Oxidation and Polymerisation

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle first. Electric wires are wrapped in plastic because most polymers do not conduct, right? But there is a polymer made from ethyne whose thin films, under special conditions, do conduct, and can even be used as electrodes in batteries. So how can a polymer conduct? Hold on to that question. Today we oxidise alkynes, burn them, and join them together. Questions on these do come up in JEE, so let's begin.

0:31Oxidation: breaking the triple bond

First, oxidation. Baeyer's reagent is cold, dilute, alkaline K M n O four, potassium permanganate; the small four written low is a subscript, okay? Alkynes decolourise it, just like alkenes do. Right? Now stronger conditions: ozonolysis, that is, treatment with ozone followed by water, or hot K M n O four. Either one breaks the triple bond completely. With me? R C triple bond C R prime: the three short lines are the triple bond, R is an alkyl group, and the little mark on the second R, a prime, means a second group, okay? The arrow means gives: RCOOH plus R prime COOH, two carboxylic acids. Each triple bond carbon becomes a COOH carbon. Clear? Now let me ask you this. In a terminal alkyne, the triple bond is at the end of the chain, so one of its carbons carries just a hydrogen in place of an alkyl group. Which acid does that carbon give? Yes, exactly, methanoic acid, HCOOH, the acid with just one carbon! On screen, the triple bond lines followed by C H show that end carbon. And methanoic acid can be oxidised further, to C O two, carbon dioxide. Keep this in mind for our second example. Okay?

1:58Burning ethyne: the oxyacetylene flame

Next, combustion. Two C two H two plus five O two gives four C O two plus two H two O, carbon dioxide and water; the big numbers in front count the molecules. Check the balance with me: four carbons, four hydrogens and ten oxygens on each side. Right? Ethyne's common name is acetylene, so ethyne burning in oxygen gives the oxyacetylene flame. It reaches over three thousand degrees Celsius; the small circle followed by C means degrees Celsius. That is why it is used for welding. Got it?

2:32Cyclic polymerisation: three alkynes, one ring

Now polymerisation, small molecules joining together. Pass ethyne through a red-hot iron tube at eight hundred and seventy three kelvin, and three molecules join to form a ring. Clear? Let me ask you this. Three molecules of C two H two join into one ring. What is the formula, and which famous compound is it? Yes, exactly, C six H six, benzene! Three times two carbons, and three times two hydrogens, gives six of each. With me? Now take propyne, C H three C triple bond C H, which is ethyne with one hydrogen replaced by a methyl group, C H three. If three propynes join in the same way, what do you get? Correct, it is benzene again, now with three methyl groups, one from each propyne, on alternate ring carbons. That is one, three, five trimethylbenzene, and the name in brackets, mesitylene, is its common name. Right? And but two yne, C H three C triple bond CCH three, has a methyl group on both triple bond carbons. So three of them put six methyl groups on the ring: hexamethylbenzene.

3:46Linear polymerisation: polyacetylene

Okay? Ethyne can also join in a long chain. This gives polyacetylene, also called polyethyne, the name in brackets. In its formula, C H double bond C H sits inside round brackets; the equals sign is the double bond. The small n written low means this unit repeats n times, and the dashes at both ends show that the chain carries on. Clear? Now, back to our puzzle. Imagine many of these units joined in a row. What pattern do the bonds make along the chain? Yes, the bonds alternate: single, double, single, double. Such a polymer is called conjugated. The Greek letter pi stands for the pi electrons of the double bonds, and here they are delocalised, spread out along the whole chain. Got it? That is why, under special conditions, thin films of polyacetylene conduct electricity, and can be used as electrodes in batteries. Puzzle solved: this polymer can conduct because its chain is conjugated, so its pi electrons can move along the chain.

4:50Example 1: mass of benzene from 7.8 g of ethyne at 873 K

Okay? Alright, let's try Example one. What is formed when ethyne is passed through a red-hot iron tube at eight hundred and seventy three kelvin? And what mass is obtained from seven point eight grams of ethyne, if the conversion is complete? The small g is grams. Pause the video and try it first. Done? Okay, let's check. The first part we just learnt: three C two H two give C six H six, benzene. Got it? Now the mass. Three molecules of ethyne become just one molecule of benzene. So the mass becomes one third: seven point eight over three gives two point six grams, right? No, no, careful, that is the trap! Fewer molecules does not mean less mass. Every carbon and every hydrogen of the three ethynes ends up inside benzene, so no atoms are lost, and mass is conserved. With me? Let us check with moles. Ethyne's molar mass is twenty six grams per mole, so seven point eight over twenty six gives zero point three mole; the slash means divided by, and mol is short for mole. Three moles of ethyne give one mole of benzene, so we get zero point one mole of C six H six. Clear? Benzene's molar mass is seventy eight grams per mole. Zero point one times seventy eight gives seven point eight grams, exactly the mass we started with! Okay?

6:22Example 2: identify C5H8 alkynes from hot KMnO4 products

Now a harder one. An alkyne, C five H eight, gives ethanoic acid and propanoic acid on oxidation with hot K M n O four. Identify it. Then, which isomer would give butanoic acid and C O two? Pause the video and try it first. Done? Okay, let's check. Here is a topper's trick: count the carbons in each product. Ethanoic acid, C H three COOH, has two carbons, shown as two C, and propanoic acid, C H three C H two COOH, has three. Right? Two plus three is five, which matches C five H eight. Each COOH carbon was a triple bond carbon, so we join these two carbons back with a triple bond. Got it? That gives C H three C triple bond CCH two C H three. Five carbons, with the triple bond starting at carbon two, so this is pent two yne. Okay? Now the second part. Butanoic acid has four carbons, and the fifth carbon comes out as C O two, not as a second acid. Where does that C O two come from? Yes, exactly, from a terminal carbon, the triple bond followed by C H, at the end of the line! That carbon gives methanoic acid, which is oxidised further to C O two. So four carbons, plus one, make five. With me? So the triple bond sits at the end of the chain: H C triple bond CCH two C H two C H three. That is pent one yne. See how counting carbons solved both parts? Clear?

8:08Quick revision: hot KMnO4, ethyne to benzene, polyacetylene

So, quick revision. Hot K M n O four, or ozonolysis followed by water, breaks the triple bond into two acids, and a terminal carbon gives methanoic acid, then carbon dioxide. Okay? In a red-hot iron tube at eight hundred and seventy three kelvin, three ethyne molecules give benzene. Right? And polyacetylene is a conjugated chain that can conduct electricity. Now go and try the practice problems on this page. All the best!