IIT-JEE Questions (1982–2010): Phenols

Transcript

0:00Introduction

Hello everyone! Okay, here is a small detective puzzle before we start. There is a compound, C seven H eight O, that does not dissolve in water. It does not dissolve in sodium bicarbonate solution either. But put it in dilute sodium hydroxide, and it dissolves. And with bromine water, it quickly picks up three bromine atoms. So tell me, which compound is this? This came in IITJEE, nineteen eighty five. Today we solve older IITJEE questions on phenols, and by the end you will crack this puzzle yourself. So, let's begin.

0:41Why is phenol an acid?

First, let us look at the numbers. Phenol has a p K a of ten point zero, and ethanol has a p K a of fifteen point nine. That is a small p, a capital K, and a small a written low. The smaller the p K a, the stronger the acid, so phenol is by far the stronger acid. And the picture on the right will tell us why. Okay? Now, IITJEE nineteen eighty four asked exactly this. To what is the acidity of phenol due? Look at the picture for a clue. It is due to the resonance stabilisation of its anion, the phenoxide ion, very good! When phenol loses its proton, the negative charge, the small minus sign in the picture, does not stay on the oxygen alone. The double headed arrows join the resonance structures, and they share the charge with the two ortho carbons and the para carbon too. In an alkoxide ion, the charge sits on the oxygen alone. Clear? So phenol reacts with aqueous sodium hydroxide. C six H five O H, plus N a O H, gives C six H five O N a, which is sodium phenoxide, plus water, H two O, at the end. The small numbers written low count the atoms, and the arrow means gives. Got it? But here is IITJEE nineteen eighty seven. Phenol is an acid, but it does not react with sodium hydrogencarbonate, N a HCO three. So can you tell me why it does not react? Because phenol is a weaker acid than carbonic acid, that is correct! Carbonic acid has a p K a of six point four, much smaller than ten. Okay? And only an acid stronger than carbonic acid can drive carbon dioxide, C O two, out of sodium hydrogencarbonate. So with phenol there is no carbon dioxide, and no fizz at all. Remember this test, because our puzzle uses it. Right?

2:38Bromination: where does bromine go?

Now let us look at the ring itself. The O H group activates the benzene ring strongly, and it directs new groups to the ortho and para positions. Ortho means the carbons right next to the O H carbon, and para means the carbon straight across from it. With me? Now IITJEE two thousand and ten asked this. Phenol is treated with aqueous sodium hydroxide and bromine, so the bromine meets the phenoxide ion. At which positions of the phenoxide ion does bromine attack? Use the same picture. At the ortho and para positions only, yes, exactly! The phenoxide ion is attacked where its negative charge is spread, and in the picture, the only ring carbons that carry the charge are the two ortho carbons and the para carbon. B r on screen is bromine. So no intermediate comes from attack at the meta position, okay? Now IITJEE nineteen eighty four: phenol with excess bromine water. All three of these positions take a bromine, and we get two four six tribromophenol. The O H carbon is carbon one, so two and six are the ortho carbons, and four is the para carbon. It comes out as a white precipitate, and p p t is short for precipitate. Clear? But why all three bromines, and so easily? Water ionises phenol to the phenoxide ion, which is very reactive, far more than phenol itself. Right? And water also polarises the bromine molecule, B r two, with the small two written low. Keep these two reasons in mind, because our first example takes both of them away. Got it?

4:16Example 1: phenol with bromine in CS2 at low temperature

Time for Example one, from IITJEE nineteen eighty eight. What is formed when phenol reacts with bromine in carbon disulphide at low temperature? Pause the video and try it first. Done? Okay, let's check. Phenol plus bromine, we just did this, so it is two four six tribromophenol again, right? No, no, careful, that is the trap in this question! Look at the solvent. Carbon disulphide, C S two, is a solvent of low polarity, and it is not water. Okay? So both our reasons are gone. Phenol is not ionised to the phenoxide ion, and the solvent does not polarise the bromine molecule. The ring, activated by the O H group, polarises it only a little, so the reaction is much milder. With me? So only one bromine atom enters the ring, and the O H group sends it to an ortho position or to the para position. Right? The answer is ortho bromophenol and para bromophenol, mainly the para one. The small italic o and p in front of the names are short for ortho and para. One word, the solvent, changed the whole answer.

5:31Example 2: identify C7H8O from NaOH, NaHCO3 and bromine

Clear? And now, our puzzle from the start, from IITJEE nineteen eighty five. There is a compound, C seven H eight O, that does not dissolve in water or in dilute sodium bicarbonate, but it dissolves in dilute sodium hydroxide. With bromine water it quickly becomes C seven H five O B r three. Your job is to identify it. Pause the video and try it first. Done? Okay, let's check. Sodium bicarbonate is just another name for sodium hydrogencarbonate. The compound dissolves in sodium hydroxide, so it is an acid, but sodium bicarbonate cannot dissolve it, so it is weaker than carbonic acid. Exactly like phenol, so our compound is a phenol. Okay? Now let us look at the formula. Take a benzene ring with an O H group, and add one methyl group, C H three. That gives C H three, C six H four, O H, joined by the short dashes, and the atoms add up to C seven H eight O exactly. This is a cresol, a methylphenol. With me? Next, look at what the bromine water does. C seven H eight O becomes C seven H five O B r three, so three hydrogens go out and three bromines come in. Three positions on the ring took a bromine. Okay? So any cresol will do, the ortho one, the meta one or the para one, right? No, no, wait, think again, because that is the trap! Bromine goes only to the two ortho positions and the para position of the O H group. In ortho cresol, the methyl group sits on one ortho position, and in para cresol, it sits on the para position. Either way, a position is blocked, and three bromines cannot enter. Clear? Only meta cresol keeps both ortho positions and the para position free. So the compound is meta cresol, which is three methylphenol, and the italic m means meta. And that solves our puzzle, with each clue using one fact from today's lesson. Got it?

7:33Quick revision: phenol acidity and bromination products

So, quick revision. Phenol is acidic because the phenoxide ion is stabilised by resonance. It is weaker than carbonic acid, so it dissolves in sodium hydroxide, but not in sodium hydrogencarbonate. With bromine water it gives two four six tribromophenol, but with bromine in carbon disulphide only one bromine enters, mainly para. Now go through the other past questions on this page, and try each one yourself before you read the solution. All the best!