AIEEE and Earlier JEE Main Questions: Phenols

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Two bottles have lost their labels. One holds phenol, and the other holds benzoic acid, okay? Your friend says: just add sodium hydroxide solution; the acid will dissolve, so that bottle must be benzoic acid. Will that tell them apart? Hmm, hold that thought. Today we solve two questions on phenols from the AIEEE paper of two thousand and eleven; that exam later became JEE Main. Questions like these come up often, and each one needs just a few ideas from this chapter. So, let's begin.

0:38Idea 1: phenol is an acid, but a weak one

So, idea number one: phenol is an acid. Phenol is C six H five O H; the small numbers written low count the atoms. Its O H group sits directly on a carbon of a benzene ring. Okay? Now, when phenol loses its hydrogen ion, H plus, its oxygen is left with a negative charge. Look at the ring right next to it. Where else can that charge go? It goes into the ring, yes, exactly! This is the phenoxide ion, C six H five O minus; the small minus sign written high is its charge. In the figure, double headed arrows join its resonance structures, and the coloured minus signs put the charge on the two ortho carbons and the para carbon. The charge is spread out, the ion is stabilised, and so phenol is an acid. Clear? So phenol reacts with N a O H, aqueous sodium hydroxide. C six H five O H plus N a O H gives C six H five O N a, sodium phenoxide, plus H two O, water; the arrow means gives. Got it? But do not overrate it. Phenol is a weak acid, and carboxylic acids, like benzoic acid, are stronger than phenol. Right? That is why phenol gives no carbon dioxide, C O two, with sodium hydrogencarbonate, N a HCO three, while carboxylic acids do give it, with a fizz. So that fizz can tell a carboxylic acid from a phenol; keep it in mind. With me?

2:08Idea 2: tests that pick out one group

Now idea number two: tests. The classic test for phenols is neutral ferric chloride, F e C l three. Phenols give a violet colour with it, and so do enols, as the brackets say. Okay? Two more reagents appear in our first question. Let me ask you this: Tollens' reagent, which group does it detect? It detects aldehydes, very good! Tollens' reagent gives a silver mirror with aldehydes, and you will meet it again in the aldehydes chapter. Right? And Molisch's test, from the biomolecules unit, gives a violet ring with all carbohydrates. Keep these three tests ready. Clear?

2:50Idea 3: bromine and the activated ring

Now idea number three: the ring itself. The O H group activates the benzene ring strongly, and directs new groups to the ortho and para positions. Look at the figure again: in the phenoxide ion, the charge sits on the ortho and para carbons, and that is where an electrophile like bromine attacks. Okay? And the phenoxide ion is even more reactive than phenol itself. Remember this one; it decides both bromine reactions. Got it? First, bromine, B r two, in carbon disulphide, C S two, at two hundred and seventy three kelvin; the capital K is kelvin. This solvent has low polarity, and we get monobromophenols, mainly p bromophenol; the small p in italics means para. With me? Now the same phenol, but with bromine water. Do we again get just one bromine on the ring? No, this time we get three bromine atoms on the ring! The product is two four six tribromophenol, with bromine at positions two, four and six: both ortho positions and the para position. It comes out as a white precipitate, that is, a white solid. Right? So why the big difference? Water ionises phenol to the very reactive phenoxide ion, and in carbon disulphide that does not happen. Clear?

4:08Example 1: neutral FeCl3 tells phenol from benzoic acid

Time for Example one, from the AIEEE paper of two thousand and eleven. Which reagent can be used to distinguish phenol from benzoic acid? Aqueous N a O H, Tollens' reagent, Molisch reagent, or neutral F e C l three? Pause the video and try it first. Done? Okay, let's check. Our friend from the start chose sodium hydroxide. Benzoic acid is an acid, so it dissolves, and phenol stays as it is, right? Oh no, careful, that is the trap! Phenol also reacts with sodium hydroxide, remember? It forms sodium phenoxide. Both are acids, both dissolve, and you see no difference at all. So the answer to our puzzle is no; that friend needs a better reagent. Got it? What about Tollens' reagent? It is a test for aldehydes, and neither compound is an aldehyde, so nothing happens with either. Okay? And Molisch reagent is for carbohydrates, and neither compound is a carbohydrate. Right? But neutral ferric chloride gives a violet colour with phenol, and benzoic acid does not give it. So the answer is neutral F e C l three, the test for phenols from idea two. Easy, isn't it?

5:21Example 2: phenol with KBr and KBrO3 gives tribromophenol

Now Example two, from the same paper. Phenol is heated with a solution of a mixture of K B r and K B r O three, potassium bromide and potassium bromate. What is the major product? Pause the video and try it first. Done? Okay, let's check. Let me ask you this first: we never added any bromine, only two potassium salts. So where does the bromine come from? It comes from the salts themselves! In acid, five bromide ions, B r minus, react with one bromate ion, B r O three minus, and six hydrogen ions, H plus; the small signs written high are the charges. They give three B r two, bromine, plus three H two O, water. Check it: six bromine atoms on each side, and the charges, minus five, minus one and plus six, add up to zero. With me? So bromine is formed right there, in the water, just like bromine water. Okay? Now, many students write p bromophenol here, one bromine at the para position. Is that right? No, no, that is the answer in carbon disulphide at low temperature, not here. Here we are in water, and water ionises phenol to the very reactive phenoxide ion. Got it? So bromine enters both ortho positions and the para position. The major product is two four six tribromophenol, the white precipitate. That is three bromine atoms in one go. Clear?

6:54Quick revision: phenol acidity, FeCl3 test, bromination

So, quick revision. Phenol is an acid, but weaker than carboxylic acids. Neutral ferric chloride gives a violet colour, the test for phenols. And bromine water gives two four six tribromophenol, while bromine in carbon disulphide gives mainly the para product. There are seven more past questions solved on this page. Try each one yourself before you read its solution. All the best!