Overview

Transcript

0:00Introduction

Hello everyone! Phenol has an old name, carbolic acid. It is about a million times stronger as an acid than ethanol, and it dissolves in sodium hydroxide. But put it into sodium bicarbonate solution, and nothing happens, no fizz at all. So, is phenol an acid or not? Today we see the whole chapter on one map, and solve this puzzle too. This chapter comes up often in JEE, so let's begin!

0:27The whole chapter on one map

So, here is the whole chapter on one page. In the centre sits phenol, C six H five O H, where the small numbers below the letters count the atoms. On the left, four arrows point in, four ways to make it. On the right, eleven arrows point out, its reactions. Okay? Now let me ask you this. Benzyl alcohol, C six H five C H two O H, has a benzene ring and an O H group. Is it a phenol? No, it is not a phenol! In a phenol, the O H sits directly on a ring carbon. In benzyl alcohol it sits on the C H two group, outside the ring, so it is an alcohol, right?

1:04Four ways to make phenol

Let's start on the left. Cumene, that is isopropylbenzene, plus oxygen, O two, from the air. On these four arrows, the semicolon means then, so next comes dilute acid, H plus slash H two O, where the raised plus is the charge on the proton, and the slash means with. We get phenol and acetone, and this is the industrial method. With me? The second is Dow's process. Chlorobenzene, C six H five C l, with sodium hydroxide, N a O H, at six hundred and twenty three kelvin and three hundred atmospheres, atm for short. That gives sodium phenoxide, so acid, H plus, sets phenol free. Right? Third, sodium benzenesulphonate, C six H five S O three N a, fused with sodium hydroxide, then acidified. Clear? And fourth, benzenediazonium chloride, C six H five N two plus C l minus, the raised signs being the charges on the two ions. Warm it with water, nitrogen gas, N two, leaves, and phenol forms. Got it?

2:01Phenol is an acid, but a weak one

Now the right side, from the top. Phenol reacts with aqueous sodium hydroxide to give sodium phenoxide, C six H five O N a, a salt, so phenol behaves as an acid. Okay? How strong an acid is it? Look at p K a, the K with a small a below it. Smaller p K a means a stronger acid. Phenol has ten point zero and ethanol fifteen point nine, so phenol is about a million times stronger. And why is it stronger? In the phenoxide ion, C six H five O minus, resonance shares the negative charge between the oxygen and three carbons of the ring. Clear? So, back to our puzzle. Will phenol give a fizz of carbon dioxide with sodium bicarbonate, N a HCO three? No, it will not: the map says no reaction. Phenol is weaker than carbonic acid, and only acids stronger than carbonic acid set carbon dioxide, C O two, free. So phenol is an acid, but a weak one, and that solves our puzzle. Got it?

2:58The ring: OH activates, ortho and para

Now the ring. The O H group activates the ring, and directs new groups to ortho, the positions next to it, and para, the one opposite. So bromine water, B r two in water, puts three bromines in: two, four, six tribromophenol, a white precipitate, p p t for short. Okay? Now change the solvent: bromine in carbon disulphide, C S two, at two hundred and seventy three kelvin. Do we still get three bromines? No, this time only one bromine goes in: mainly para bromophenol, with some ortho; the italic p and o mean para and ortho. In water, phenol ionises to the far more reactive phenoxide ion; in this solvent of low polarity, it does not. Don't mix these two up, right? Nitration is similar. Dilute nitric acid, HNO three, gives ortho and para nitrophenol, while concentrated nitric acid, conc. for short, gives picric acid. With me?

3:53Kolbe and Reimer–Tiemann reactions

Now come two named reactions. In Kolbe's reaction, sodium hydroxide first makes the phenoxide ion, which is far more reactive than phenol. Then carbon dioxide at four hundred kelvin under pressure, then acid, gives salicylic acid. Okay? The second is the Reimer Tiemann reaction. Phenol with chloroform, CHC l three, and aqueous sodium hydroxide, then acid, gives salicylaldehyde. Right? Its electrophile is dichlorocarbene, C C l two, where the two dots in front are a lone pair on the carbon. Got it? And with carbon tetrachloride, C C l four, in place of chloroform, the product is salicylic acid instead. Keep that swap in mind, okay?

4:32More arrows: zinc, oxidation, test, ethers

Now the bottom of the map. Heat phenol with zinc dust, Z n, and you get benzene. Separately, oxidise phenol with chromic acid, N a two C r two O seven slash H two S O four, and you get benzoquinone. With me? Neutral ferric chloride, F e C l three, gives a violet colour, a test for phenols, and for enols too. Clear? The O H group reacts too. Sodium hydroxide, then an alkyl halide R X, gives an ether, A r O R, where X is a halogen and A r is an aryl group, like the phenyl ring. And ethanoic anhydride, C H three C O in brackets, taken twice, then O, gives an ester. Right?

5:14Example 1: mass of tribromophenol from 0.47 g phenol

Now Example one, from the concept Reactions of Phenol. Find the mass of two, four, six tribromophenol, molar mass three hundred and thirty one, formed from zero point four seven grams of phenol with excess bromine water. Capital M is the molar mass, and g is grams. Pause the video and try it first. Done? Okay, let's check. Phenol, C six H five O H, has a molar mass of ninety four, so the moles, small n, are zero point four seven over ninety four, which gives zero point zero zero five mole. Right? Now be careful here, because the product has three bromine atoms, and many students multiply the moles by three. We do not! Each phenol molecule becomes one tribromophenol molecule, so one mole gives one mole. Clear? So the mass is zero point zero zero five times three hundred and thirty one, the cross meaning times. That gives one point six five five, which is about one point six six grams; the wavy equals sign means about. Got it?

6:16Example 2: phenol to anisole, salicylic acid, ester, quinone

And now Example two, a harder one, from the same concept. How would you convert phenol into, part a, anisole; part b, two hydroxybenzoic acid; part c, phenyl ethanoate; and part d, para benzoquinone? Pause the video and try it first. Done? Okay, let's check. Anisole, C six H five OCH three, is an ether: sodium hydroxide first, to make the phenoxide, then methyl iodide, C H three I, the Williamson synthesis. Okay? Now part b, two hydroxybenzoic acid, is salicylic acid, so this is Kolbe's reaction: sodium hydroxide, then carbon dioxide at four hundred kelvin under pressure, then acid. Right? Part c, phenyl ethanoate, is an ester. So why not simply heat phenol with ethanoic acid? Because phenol does not form esters easily with acids! So we use ethanoic anhydride instead, and it gives phenyl ethanoate. Clear? And part d, para benzoquinone: oxidise phenol with chromic acid, that is, sodium dichromate with sulphuric acid. Four targets, four arrows from one map, that's all!

7:27Quick revision: making phenol, acidity, ring reactions

So, let us do a quick revision. Make phenol from cumene, with acetone, by Dow's process, by sulphonate fusion, or from a diazonium salt and water. Phenol dissolves in sodium hydroxide, but gives no carbon dioxide with sodium bicarbonate. On the ring, bromine water gives tribromophenol, Kolbe gives salicylic acid, and Reimer Tiemann gives salicylaldehyde. Now try the practice problems in this chapter's concepts. All the best!