The Alcoholic Hydroxyl Group

Transcript

0:00Introduction

Hello everyone! Let me start with a small lab puzzle. Three bottles sit on the bench, and their labels have fallen off. One holds butan one ol, one holds butan two ol, and one holds two methylpropan two ol. So how will you tell which bottle is which, with just one reagent and a clock? Sounds tricky, doesn't it? By the end of this video, you will do it easily. And these tests for alcohols come up often in JEE practical chemistry. So, let's begin.

0:31First question: is it an alcohol at all?

So, first question: is the liquid an alcohol at all? Test number one is ceric ammonium nitrate. In the formula, two ammonium ions, N H four, sit in round brackets with a small two written low, and the square brackets hold cerium, C e, with six nitrate groups, N O three. This yellow reagent turns red, wine red, with an alcohol. Okay? Test number two is sodium metal. Two ROH plus two sodium gives two RON a, the sodium alkoxide, plus H two, hydrogen gas. Here R is the alkyl group, and the arrow means gives. You see a brisk evolution of hydrogen. Right? Now let me ask you something. Your liquid gives hydrogen with sodium. Does that prove it is an alcohol? No, it does not, and this is a classic trap. Acids, phenols and water also give hydrogen with sodium. So test the dry liquid. In a wet sample, the water gives hydrogen, and any liquid would appear to be an alcohol. Got it? Test number three is the ester test. Warm the liquid with acetic acid, and add a few drops of concentrated sulphuric acid, H two S O four. With me? An alcohol gives an ester with a fruity smell. So a fruity smell tells you an alcohol was there. Clear?

1:56Lucas test: primary, secondary or tertiary?

Now, which class of alcohol is it? For that, we use the Lucas test. Lucas reagent is concentrated hydrochloric acid, H C l, with anhydrous zinc chloride, Z n C l two. Anhydrous means completely dry. Okay? The alcohol dissolves in the reagent, so the solution starts clear. But the alkyl chloride that forms is insoluble. It separates as fine droplets, and the solution turns turbid, which means cloudy. Right? So here is my question. Primary, secondary or tertiary: which alcohol turns the solution turbid fastest? Yes, exactly, the tertiary alcohol is the fastest. The small circle written high is read as degree, so three degree means tertiary. It turns turbid at once. With me? Two degree, the secondary alcohol, takes a little longer. It turns turbid within about five minutes. Clear? So the primary alcohol simply takes a bit longer, maybe ten or fifteen minutes, right? No, no, careful, that is exactly the trap students fall into. One degree, the primary alcohol, gives no turbidity at room temperature at all. The solution just stays clear. Got it? And why this order? It follows the stability of the carbocation, a carbon with a positive charge. The sign on screen is read as greater than: three degree, greater than two degree, greater than one degree. Okay?

3:26Lucas trap: benzyl alcohol

Now a twist that comes up often. Benzyl alcohol is C six H five, the benzene ring, joined to a C H two O H group. Its O H carbon is joined to one carbon only, so it is primary. So with Lucas reagent, it stays clear at room temperature, right? Oh no, careful, it turns turbid at once, even though it is primary. The benzyl carbocation is stabilised by resonance with the benzene ring, so it forms as readily as a tertiary carbocation. Clear?

4:01Iodoform test and acidified dichromate

Next comes the iodoform test. The reagent is iodine, I two, with N a O H, sodium hydroxide. Okay? But only some alcohols give it. They need the group on screen: a C H three joined to a carbon carrying an H and an O H group, and the dash at the end is a bond to the rest of the molecule. These alcohols give a yellow precipitate of CHI three, iodoform. Right? Now tell me, among methanol, ethanol and propan one ol, which one gives the iodoform test? It is only ethanol, although ethanol is a primary alcohol. Its O H carbon is joined to a C H three, so it has the group. Methanol has no C H three there, and in propan one ol, that carbon carries an ethyl group instead. Clear? The last test uses acidified dichromate. Primary and secondary alcohols are oxidised, and the orange colour turns green. With me? But tertiary alcohols are not oxidised, so there is no colour change. So this also picks out a tertiary alcohol, just like the Lucas test. Got it?

5:12Example 1: telling three butanols apart with Lucas test

Now, remember our three bottles? Let's solve that puzzle. Here is example one. Three bottles hold butan one ol, butan two ol and two methylpropan two ol. How does Lucas reagent sort them out? Pause the video and try it first. Done? Okay, let's check. First, find the class of each alcohol. In two methylpropan two ol, the bracket with a small three means three methyl groups, all on one carbon, and the dash is the bond to the O H group. That carbon is joined to three carbons, so the alcohol is tertiary, and it turns turbid at once. Okay? Butan two ol has its O H group on carbon two, which is joined to two carbons, so it is secondary. It turns turbid in about five minutes. Right? And butan one ol? Its O H carbon is joined to just one carbon, so it is primary. What will you see in its bottle? Yes, exactly, it stays clear at room temperature. So, one reagent and a clock: the bottle that turns turbid at once is two methylpropan two ol, the one that takes about five minutes is butan two ol, and the one that stays clear is butan one ol. So our puzzle is solved!

6:32Example 2: identify C3H8O from Na, Lucas and iodoform

Now a harder one, where you combine the tests. Here is example two: a liquid with formula C three H eight O gives hydrogen with sodium, turbidity with Lucas reagent in five minutes, and a yellow precipitate with iodine and sodium hydroxide; the slash on screen just means with. Identify it. Pause the video and try it first, one clue at a time. Done? Okay, let's check. Clue one: hydrogen with sodium, so it has an O H group, the dash showing its bond. With three carbons, that means propan one ol or propan two ol. Okay? Now look at clue two: turbidity with Lucas reagent in five minutes. What does that tell you? Yes, exactly, it is a secondary alcohol. Propan one ol is primary and would stay clear at room temperature. So the liquid is propan two ol. Right? And clue three checks our answer. Propan two ol has the C H three, C H, O H group, so it gives the yellow precipitate of iodoform, CHI three. So every clue fits. With me? So the answer is propan two ol, with its O H group on the middle carbon and a methyl group on each side. See how three simple tests, one clue at a time, fixed the structure? Well done if you got it!

7:56Quick revision: CAN, sodium, Lucas, iodoform, dichromate

So, quick revision. For an alcohol, ceric ammonium nitrate turns red, and the dry liquid gives hydrogen with sodium. In the Lucas test, tertiary turns turbid at once, secondary in about five minutes, and primary not at room temperature. The iodoform test needs the C H three, C H, O H group, and dichromate goes from orange to green, but not with tertiary alcohols. Now go and try the practice problems on this page. All the best!