The Phenolic Hydroxyl Group

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Here is a compound that turns blue litmus red, and it dissolves in sodium hydroxide. So it is an acid, right? Now add sodium hydrogencarbonate solution, which fizzes with carboxylic acids. And there is not a single bubble of gas. How can an acid refuse to fizz? The answer is hidden in today's topic, the tests for the phenolic hydroxyl group, that is, the O H group on a benzene ring. Questions on these tests come up often in JEE practical chemistry, so let's begin.

0:39Test 1: neutral ferric chloride

So, test number one, and the most famous one. Take the compound in water and add neutral ferric chloride solution. On screen it is F e C l three; the small three written low is a subscript, and it counts the chlorine atoms. With a phenol, you get a violet colour. Okay? Now, the colour is not always violet. Sometimes it is blue, green or red instead. So do not look only for violet. Right? And where does this colour come from? From a coloured complex, such as the one on screen. Inside the square brackets, one iron atom, F e, holds six phenoxide groups, O C six H five, written in round brackets; the small six outside counts them. The three minus, written up high, is the charge of the complex. Clear? You add neutral ferric chloride, and you see a violet colour. Can you say for sure that the compound is a phenol? No, you cannot, and this is a common trap. The test is also given by compounds with an enolic O H group. The little dash before O H means the group is joined to the rest of the molecule. So a violet colour tells you there is a phenolic or an enolic O H group. Got it? And why must the ferric chloride be neutral? If free acid is present, the coloured complex does not form. And if alkali is added, it precipitates ferric hydroxide, F e, then O H in round brackets with a small three. So it is kept just neutral. With me?

2:13Test 2: bromine water

Test number two is bromine water. Add bromine water to a solution of phenol, and the colour of the bromine water is discharged; it simply disappears. Okay? And at the same time, a white precipitate forms. This is two, four, six tribromophenol. The numbers two, four and six are positions on the ring, counting the carbon that holds the O H as position one, and each of them now carries a bromine atom. Clear? Now think about this. Benzene needs a catalyst to react with bromine. But phenol reacts with plain bromine water, easily, with no catalyst at all. So what makes phenol so much faster? It is the O H group. The O H group, and even more the phenoxide ion present in water, strongly activates the ortho and para positions. Positions two and six are ortho, and position four is para. And that is exactly where the three bromine atoms went! See? But careful, here is a trap. Aniline also gives a white precipitate with bromine water. So how do you tell them apart? Use neutral ferric chloride, which gives a violet colour with phenol only. Right?

3:26Test 3: acidic, but only weakly

Test number three asks a simple question: is phenol an acid? It turns blue litmus red, and it dissolves in sodium hydroxide, written N a O H on screen. So yes, phenol is acidic. Okay? Now, back to our puzzle. Phenol is acidic. So if you add sodium hydrogencarbonate, will it give bubbles of carbon dioxide? No, it will not, and that solves our puzzle! Sodium hydrogencarbonate is N a HCO three, and carbon dioxide is C O two. Phenol is too weak an acid to give carbon dioxide with it. Acidic enough for sodium hydroxide, but not for sodium hydrogencarbonate. Clear? But one exception is worth knowing. Picric acid, which is two, four, six trinitrophenol, does give effervescence. Its three nitro groups, N O two, stabilise the phenoxide ion so strongly that picric acid is a stronger acid than carbonic acid. Got it?

4:28Two colour tests: phthalein and Liebermann

Now two colour tests. First, the phthalein dye test. Heat the phenol with phthalic anhydride and concentrated sulphuric acid. On screen, H two S O four is sulphuric acid, and the short word before it means concentrated. Okay? Now, the name of the test is a hint. Which dye, one you already use in titrations, could this test make? Yes, exactly, it makes phenolphthalein. Pour the product into sodium hydroxide, and you see its pink colour. So a pink colour here means a phenol. Right? Second, Liebermann's test. Add sodium nitrite, N a N O two, and concentrated sulphuric acid. Then a deep blue or green colour forms. With me? Now watch the colour change. On dilution, it turns red. And in alkali, it becomes blue or green again. The colour changes twice in one test, so read such questions carefully. Okay?

5:29Example 1: identify a solid from NaOH, NaHCO3, bromine water

Time for the first example. We have a solid that dissolves in sodium hydroxide, but not in sodium hydrogencarbonate solution, and it gives a white precipitate with bromine water. What is it likely to be? Pause the video and try it first. Done? Okay, let's check. It dissolves in sodium hydroxide, so it is acidic. Right? But it does not dissolve in sodium hydrogencarbonate, so it is too weak an acid to give carbon dioxide. That is exactly our puzzle from the start. Clear? And the white precipitate with bromine water is just what phenol gives. So the answer is a phenol. Simple, right?

6:11Example 2: bromine water precipitate mass, 4-methylphenol

Now a harder one, with numbers. Four methylphenol, with molar mass capital M equal to one hundred and eight, has only two positions open to bromine water. Find the mass of the precipitate from zero point five four grams of it; the small g on screen means grams. Take the atomic mass of bromine, B r, as eighty. Pause the video and try it first. Done? Okay, let's check. Phenol gave two, four, six tribromophenol. So here also we get three bromine atoms, right? No, that is the trap, so look at position four. It already holds the methyl group, C H three, and position four is the para position. So only the two ortho positions, two and six, are open. That is why the question says only two positions. Clear? So the precipitate is two, six dibromo, four methylphenol. Okay? Now its molar mass. Each bromine atom replaces one hydrogen atom. So start from one hundred and eight, subtract two for the two hydrogens, and add two times eighty for the two bromines; the cross means times. That gives two hundred and sixty six. Got it? Next, the moles of four methylphenol. Small n stands for the number of moles, and mol is short for moles. Zero point five four over one hundred and eight gives zero point zero zero five moles. Right? Each mole of four methylphenol gives one mole of this precipitate. So the mass is zero point zero zero five times two hundred and sixty six, which is one point three three grams. Right positions, right molar mass, right answer. See how it all fits?

7:59Quick revision: FeCl3 violet, white tribromophenol, no CO2

So, quick revision. Neutral ferric chloride gives a violet colour, and an enolic O H group gives it too. Bromine water gives a white precipitate of two, four, six tribromophenol. And phenol is acidic enough for sodium hydroxide, but too weak to give carbon dioxide with sodium hydrogencarbonate. Now go and try the practice problems on this page. You will find them easy. All the best!