Preparation of Phenol

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle before we start. There is a factory built to make phenol. But out of the very same process comes a second product, acetone. And the factory is happy about it. Now why would the factory be happy? Today we learn four main ways to prepare phenol, with their reagents and conditions. Questions on these routes do come up in JEE, so let's begin.

0:28Four routes into phenol

First, let us see what we are making. Phenol is C six H five O H: a benzene ring, C six H five, carrying one O H group. The small numbers written low count the atoms, okay? Now look at the map. On its left, four arrows lead into phenol: from cumene, from chlorobenzene, from sodium benzenesulphonate, the sodium salt of benzenesulphonic acid, and from a diazonium salt. The right side shows its reactions, for a later lesson. Let us take them one at a time. With me?

1:02Route 1: the cumene process (industrial)

Route one is the industrial method, the cumene process. Benzene and propene give cumene, also called isopropylbenzene; the arrow means gives. Its formula is C six H five, C H, then C H three in brackets with a small two after the bracket, which means two methyl groups on that carbon. Okay? Now air oxidises cumene. Cumene has many C H bonds, on the ring and on the side chain. Let me ask you this: which C H bond does oxygen attack? The C H bond on the central carbon of the side chain, very good! That carbon is tertiary, and it is benzylic, right next to the ring. This gives the most stable radical, so oxygen goes in there. Right? So oxygen, O two, written over the arrow, turns cumene into cumene hydroperoxide. Look at the end of the formula: dash, O, dash, O H; the dashes are single bonds, so that carbon now carries an OOH group. Clear? Then comes dilute acid: H plus and water, written over the arrow; the plus sign written high is the charge. The hydroperoxide breaks into phenol, C six H five O H, and acetone, C H three COCH three. So, back to our puzzle. Why is the factory happy to get acetone as well? Yes, exactly, both products are valuable, and the raw materials, benzene, propene and air, are cheap. So the process is economical, and our puzzle is solved. Got it?

2:37Route 2: Dow's process, from chlorobenzene

Route two is Dow's process. Chlorobenzene, C six H five C l, is heated with sodium hydroxide, N a O H, at six hundred and twenty three kelvin, the capital K, and three hundred atmospheres. Note both numbers; the practice set asks for them. The product is sodium phenoxide, C six H five O N a. Okay? But sodium phenoxide is a salt, not phenol yet. So the last step is acid, H plus over the arrow, which gives phenol. We see this last step again in route three. With me?

3:11Route 3: from benzenesulphonic acid

Route three starts from benzene itself. Benzene is sulphonated with oleum, which is fuming sulphuric acid, written over the arrow. That gives C six H five S O three H, benzenesulphonic acid. Okay? Next, we neutralise it to the sodium salt, sodium benzenesulphonate, C six H five S O three N a. Now this salt is fused, that is, heated strongly until it melts, with a reagent you have already met today. Can you tell me which one? It is sodium hydroxide again, perfect! Fused with sodium hydroxide at about five hundred and seventy three kelvin, it gives sodium phenoxide. Careful, do not mix this up with the conditions of Dow's process. Right? Finally, we acidify, and the phenoxide gives phenol. Notice the pattern: routes two and three end the same way, sodium phenoxide and then acid. Clear?

4:08Route 4: from diazonium salts

Route four starts from aniline. We diazotise it with sodium nitrite and hydrochloric acid, N a N O two, slash, H C l, the slash meaning together, at two hundred and seventy three to two hundred and seventy eight kelvin. This gives benzenediazonium chloride, C six H five N two plus, with C l minus; the plus and minus written high are the charges. Okay? Now we warm this solution with water. Something escapes as a gas. Can you name that gas? It is nitrogen gas, N two, that is correct! The diazonium salt plus water gives phenol, nitrogen gas and H C l, and water or dilute acid both work. And notice, the O H group goes exactly where the diazonium group was. We need that in example two. Got it?

4:59Example 1: acetone formed with 47 g phenol, cumene process

Now, Example one, your turn. Find the mass of acetone formed along with forty seven grams of phenol in the cumene process. Pause the video and try it first. Done? Okay, let's check. From our equation, one hydroperoxide gives one phenol and one acetone. So the mole ratio is one to one; the colon is read as to. Right? Phenol, C six H five O H: six carbons times twelve is seventy two; add six for the hydrogens and sixteen for the oxygen. So capital M, the molar mass, is ninety four grams per mole; the minus one written high on mol means per mole. Forty seven over ninety four gives zero point five mole. Okay? So we also get zero point five mole of acetone. Same moles, so the same forty seven grams of acetone, right? No, no, wait, equal moles do not mean equal masses! Acetone, C H three COCH three, has three carbons, six hydrogens and one oxygen. Thirty six plus six plus sixteen gives fifty eight grams per mole. Clear? So the mass of acetone is zero point five times fifty eight, which gives twenty nine grams. Twenty nine grams, not forty seven. That was the trap. Got it?

6:20Example 2: why m-cresol needs the diazonium salt route

Now a harder one, Example two. Meta cresol cannot be made by sulphonating toluene and fusing the product with alkali. Why can it not be made this way? And suggest a starting material from which it can be made in two steps. Cresol is methylphenol, and the small m in front stands for meta. Pause the video and try it first. Done? Okay, let's check. Toluene has a methyl group on the ring. When we sulphonate it, where does the methyl group send the sulphonic acid group? It sends it to the ortho and para positions, very good! On screen, dash C H three is the methyl group, and dash S O three H is the sulphonic acid group. Okay? So alkali fusion gives ortho and para cresol, written with a small o and a small p. Meta cresol does not form, and that answers the first part. Right? Now the second part. We need a route where the O H group goes exactly where we choose. Which of our four routes can do that? It is the diazonium route, yes, exactly! So we start from three methylaniline, also called meta toluidine, where the amino group already sits meta to the methyl group. Clear? Step one, diazotise with sodium nitrite and hydrochloric acid at two hundred and seventy three to two hundred and seventy eight kelvin. Step two, warm the solution with water. The O H takes the place of the diazonium group, giving meta cresol in two steps. Got it?

7:56Quick revision: four routes to phenol and their conditions

So, quick revision. Cumene with oxygen, then acid, gives phenol and acetone: the industrial method. Dow's process heats chlorobenzene with sodium hydroxide at six hundred and twenty three kelvin and three hundred atmospheres, and the sulphonate route fuses the sodium salt with sodium hydroxide at about five hundred and seventy three kelvin. And a diazonium salt from aniline, warmed with water, gives phenol and nitrogen. Now go and try the practice problems on this page. All the best!