Methods for Aldehydes Only

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. An aldehyde sits in the middle of a ladder. Reduce it, and you get an alcohol; oxidise it, and you get a carboxylic acid. So the reaction does not want to stop at the aldehyde, right? How do chemists make it stop exactly there? That is today's story: the methods that give aldehydes only, each with its own trick for stopping halfway. Questions on these named reactions come up often in JEE, so let's begin.

0:32Rosenmund reduction: acyl chloride to aldehyde

So, method number one, the Rosenmund reduction. An acyl chloride, RCOC l, where R is the rest of the molecule, plus hydrogen, H two; the small two written low counts the atoms. Above the arrow is the catalyst: palladium, P d, on barium sulphate, B a S O four, and the slash means supported on. The arrow means gives, so we get RCHO, the aldehyde, plus H C l. Okay? Now, palladium is an excellent catalyst for adding hydrogen. So why would anybody spoil it with barium sulphate, on purpose? To stop at the aldehyde, yes, exactly! Barium sulphate poisons the palladium, and this poisoned catalyst does not reduce the aldehyde any further. So that is the first answer to our puzzle. Right? Now let me ask you this. Methanal is the simplest aldehyde of all. Can the Rosenmund reduction make it? No, it cannot, and this is a classic trap. For methanal we would need methanoyl chloride, HCOC l, and that acyl chloride is unstable. So no methanal by this route. Clear?

1:42From nitriles and esters: Stephen and DIBAL-H

Method number two is the Stephen reaction, which starts from RCN, a nitrile. Above the arrow are the reagents: S n C l two, stannous chloride, and H C l. The product is RCH, double bonded to N H; the equals sign is that double bond, and this is an imine. The dot and the H C l after it mean the imine is held as its hydrochloride, a salt. Okay? Then hydrolysis, which means reaction with water, turns it into RCHO, the aldehyde. Notice that the nitrile carbon becomes the aldehyde carbon. Keep this in mind for the examples. Right? Now a modern reagent, DIBALH, diisobutylaluminium hydride. Its formula is A l H, then i dash B u in brackets with a small two, which means two isobutyl groups. It also reduces a nitrile to an imine, and hydrolysis then gives the aldehyde. Got it? Now give DIBALH an ester. Does it go all the way to an alcohol, or stop at the aldehyde? It stops at the aldehyde, at one hundred and ninety five kelvin, where capital K is the kelvin. So the low temperature is part of the condition. With me? And one more property. DIBALH does not touch a carbon carbon double bond, the C, equals sign, C on screen. So such a double bond in the molecule survives. Clear?

3:08Benzaldehyde from toluene: stop before the acid

Now aromatic aldehydes. Toluene is a methyl group on a benzene ring, and we want benzaldehyde. So we just oxidise the methyl group with K M n O four, potassium permanganate, and we are done, right? No, no, careful, that is the trap! Strong oxidants like K M n O four, potassium dichromate, K two C r two O seven, or nitric acid, HNO three, take toluene straight to benzoic acid. So we need routes that stop halfway. Okay? Route one is the Etard reaction. Toluene reacts with chromyl chloride, C r O two C l two, in carbon disulphide, C S two. That gives a chromium complex, and its hydrolysis gives benzaldehyde. Got it? Route two uses chromic oxide, C r O three, in ethanoic anhydride. Now toluene gives benzylidene diacetate: C six H five C H, then OCOCH three in brackets, with a small two outside. So two acetate groups sit on one carbon. Right? So tell me, why do we add the ethanoic anhydride at all? Very good, it traps the aldehyde! Benzylidene diacetate cannot be oxidised further, so it does not go on to the acid. Then acid hydrolysis gives C six H five CHO, which is benzaldehyde. Clear? And route three: side chain chlorination. Toluene with chlorine, C l two, in light gives benzal chloride. Its hydrolysis at three hundred and seventy three kelvin gives benzaldehyde, and this is a commercial method. With me?

4:44Gattermann–Koch: benzene straight to benzaldehyde

And the last method, the Gattermann-Koch reaction, starts from benzene itself, C six H six. Above the arrow are carbon monoxide, C O, and hydrogen chloride, H C l. The product is C six H five CHO, benzaldehyde, so the aldehyde group goes straight onto the ring. Okay? But it needs its helpers: anhydrous aluminium chloride, A l C l three, and cuprous chloride, C u C l. Anhydrous means completely dry. Right? And here is a nice link. Carbon monoxide and H C l together behave as methanoyl chloride, HCOC l. That is the same unstable acyl chloride the Rosenmund reduction could not use! Got it?

5:28Example 1: propanenitrile and ethanoyl chloride to aldehydes

Time for Example one, an easy one to warm up. Name the aldehyde obtained from, part a, propanenitrile by the Stephen reaction, and part b, ethanoyl chloride by the Rosenmund reduction. Pause the video and try it first. Done? Okay, let's check part a. The name propanenitrile says three carbons, counting the carbon of the nitrile group. Some students see only the ethyl part and write ethanal. So which is it, ethanal or propanal? Propanal, correct! C H three C H two C N becomes C H three C H two CHO, so three carbons in, three carbons out. The carbon of the C N group simply becomes the carbon of the CHO group. Right? Part b is direct. Ethanoyl chloride, C H three COC l, by the Rosenmund reduction gives C H three CHO, which is ethanal. Two carbons in, two carbons out. Easy, isn't it?

6:28Example 2: ethanol to propanal via bromoethane and nitrile

Now Example two, a harder one. How would you convert ethanol into propanal, increasing the chain by one carbon? Here is a hint: work backwards from propanal. Pause the video and try it first. Done? Okay, let's work backwards. Ethanol has two carbons, and propanal has three. From Example one, which compound gives propanal by the Stephen reaction? Propanenitrile, yes, exactly! So, working backwards on screen: propanenitrile comes from bromoethane, and bromoethane comes from ethanol. These arrows point to the left, from each product back to where it comes from. With me? Now we go forwards. Step one: ethanol, C two H five O H, with phosphorus tribromide, P B r three, gives bromoethane, C two H five B r. Okay? Step two: bromoethane with potassium cyanide, KCN, gives propanenitrile, C two H five C N, and this is the step that adds the extra carbon. Right? Step three: stannous chloride and H C l, then water, which is the Stephen reaction, or DIBALH, then water. Either way we get C two H five CHO, propanal. Three steps, and the chain has grown by one carbon. That's it!

7:51Quick revision: Rosenmund, Stephen, Etard, Gattermann-Koch

So, quick revision. Rosenmund: an acyl chloride and hydrogen over palladium on barium sulphate, and no methanal. Stephen reaction or DIBALH: nitrile, to imine, to aldehyde. And for benzaldehyde: from toluene by the Etard reaction, by chromic oxide in the anhydride, or by chlorine in light, and from benzene by Gattermann-Koch. Now go and try the four practice problems on this page. All the best!