Carbon Tetrachloride, Freons, DDT and BHC

Transcript

0:00Introduction

Hello everyone! First, a small puzzle. One well known refrigerant is called freon twelve, and its cousins are freon eleven and freon twenty two. Are these just model numbers? No, they are not! Each number is a code that hides the full formula. Today we meet carbon tetrachloride, the freons, DDT and BHC, and at the end you will crack that code yourself. Questions on these compounds do appear in JEE papers, so let's begin.

0:31Carbon tetrachloride, CCl₄

So, first, carbon tetrachloride, C Cl four, where the small numbers written low count the atoms. From methane, C H four plus four Cl two gives C Cl four plus four H Cl. The arrow means gives. Okay? Or from carbon disulphide: C S two plus three Cl two gives C Cl four plus S two Cl two, with aluminium chloride, Al Cl three, or iron, F e, present. Right? It is a solvent, and the feedstock for C F Cs, the chlorofluorocarbons. Earlier it was even a cleaning fluid and a fire extinguisher. Clear? But careful, it causes liver damage, even liver cancer, along with dizziness and nausea. And it rises into the atmosphere and depletes the ozone layer. Got it?

1:20Why water does not attack CCl₄

Now a lovely comparison. Silicon tetrachloride, Si Cl four, looks just like C Cl four, and water hydrolyses it violently. So will water attack C Cl four too? No, it will not, and that is the trap! Water does not attack C Cl four at all. So what does silicon have that carbon lacks? Silicon has vacant three d orbitals, empty d orbitals in its third shell, which can accept a lone pair from water. With me? Carbon has none in its valence shell, and the small carbon atom is shielded by four chlorines. So water simply cannot get in.

2:01Freons: the perfect refrigerants

Okay? Now the freons. They are C F Cs, chlorofluorocarbons of methane and ethane. They serve as refrigerants, in air conditioning, and as aerosol propellants. Clear? And why are they so useful? They are very stable, unreactive, non toxic, non corrosive and easily liquefied. Remember that word stable, it comes back soon. Right? Freon twelve is made from C Cl four. Three C Cl four plus two S b F three gives three C Cl two F two, plus two S b Cl three, where S b is antimony, and the S b Cl five over the arrow is the catalyst. Now, what has changed from C Cl four to C Cl two F two? Yes, exactly, two chlorines have been swapped for two fluorines! That is a halogen exchange, called the Swarts reaction. With me?

2:56How freons destroy ozone

Now the dark side of that stability. Freons drift up unchanged into the stratosphere, where ultraviolet light breaks them; the h nu over the arrow, with the Greek letter nu, stands for light. C F two Cl two gives C F two Cl with a dot, plus Cl dot. The dot, written high, marks an unpaired electron, so Cl dot is a free chlorine atom. Clear? This chlorine atom attacks ozone, O three. Cl dot plus O three gives Cl O dot plus O two. So one ozone molecule is gone. Got it? Then Cl O dot meets an oxygen atom, O, and gives Cl dot plus O two. Look, the chlorine atom is back again. So how many ozone molecules can one chlorine atom destroy? It can destroy thousands, because Cl dot is regenerated, so the two steps repeat as a chain. See the irony? The same stability that makes freons safe lets them reach the stratosphere. Okay?

4:00DDT: a useful insecticide that stays

Now the famous insecticide DDT, the third structure in the figure; the first two are chloroform and our freon twelve. Its full name is p p prime dichlorodiphenyltrichloroethane, where p means para, and the prime, the small mark, points to the second ring. It is made from chloral, C Cl three CHO, and two moles of chlorobenzene. With me? The reagent is concentrated sulphuric acid, H two S O four. Right? DDT was the first chlorinated organic insecticide, used against malaria mosquitoes and typhus lice. So why have many countries banned it? 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. Okay? It is very toxic to fish, and many insects have become resistant to it. That is why it is banned. Clear?

4:58BHC: benzene hexachloride

And BHC, benzene hexachloride, is the ring at the far right of the figure. Benzene, C six H six, adds three molecules of chlorine in ultraviolet light, U V over the arrow, giving C six H six Cl six. Is this a substitution, or an addition? It is an addition, because all six hydrogens are still there, and six chlorines have joined them. So the product is no longer aromatic. Got it? Its gamma isomer, written with the Greek letter gamma, is gammaxene, or lindane, an insecticide. Right?

5:37Example 1: percentage of chlorine in BHC, C6H6Cl6

Now our first example. Find the percentage of chlorine in BHC, C six H six Cl six, the ring on the right of the figure. Take carbon as twelve, hydrogen as one, and chlorine as thirty five point five. Pause the video and try it first. Done? Okay, let's check. First M, the molar mass, keeping all six hydrogens, since BHC came from addition. Six times twelve, plus six times one, plus six times thirty five point five, gives two hundred and ninety one; the cross means times. Now, which chlorine mass goes on top? All six chlorines, so six times thirty five point five, which is two hundred and thirteen. Use only one chlorine and you fall into the trap. Clear? So the percentage is two hundred and thirteen over two hundred and ninety one, times one hundred, which gives seventy three point two percent. Almost three quarters of BHC is chlorine!

6:43Example 2: formulae of freon-11, freon-12 and freon-22

Now the harder one, our puzzle from the start. Read a freon number from the right: the digits give the number of fluorine atoms, then hydrogen atoms plus one, then carbon atoms minus one. Write the formulae of freon eleven, freon twelve and freon twenty two. Pause the video and try it first. Done? Okay, let's decode it. From the right we read fluorines, hydrogens plus one, and carbons minus one, and chlorine fills the valencies of carbon that are left. Now, freon eleven has only two digits. So how many carbons does it have? The missing third digit is zero, so carbons minus one is zero, and there is one carbon. The digit on the right is one, so there is one fluorine. The other digit is also one, so hydrogens plus one is one, which means no hydrogen. The other three valencies go to chlorine. The double arrow means therefore, and we get C Cl three F, freon eleven. Okay? For freon twelve, one carbon, no hydrogen and two fluorines leave two chlorines, so C Cl two F two, the same freon twelve as in the Swarts reaction. Right? For freon twenty two, the left digit is two, so hydrogens plus one is two, which means one hydrogen. With two fluorines, one chlorine fills the last valency, giving C H Cl F two. Not a model number at all, but a full formula. Got it?

8:13Quick revision: CCl4, freons, ozone chain, DDT, BHC

So, quick revision. Carbon tetrachloride comes from methane or carbon disulphide, and water cannot hydrolyse it, because carbon has no vacant d orbitals. Freon twelve comes from the Swarts reaction, and it is so stable that it reaches the stratosphere, where one chlorine atom destroys ozone in a chain. DDT persists and builds up the food chain, and BHC is an addition product. And the freon code, from the right, gives fluorines, hydrogens plus one, and carbons minus one. Now try the practice problems on this page. All the best!