JEE Advanced Questions (2012–2016): Acids

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Phenol has another name, carbolic acid, so it is an acid right there in its name. Yet stir it into sodium hydrogen carbonate solution, and not a single bubble of carbon dioxide comes out. Benzoic acid, in the same solution, gives effervescence, bubbles of carbon dioxide. So why does this acid give no carbon dioxide? Today two past JEE questions on acids, from JEE Advanced twenty thirteen and IITJEE twenty twelve, will settle it. Questions like these come up often, so let's begin.

0:38Why a carboxylic acid is an acid

So, first, why is a carboxylic acid an acid at all? RCOOH, where R is the rest of the molecule, gives a proton to water, forming RCOO minus and H three O plus. The small numbers written low are subscripts, the signs written high are charges, and the two half arrows mean equilibrium. Its p K a, a small p, a capital K and a small a below, is about four to five. Okay? Now, the proton has gone, and a negative charge is left behind. But the carboxylate ion, RCOO minus, has two oxygens. So which oxygen carries the charge? Both of them, yes, exactly! Two equivalent resonance structures share the charge between the two oxygens, and this sharing stabilises the ion. So the acid gives up its proton quite easily. Right? So where do carboxylic acids stand? They are weaker than mineral acids, but stronger than phenols and alcohols. With me? And one more fact for later. Put a group on the benzene ring of benzoic acid, right next to the COOH group, in the ortho position. Nearly every such ortho substituted benzoic acid is stronger than benzoic acid itself. This is the ortho effect. Keep it in mind for example one, okay?

2:02Stronger than carbonic acid, or not?

Now look at the course's p K a ladder, on the right. The red arrow pointing left means a stronger acid, and the blue arrow pointing right means a stronger conjugate base. Ethanoic acid, C H three COOH, sits at four point eight; carbonic acid, H two C O three, at six point four; and phenol at ten point zero. Further right are water and alcohols, and then much weaker acids that we do not need today. Right? So tell me, phenol or carbonic acid: which one is the stronger acid? Carbonic acid, very good! The smaller p K a means the stronger acid. So here is the order, strongest first: sulphonic acids, RSO three H, then carboxylic acids, carbonic acid, phenol, water, and alcohols, ROH, last. Each greater than sign means more acidic than. Got it? And now the rule behind every acid test today: the stronger acid displaces the weaker acid from its salt. Clear? Sodium hydrogen carbonate, N a HCO three, is a salt of carbonic acid. So only acids above carbonic acid in our order can displace it and liberate carbon dioxide, C O two. That means carboxylic acids and sulphonic acids. With me? And phenol sits below carbonic acid, so it gives no carbon dioxide at all. That is our puzzle solved! But careful, picric acid, at zero point four on the far left of the ladder, is strong enough, so it does give carbon dioxide. Right? Now the second reagent, N a O H, which is sodium hydroxide. Carboxylic acids and phenols both react with it, and form soluble sodium salts. Okay? And what about alcohols, ROH? Will they form a salt with sodium hydroxide too? No, they will not. Alcohols are too weakly acidic, so no salt forms. Keep these two reagents in mind, because example two needs both. Got it?

4:10Example 1: which acid gives no CO2 with NaHCO3

Alright, example one, from JEE Advanced twenty thirteen. Which of these does not liberate carbon dioxide from aqueous sodium hydrogen carbonate: benzoic acid, benzenesulphonic acid, salicylic acid, or phenol? Pause the video and try it first. Done? Okay, let's check. Benzoic acid, C six H five COOH, is a carboxylic acid, stronger than carbonic acid, so it liberates carbon dioxide. Right? Benzenesulphonic acid, C six H five S O three H, is a sulphonic acid, at the very top of our order. It is even stronger, so it gives carbon dioxide too. Okay? Now salicylic acid, which is two hydroxybenzoic acid. It has an O H group on the ring, just like phenol. So it behaves like phenol and gives no carbon dioxide, right? No, no, careful, that is the trap! Read the name again: hydroxybenzoic acid. It still has a COOH group, so it is a carboxylic acid. And its O H group is ortho, so by the ortho effect it is even stronger than benzoic acid. It gives carbon dioxide, clear? That leaves phenol, C six H five O H, which is weaker than carbonic acid, so it gives no carbon dioxide. So the answer is phenol, our carbolic acid from the start. That's it!

5:36Example 2: separating mixtures with NaOH and NaHCO3

Now the harder one, from IITJEE twenty twelve. Which of these binary mixtures, that is, mixtures of two compounds, can be separated by extraction with aqueous sodium hydroxide, and also with aqueous sodium hydrogen carbonate? The Roman numerals in brackets number the mixtures, and each plus sign means mixed with. Mixture one is phenol and benzoic acid; mixture two, benzoic acid and benzyl alcohol; mixture three, benzyl alcohol and phenol; and mixture four, benzyl alcohol and phenylethanoic acid. Pause the video and try it first. Done? Okay, let's check. First, for a reagent to separate two compounds by extraction, how many of the two should it dissolve? Exactly one, very good! If it dissolves both, or neither, the two stay together. And here, both reagents must pass this test. Okay? Now bring in our two reagents. Sodium hydroxide dissolves carboxylic acids and phenol; sodium hydrogen carbonate dissolves only the carboxylic acids; and an alcohol dissolves in neither. With me? Mixture one, phenol and benzoic acid. Phenol is the weaker acid, so sodium hydroxide takes only the benzoic acid and leaves the phenol behind, right? Oh no, careful, sodium hydroxide dissolves both of them! Phenol is weak, but it still forms its sodium salt with sodium hydroxide. So sodium hydroxide cannot separate them, and since both reagents must work, mixture one fails. Okay? Mixture two, benzoic acid and benzyl alcohol. Sodium hydroxide takes only the benzoic acid, and so does sodium hydrogen carbonate, while benzyl alcohol stays behind both times. So mixture two works. Got it? Mixture three, benzyl alcohol and phenol. Sodium hydroxide takes the phenol, fine. But sodium hydrogen carbonate dissolves neither of them, because phenol is weaker than carbonic acid. So mixture three fails. Clear? And mixture four. Phenylethanoic acid is C six H five C H two COOH, a carboxylic acid, so this mixture behaves just like mixture two. Both reagents take only the acid. So the answer is mixtures two and four. That's it!

8:04Quick revision: acidity order, NaHCO3 and NaOH tests

So, quick revision. The acidity order is sulphonic acids, carboxylic acids, carbonic acid, phenol, water, and then alcohols. Sodium hydrogen carbonate gives carbon dioxide only with acids stronger than carbonic acid, so not with phenol. And sodium hydroxide dissolves carboxylic acids and phenols, but not alcohols. Now try the other JEE Advanced questions on this page. All the best!