Earlier JEE Main Questions: Structure and Preparation

Transcript

0:00Introduction

Hello everyone! Here is a strange recipe from a JEE Main paper. We start with toluene, and we want benzaldehyde. But the recipe first oxidises toluene all the way up to benzoic acid, and then brings it back down to the aldehyde. Why would a chemist climb all the way up to the acid, only to come back down? Sounds strange, right? Today we solve two earlier JEE Main questions, from twenty fourteen and twenty fifteen. Both test one skill, stopping exactly at the aldehyde, and it comes up often. So, let's begin.

0:35Primary alcohol: the aldehyde is only halfway

So, first the alcohols. On screen is a primary alcohol, RCH two O H, where R is the rest of the molecule, and the small two written low counts the hydrogens. Each arrow means gives. It is oxidised first to the aldehyde, RCHO, and then on to RCOOH, the carboxylic acid. So the aldehyde sits only halfway up. Okay? Now suppose we use a strong oxidising agent. Where will the oxidation stop, at the aldehyde or at the acid? It goes all the way to the acid, yes, exactly! Acidified potassium permanganate, K M n O four, or acidified potassium dichromate, K two C r two O seven, carries it right up to the acid. On the map, it is the last box on the right, where slash H plus means in acid. Clear? So, to stop at RCHO, the course gives us PCC, pyridinium chlorochromate, used in dichloromethane, C H two C l two. It is the first box on the left of the map, where the number one with a small raised circle, the degree sign, means a primary alcohol. With me? Or chromium trioxide, C r O three, but only in an anhydrous medium, which means no water at all, because in water the aldehyde is oxidised further, to the acid. Keep this condition in mind; our first example tests it. Got it?

1:52Up to the acid, then on to the acid chloride

Now the acid side, with the map of carboxylic acids from a later chapter. Second box on the left: an alkylbenzene, a benzene ring carrying an alkyl group, is heated with alkaline K M n O four, and then treated with acid, H three O plus; the raised plus is its positive charge. Okay? Now, ethylbenzene has two carbons in its side chain. After this oxidation, does the side chain keep both of its carbons? No, it keeps only one of them, because the whole side chain, whatever its length, becomes a single COOH group on the ring. So ethylbenzene also gives C six H five COOH, benzoic acid. Right? But there is one condition. The carbon joined to the ring, the benzylic carbon, must carry at least one hydrogen atom, the H on screen. If it has none, the side chain is not oxidised. Clear? Now the right side of this map. An acid, RCOOH, with thionyl chloride, SOC l two, gives the acid chloride, RCOC l, plus sulphur dioxide, S O two, plus hydrogen chloride, H C l. With me? The box also lists phosphorus chlorides, but thionyl chloride is preferred, because its other two products are gases, so they escape easily. Got it?

3:06Rosenmund: back down from RCOCl to RCHO

Now back to the aldehyde map, fifth box on the left: the Rosenmund reduction. The acid chloride, RCOC l, plus hydrogen, H two, over palladium on barium sulphate, written over the arrow as P d, slash, B a S O four. It gives the aldehyde, RCHO, plus H C l. Okay? Now, palladium is an excellent catalyst for adding hydrogen. So what would plain palladium do to our aldehyde? It would reduce it further, exactly! Barium sulphate poisons the palladium, so the weakened catalyst stops at the aldehyde, and RCHO is not reduced any further. Right? So, the big picture: there are two roads to the aldehyde. Up from the primary alcohol, RCH two O H, with a gentle oxidant, or down from the acid chloride, RCOC l, with a poisoned catalyst. Our two questions use one road each. Clear?

4:01Example 1: best reagent for primary alcohol to aldehyde

Time for Example one, an easy one, from JEE Main twenty fourteen. Which is the most suitable reagent to convert RCH two O H into RCHO? The short dashes are just single bonds. The choices are K M n O four, K two C r two O seven, C r O three, or PCC, okay? Pause the video and try it first. Done? Okay, let's check. Potassium permanganate is the strongest oxidant in the list, so it should do the job best, right? No, no, careful, that is the trap, because strong is exactly the problem here! Permanganate and dichromate do not stop at RCHO; they push it on to the acid, RCOOH, so both are out. Okay? Now chromium trioxide. We saw that it can give the aldehyde. So is it a safe answer here? Not quite, and this is the subtle part. Chromium trioxide stops at the aldehyde only in an anhydrous medium, and the question gives no such condition. In water, the aldehyde goes on to the acid. Right? And PCC, pyridinium chlorochromate, stops at RCHO, which is exactly what we want. With me? So the answer is PCC, the most suitable reagent. For a primary alcohol to an aldehyde, think of it first. Clear?

5:22Example 2: toluene to benzaldehyde via Rosenmund reduction

And now Example two, from JEE Main twenty fifteen: our strange recipe from the start. Toluene, with K M n O four written over the first arrow, gives compound A; then thionyl chloride turns A into B, and hydrogen over palladium and barium sulphate turns B into C, the compound we must identify, okay? Pause the video and try it first. Done? Okay, let's check. Permanganate attacks the side chain of toluene, so compound A is just benzaldehyde, right? No, no, wait, that is the trap! Toluene is C six H five C H three. Permanganate is a strong oxidant, and it takes the whole side chain to COOH, since the carbon next to the ring has hydrogens. So compound A is C six H five COOH, benzoic acid. Okay? Next, thionyl chloride turns the acid into its acid chloride. So compound B is benzoyl chloride, C six H five COC l. Right? Now the last step, hydrogen over palladium. Will it stop at the aldehyde, or go further? Look at what is written over the last arrow. It stops at the aldehyde, because the palladium sits on barium sulphate! This is the Rosenmund reduction: the poisoned catalyst turns benzoyl chloride into C six H five CHO, the aldehyde we wanted. Got it? So compound C is benzaldehyde, and our puzzle is solved. Permanganate cannot stop at the aldehyde, so the route climbs up to the acid, and the Rosenmund reduction brings it back down, exactly to the aldehyde. Clear?

7:04Quick revision: PCC, SOCl2 and Rosenmund reduction

So, quick revision. For a primary alcohol to an aldehyde, use PCC, or chromium trioxide in an anhydrous medium. Permanganate and dichromate go on to the acid, and they take toluene to benzoic acid. From an acid, thionyl chloride gives the acid chloride, and the Rosenmund reduction, over palladium on barium sulphate, brings it down to the aldehyde. Now try both questions again on this page without looking, and then move on to the recent JEE Main questions. All the best!