Overview

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. You have a solid compound, and it turns blue litmus red. So it is an acid, right? But drop it into sodium hydrogencarbonate solution, and not a single bubble comes out. So is it an acid, or is it not? Keep that thought; we will settle it soon. Today we map this whole chapter: which test finds which functional group, and what you see. These tests are part of the JEE syllabus, so let's begin.

0:32Alcohols: ceric ammonium nitrate, sodium and Lucas test

So, here is our map, a figure from the course. On the left are the groups, and each arrow points to their tests on the right, with the result to look for in bold. First, the alcohol, ROH, where R is an alkyl group and the dash is a bond. Ceric ammonium nitrate, a yellow reagent, turns red with an alcohol. Okay? Next, sodium with the dry liquid gives hydrogen gas, H two; the small lowered two means two atoms. But careful, because acids, phenols and even water also give hydrogen. So the liquid must be dry, and sodium alone is never enough. Right? Then the Lucas test, where the solution turns turbid. The small circle in brackets is a degree sign. Three with a degree sign means a tertiary alcohol, which goes turbid at once, and two with a degree sign means a secondary alcohol, which takes about five minutes. And the figure says a primary alcohol goes turbid only on heating. With me?

1:32Phenols: neutral FeCl3 and bromine water tests

Now let us move to the phenol, in the second box. In the heading, the letters A r stand for an aryl group, such as a benzene ring, so the O H sits right on the ring. Neutral ferric chloride, F e C l three, gives a violet colour with a phenol. Okay? Next, with bromine water, the colour of the bromine goes, and a white precipitate forms. Got it? And a phenol turns blue litmus red, so it is acidic. Now think about our puzzle from the start. Will a phenol give bubbles of carbon dioxide with sodium hydrogencarbonate, just like an acid? No, it will not, and if you said no, very good, because that settles our puzzle! Phenol is too weak an acid to give C O two, carbon dioxide, with N a HCO three, sodium hydrogencarbonate. So our solid, acidic but with no bubbles, points to a phenol. Right?

2:29Aldehydes and ketones: 2,4-DNP, Tollens, Fehling, iodoform

Next, aldehydes and ketones, with the carbonyl group, C double bond O; the two short lines are a double bond. Their common test is two four DNP, two four dinitrophenylhydrazine. It gives a yellow, orange or red precipitate with both. With me? So how do we tell them apart? Tollens' reagent gives a silver mirror with an aldehyde, but not with a ketone. Clear? Fehling's solution also finds aldehydes, with a red precipitate. So will benzaldehyde, an aromatic aldehyde, give it? No, and this is a classic trap. Aromatic aldehydes do not respond to Fehling's solution. The red cuprous oxide, C u two O on screen, comes only with aliphatic aldehydes. Okay? Then Schiff's reagent, where an aldehyde brings back the pink colour. And ketones give none of these three tests. Right? And the iodoform test, with iodine and sodium hydroxide in the figure. Aldehydes and ketones with C H three C O, a methyl group on the carbonyl carbon, give yellow iodoform, CHI three; the dash is the bond to the rest of the molecule. So ethanal and the methyl ketones give it. Got it?

3:44Carboxylic acids and amines

Now the carboxylic acid, with the COOH group you see in the figure. With sodium hydrogencarbonate it gives brisk effervescence of carbon dioxide. This is exactly the test that separates an acid from a phenol. Okay? Last, the primary amine, N H two in the figure, and its carbylamine test. Warm it with chloroform, CHC l three, and alcoholic KOH, written in short on screen, potassium hydroxide, and you get an isocyanide with a really foul smell. Only primary amines give this test, which is why the brackets say one with a degree sign, for primary. Clear? And an aromatic primary amine, diazotised and coupled with two naphthol as in the figure, gives an orange red azo dye. Right?

4:30Topper's trick: test in a fixed order

Now a trick that toppers use for an unknown compound: do the tests in a fixed order. The word in brackets is the group each test finds. First litmus and sodium hydrogencarbonate, then neutral ferric chloride. Okay? Then two four DNP, and Tollens' reagent, to decide between aldehyde and ketone. Got it? Then ceric ammonium nitrate, and finally solubility in dilute H C l, hydrochloric acid, and the carbylamine test. With me? One positive test fixes the group, and the molecular formula then fixes the structure. We will use this order in the examples. Clear?

5:10Example 1: one test to tell apart each of 3 compound pairs

Now example one, from the concept on identifying an unknown compound, later in this chapter. Which single test distinguishes each pair? The letters a, b and c in brackets label the pairs: ethanol and phenol, acetone and acetaldehyde, and aniline and N, N-dimethylaniline. Pause the video and try it first. Done? Okay, let's check. For ethanol and phenol, if you picked neutral ferric chloride, very good: it gives the violet colour with phenol, and nothing with ethanol. Right? Now acetone and acetaldehyde. Many students reach for two four DNP, or the iodoform test. Will either of them work here? No, careful, because both of them fail here. Both compounds have a carbonyl group, so both give the two four DNP precipitate. And acetaldehyde is ethanal, while acetone is a methyl ketone, so both give iodoform too. Okay? So we need a test for aldehydes only, and the course uses Tollens' reagent: a silver mirror with acetaldehyde, and nothing with acetone. Got it? In the last pair, aniline is a primary amine, but in N, N-dimethylaniline both hydrogens on the nitrogen are replaced by methyl groups, so it is tertiary. So the carbylamine test gives the foul smell with aniline only. With me?

6:34Example 2: structure of C3H6O2 from four test results

Now a harder one, example two, from the same concept. There is a compound, C three H six O two, which is neutral to litmus. It gives a precipitate with two four DNP, a silver mirror with Tollens' reagent, and hydrogen with sodium. Suggest a structure. Here is a hint: use our fixed order. Pause the video and try it first. Done? Okay, let's check. Some students see hydrogen with sodium and two oxygen atoms, and write propanoic acid, which is also C three H six O two. Is that the answer? No, and that is exactly the trap. The compound is neutral to litmus, and a carboxylic acid turns blue litmus red. So it is not an acid. Right? Next, the precipitate with two four DNP tells us there is a carbonyl group, C double bond O, in it. Clear? And the silver mirror says this carbonyl group is an aldehyde, the CHO group; the dash is its bond to the chain. Got it? Now, hydrogen with sodium, and yet it is not an acid. So it must also carry an alcoholic O H group. Okay? So, a chain of three carbons, with the aldehyde at one end, and the O H, shown in brackets, as a branch on the middle carbon. That is two hydroxypropanal. Three carbons, six hydrogens and two oxygens, exactly C three H six O two. And as the brackets say, three hydroxypropanal, with the O H on the end carbon, fits too. If you got either one, well done!

8:12Quick revision: tests for OH, C=O, acids and amines

So, quick revision. For the O H group, ceric ammonium nitrate turns red with an alcohol, and neutral ferric chloride violet with a phenol. For the carbonyl group, two four DNP gives a precipitate, and Tollens' reagent a silver mirror with aldehydes only. And an acid gives carbon dioxide with sodium hydrogencarbonate, while a primary amine gives the foul smell in the carbylamine test. Now try the practice problems on this page, and then go through each group in the concepts that follow. All the best!