Oxidation and Tests for Alcohols

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Three bottles in the lab have lost their labels. Each holds an alcohol with the same formula, C four H ten O, so they look exactly alike. One is primary, one is secondary, and one is tertiary. With just test tubes and a few reagents, how would you find out which is which? Keep that question in mind. Today we learn how alcohols are oxidised, and the tests that tell them apart. This comes up often in JEE, so let's begin.

0:34Primary alcohols: aldehyde first, then acid

So, first a primary alcohol. The small circle written high means degree, so one degree means primary, and each arrow means is oxidised to. So a primary alcohol goes first to an aldehyde, and then on to a carboxylic acid. Okay? Now we oxidise it with acidified K M n O four, potassium permanganate; the small four written low means four oxygen atoms. Where does it stop: at the aldehyde, or at the acid? All the way to the acid, yes! Acidified permanganate, or acidified K two C r two O seven, potassium dichromate, carries a primary alcohol right on to the acid. Write aldehyde here, and you lose the marks. Got it? So how do we stop at the aldehyde? With PCC, pyridinium chlorochromate, in C H two C l two, dichloromethane. As the note in brackets says, PCC does not attack a carbon carbon double bond, the two short lines between the carbons. Or with C r O three in an anhydrous medium, which means no water present. Clear?

1:43Secondary and tertiary alcohols

Now, a secondary alcohol, two degree, gives a ketone. The reagents in brackets are C r O three, K two C r two O seven slash H plus, or PCC again; the slash H plus, with the plus sign written high, means acidified. And under these conditions, the ketone is not oxidised further. Okay? So, primary gives an aldehyde, and secondary gives a ketone. Now let me ask you this: what does a tertiary alcohol give with acidified dichromate? It is not oxidised at all, and if you said so, very good! Tertiary alcohols resist oxidation. The carbinol carbon is the carbon carrying the O H group, and in a tertiary alcohol it has no hydrogen atom to lose. Clear? Only under drastic conditions do the carbon carbon bonds break, the C dash C on screen, giving acids with fewer carbon atoms. Right?

2:41Hot copper at 573 K

Another method is to pass the alcohol vapour over hot copper, C u, at five hundred and seventy three kelvin; the capital K stands for kelvin. Hydrogen is removed, which is dehydrogenation, the word in brackets. So a primary alcohol gives an aldehyde, and a secondary alcohol gives a ketone. With me? And a tertiary alcohol over hot copper? What do you think it gives? It gives an alkene! Read the word carefully: dehydration, not dehydrogenation, so the alcohol loses water. The two words look alike, and that is a common trap. Got it?

3:20Simple tests for an alcohol

Alright, now the tests. With sodium metal, an alcohol evolves hydrogen gas, and with ceric ammonium nitrate it gives a red colour. Okay? And with an acid chloride or an acid, it gives a sweet-smelling ester. But to tell primary, secondary and tertiary apart, we need sharper tests. Let's go.

3:39The Lucas test and the iodoform test

The Lucas test tells primary, secondary and tertiary alcohols apart. Lucas reagent is concentrated H C l with anhydrous Z n C l two, zinc chloride; the plus sign means together with. Okay? The alcohol dissolves in it, but the chloride formed does not, so the mixture turns turbid, which means cloudy. Tertiary alcohols turn turbid at once, secondary in about five minutes, which is what min means, and primary only on heating. Got it? Next, the iodoform test, with I two plus N a O H, iodine and sodium hydroxide, in brackets. It is positive for ethanol, and for alcohols of the type C H three C H, O H in brackets, R, where the O H hangs on that second carbon and R is an alkyl group. The sign of a positive test is yellow CHI three, which is iodoform. Clear? Now, a quick trap. Propan one ol has a C H three group at one end. So does it give the iodoform test? No, it does not! Its O H is on the end carbon, not next to the C H three. But in propan two ol, C H three CHOHCH three, the O H sits right next to a C H three, so it gives the yellow precipitate.

5:02Oxidise first, then test the product

Right? One more trick is to oxidise the alcohol first, and then test the product. So a primary alcohol gives an aldehyde, which reduces Tollens' reagent. It reduces Fehling's reagent too, but only if it is aliphatic, which means the CHO group is not attached to a benzene ring; benzaldehyde, for example, does not. Okay? Now, a secondary alcohol gives a ketone. Does that ketone reduce these reagents? No, it does not. That is how we tell a primary alcohol from a secondary one, and Example two uses exactly this. With me?

5:39Example 1: oxidation products of propan-1-ol and propan-2-ol

Now, Example one. Name the product of oxidation of propan one ol with acidified potassium dichromate, of propan one ol with PCC, and of propan two ol with C r O three. The letters a, b and c mark the three parts. Pause the video and try it first. Done? Okay, let's check. Propan one ol has its O H on the end carbon, so it is primary. And primary gives an aldehyde, so part a is propanal, right? Oh no, careful, acidified dichromate does not stop at the aldehyde! It goes all the way, so part a gives propanoic acid. Okay? For part b, the same alcohol with PCC stops at the aldehyde, so we get propanal. Clear? And for part c, propan two ol has its O H on the middle carbon, so it is secondary. C r O three gives the ketone, propanone. Propanoic acid, propanal and propanone. That's it!

6:44Example 2: identify C4H10O from Lucas, iodoform and Tollens

Now, remember our three unlabelled bottles? An alcohol A, C four H ten O, gives turbidity with Lucas reagent in about five minutes, and a yellow precipitate with iodine and alkali. Oxidation gives B, C four H eight O, which does not reduce Tollens' reagent. Identify A and B, okay? Pause the video and try it first. Done? Okay, let's check. Turbidity with Lucas reagent in about five minutes means a secondary alcohol. Clear? The yellow precipitate is iodoform, CHI three. So alcohol A has the C H three CHOH group; the dash at the end is the bond to the rest of the molecule. Right? So, four carbons, a secondary alcohol, and this group. Which alcohol is it? Butan two ol, exactly! That is C H three C H, O H in brackets, C H two C H three. Count it: four carbons, ten hydrogens and one oxygen, so it is C four H ten O, just as given. Got it? And a secondary alcohol is oxidised to a ketone, so B is butanone, C H three COCH two C H three, and its formula, C four H eight O, matches. And a ketone does not reduce Tollens' reagent, which fits perfectly. So the bottle that turns turbid in about five minutes holds butan two ol, and the Lucas test sorts out the other two bottles as well. So our mystery is solved!

8:25Quick revision: oxidation, hot Cu, Lucas, iodoform, Tollens

So, quick revision. Primary alcohols go to the aldehyde and then to the acid, and PCC stops at the aldehyde; secondary alcohols give a ketone. Tertiary alcohols resist oxidation, and over hot copper at five hundred and seventy three kelvin they give an alkene. The Lucas test sorts primary, secondary and tertiary; the iodoform test needs the C H three CHOH group; and Tollens' reagent picks out the aldehyde. Now go and try the practice problems on this page. All the best!