tetrachloride and the freons

Now carbon tetrachloride, C Cl four: a solvent, and the feedstock for chlorofluorocarbons, written CFC for short. It was once a fire extinguisher too, remember that. But it harms the liver. Okay? From it, the Swarts reaction, a halogen exchange, makes freon twelve. Three C Cl four plus two S b F three, with S b Cl five over the arrow as catalyst, gives three C Cl two F two, plus two S b Cl three. Right? Freons are stable, non toxic and easily liquefied, so they serve as refrigerants and aerosol propellants. So, back to our puzzle: how can such a safe gas harm the ozone layer? The answer is that same stability! Freons drift unchanged into the stratosphere, where ultraviolet light, U V, frees chlorine atoms, and the dot written high marks an unpaired electron. Cl dot takes an oxygen from ozone, O three, giving Cl O dot and O two. Then Cl O dot meets an oxygen atom, and Cl dot comes back! So the cycle repeats, a chain reaction. With me?

5:46DDT and BHC

Now, DDT is made from two moles of chlorobenzene and chloral, with concentrated sulphuric acid, H two S O four, where conc means concentrated. Okay? It was the first chlorinated organic insecticide, against the mosquitoes that spread malaria and the lice that carry typhus. So why is it banned in many countries? The reason is that it persists, because it is not broken down quickly. It is fat soluble, so it is stored in fatty tissue and builds up along the food chain. That is why many countries have banned it. Clear? And BHC is made when benzene adds three molecules of chlorine in ultraviolet light, with U V over the arrow. Careful, this is addition, not substitution. Got it? Its gamma isomer, written with the Greek letter gamma, is gammaxene, or lindane, an insecticide.

6:39Example 1: match CCl4, DDT, CFC and iodoform to their uses

Right? This first example is a JEE Main question from twenty twenty four, from the recent questions section. Match C Cl four, DDT, CFC and CHI three with their uses: antiseptic, refrigerant, insecticide and fire extinguisher. Pause the video and try it first. Done? Okay, let's check. C Cl four was once a fire extinguisher, and DDT is the insecticide. With me? Then CFC, like freon twelve, is the refrigerant, leaving CHI three, iodoform, as the antiseptic. If you got all four, very good!

7:22Example 2: percentage of chlorine in DDT, C14H9Cl5

Now a harder one, from the DDT section of this chapter. Find the percentage of chlorine in DDT, C fourteen H nine Cl five. Use carbon twelve, hydrogen one and chlorine thirty five point five. Pause the video and try it first. Done? Okay, let's check. First M, the molar mass: fourteen times twelve, plus nine times one, plus five times thirty five point five, which gives three hundred and fifty four point five. Now, what goes on top of the fraction? All five chlorines, so five times thirty five point five, which is one hundred and seventy seven point five. Use just one chlorine and you get about ten percent, and that is the trap. Clear? So the percentage is one hundred and seventy seven point five, over three hundred and fifty four point five, times one hundred, which gives fifty point one percent. Half of DDT, by mass, is chlorine!

8:27Quick revision: dihalides, chloroform, iodoform, freons, DDT

So, quick revision. Dihalides with alkali: vicinal ones give diols, geminal ones give aldehydes or ketones. Chloroform needs dark, full bottles and about one percent ethanol, against phosgene. The iodoform test needs C H three C O on a carbon or a hydrogen, or C H three CHOH, and gives yellow iodoform. Freons are so stable that they reach the stratosphere, where chlorine atoms destroy ozone. And DDT is a useful insecticide, but it persists, so many countries ban it. Now try the practice problems in the next sections. All the best!