Addition, Oxidation and Side-Chain Reactions

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Treat benzene with hot K M n O four, potassium permanganate, and nothing happens; benzene itself resists it. Now put one methyl group on the ring, which makes toluene, and try again. This time the methyl group is oxidised to an acid group, and the ring survives! So, if the side chain has two carbons, as in ethylbenzene, how many of them stay on the ring after oxidation? Hold on to your answer; Example two will settle it. These reactions come up often in JEE, so let's begin.

0:36Addition: benzene adds when pushed hard

First, reactions that are not substitution in the ring: benzene can also add. In hydrogenation, C six H six becomes cyclohexane, C six H twelve. So how many molecules of hydrogen, H two, must add to one benzene? Yes, exactly, three of them, for six extra hydrogen atoms! The small numbers written low are subscripts. Above the arrow: N i, the nickel catalyst, and four hundred and seventy three to five hundred and twenty three kelvin, with capital K for kelvin. It is run under pressure. Okay? Next, chlorine. In ultraviolet light at five hundred kelvin, three molecules of chlorine, C l two, add to benzene. The product is C six H six C l six, with six chlorines added and all six hydrogens still there. Right? This is benzene hexachloride, BHC in brackets. The Greek letter gamma marks one of its isomers, the gamma isomer, which is the insecticide gammexane, also called lindane. With me?

1:40Ozonolysis of benzene

Now ozonolysis, the reaction with ozone. Benzene gives three molecules of ethanedial, OHC, dash, CHO, also called glyoxal. Each has two carbons, so three of them use all six ring carbons. Clear? Let me ask you this. Ozonolysis cleaves each double bond, and the benzene ring is drawn with three of them. So how many moles of ozone react with one mole of benzene? Correct, it is three moles of ozone, O three, because each double bond needs its own ozone molecule! On screen, mol is short for mole.

2:17Oxidation of side chains

Got it? Now back to our puzzle. Benzene itself resists K M n O four; the ring is not oxidised. Okay? But an alkyl side chain is a different story. Hot K M n O four, or hot K two C r two O seven, potassium dichromate, oxidises it to COOH, the carboxylic acid group; the dash shows where it joins the ring. Right? So toluene, C six H five C H three, gives C six H five COOH, benzoic acid. The ring stays as it is; only the side chain carbon is oxidised. Clear? Now the condition. The side chain needs at least one benzylic hydrogen, the capital H on screen: a hydrogen on the carbon joined directly to the ring. Without one, it is not oxidised this way. And the length of the chain does not matter. Remember that for our puzzle. With me?

3:11Toluene and chlorine: the conditions decide

Next, halogenation. In toluene, chlorine has two choices: the ring, or the methyl side chain. So where will the chlorine go? It depends on the conditions! In light or heat, the free radical conditions, the halogen enters the benzylic position, on the side chain. Got it? With chlorine, the benzylic hydrogens are replaced one by one. C six H five C H three gives C six H five C H two C l, then C six H five CHC l two, and finally C six H five C C l three. Okay? In order, these are benzyl chloride, benzal chloride and benzotrichloride. Three benzylic hydrogens, so three steps. Right? But with a Lewis acid such as F e C l three, ferric chloride, in the dark, the halogen enters the ring instead. So remember it like this: light for the side chain, Lewis acid for the ring. Clear?

4:10Toluene to benzaldehyde: the Etard reaction

Now suppose we want benzaldehyde, C six H five CHO, and not the acid. Can we simply use hot K M n O four on toluene? No, no, careful, that goes all the way to benzoic acid! We need a different reagent. Toluene with chromyl chloride, C r O two C l two, in C S two, carbon disulphide, then water: this is the Etard reaction. With me? The other way is C r O three, chromium trioxide, in ethanoic anhydride, followed by hydrolysis. Both routes give benzaldehyde. Okay?

4:47A health note

One quick note. Benzene, and polynuclear hydrocarbons with more than two fused rings, are carcinogenic, which means they can cause cancer. Right? Alright, Example one. Benzene reacts with chlorine in ultraviolet light. Name the product, find its molar mass, and the percentage of chlorine in it. Pause the video and try it first. Done? Okay, let's check. U V, ultraviolet light, means addition here, not substitution: three C l two molecules add, giving C six H six C l six, which is BHC, right? Its proper name is one, two, three, four, five, six hexachlorocyclohexane: a cyclohexane ring with a chlorine on every carbon. Clear? Now the molar mass. Benzene, C six H six, is seventy eight. Three molecules of chlorine were added, so it is seventy eight plus three times thirty five point five, right? No, no, careful, that is the trap! Each C l two has two atoms, so three molecules bring six chlorine atoms, and six times thirty five point five gives two hundred and thirteen. Seventy eight plus two hundred and thirteen gives two hundred and ninety one grams per mole; capital M is the molar mass, and g mol to the power minus one is grams per mole. Got it? Finally, the percentage of chlorine is two hundred and thirteen over two hundred and ninety one, times one hundred. That gives seventy three point two percent. So nearly three quarters of BHC, by mass, is chlorine! Okay?

6:23Example 2: KMnO4 on ethylbenzene, isopropylbenzene, p-xylene

Now a harder one, and it answers our opening puzzle. What are the products of vigorous oxidation with K M n O four of ethylbenzene, isopropylbenzene, and one, four dimethylbenzene? Pause the video and try it first. Done? Okay, let's check. Ethylbenzene has two carbons in its side chain. So the acid keeps both of them, right? No, no, careful, this is the classic trap! Oxidation starts at the benzylic carbon, and the rest of the chain is cut off, whatever its length. So C six H five C H two C H three gives C six H five COOH, benzoic acid. Puzzle solved: only one carbon stays on the ring. With me? Next, isopropylbenzene. Its benzylic carbon carries two methyl groups. Does it still have a benzylic hydrogen? Yes, exactly, one hydrogen is left on that carbon, and one is enough! In the formula, C H three sits in round brackets, and the small two after them means two methyl groups. So C six H five C H, C H three in brackets twice, also gives C six H five COOH, benzoic acid. Clear? Last, one, four dimethylbenzene has two methyl groups, on opposite carbons of the ring. Here is a topper's trick: every side chain with a benzylic hydrogen becomes its own COOH group. So we get benzene one, four dicarboxylic acid, also called terephthalic acid. Okay?

7:57Quick revision: BHC, benzylic H rule, light vs Lewis acid

So, quick revision. Benzene adds three hydrogen molecules over nickel to give cyclohexane, and three chlorine molecules in ultraviolet light to give BHC, okay? Hot K M n O four turns any side chain with a benzylic hydrogen into COOH, whatever its length. And for chlorine, light for the side chain, Lewis acid for the ring. Now go and try the practice problems on this page. All the best!